src/Doc/IsarImplementation/Logic.thy
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theory Logic
imports Base
begin

chapter {* Primitive logic \label{ch:logic} *}

text {*
The logical foundations of Isabelle/Isar are that of the Pure logic,
which has been introduced as a Natural Deduction framework in
\cite{paulson700}.  This is essentially the same logic as @{text
"\<lambda>HOL"}'' in the more abstract setting of Pure Type Systems (PTS)
\cite{Barendregt-Geuvers:2001}, although there are some key
differences in the specific treatment of simple types in
Isabelle/Pure.

Following type-theoretic parlance, the Pure logic consists of three
levels of @{text "\<lambda>"}-calculus with corresponding arrows, @{text
"\<Rightarrow>"} for syntactic function space (terms depending on terms), @{text
"\<And>"} for universal quantification (proofs depending on terms), and
@{text "\<Longrightarrow>"} for implication (proofs depending on proofs).

Derivations are relative to a logical theory, which declares type
constructors, constants, and axioms.  Theory declarations support
schematic polymorphism, which is strictly speaking outside the
logic.\footnote{This is the deeper logical reason, why the theory
context @{text "\<Theta>"} is separate from the proof context @{text "\<Gamma>"}
of the core calculus: type constructors, term constants, and facts
(proof constants) may involve arbitrary type schemes, but the type
of a locally fixed term parameter is also fixed!}
*}

section {* Types \label{sec:types} *}

text {*
The language of types is an uninterpreted order-sorted first-order
algebra; types are qualified by ordered type classes.

\medskip A \emph{type class} is an abstract syntactic entity
declared in the theory context.  The \emph{subclass relation} @{text
"c\<^sub>1 \<subseteq> c\<^sub>2"} is specified by stating an acyclic
generating relation; the transitive closure is maintained
internally.  The resulting relation is an ordering: reflexive,
transitive, and antisymmetric.

A \emph{sort} is a list of type classes written as @{text "s = {c\<^sub>1,
\<dots>, c\<^sub>m}"}, it represents symbolic intersection.  Notationally, the
curly braces are omitted for singleton intersections, i.e.\ any
class @{text "c"} may be read as a sort @{text "{c}"}.  The ordering
on type classes is extended to sorts according to the meaning of
intersections: @{text "{c\<^sub>1, \<dots> c\<^sub>m} \<subseteq> {d\<^sub>1, \<dots>, d\<^sub>n}"} iff @{text
"\<forall>j. \<exists>i. c\<^sub>i \<subseteq> d\<^sub>j"}.  The empty intersection @{text "{}"} refers to
the universal sort, which is the largest element wrt.\ the sort
order.  Thus @{text "{}"} represents the full sort'', not the
empty one!  The intersection of all (finitely many) classes declared
in the current theory is the least element wrt.\ the sort ordering.

\medskip A \emph{fixed type variable} is a pair of a basic name
(starting with a @{text "'"} character) and a sort constraint, e.g.\
@{text "('a, s)"} which is usually printed as @{text "\<alpha>\<^sub>s"}.
A \emph{schematic type variable} is a pair of an indexname and a
sort constraint, e.g.\ @{text "(('a, 0), s)"} which is usually
printed as @{text "?\<alpha>\<^sub>s"}.

Note that \emph{all} syntactic components contribute to the identity
of type variables: basic name, index, and sort constraint.  The core
logic handles type variables with the same name but different sorts
as different, although the type-inference layer (which is outside
the core) rejects anything like that.

A \emph{type constructor} @{text "\<kappa>"} is a @{text "k"}-ary operator
on types declared in the theory.  Type constructor application is
written postfix as @{text "(\<alpha>\<^sub>1, \<dots>, \<alpha>\<^sub>k)\<kappa>"}.  For
@{text "k = 0"} the argument tuple is omitted, e.g.\ @{text "prop"}
instead of @{text "()prop"}.  For @{text "k = 1"} the parentheses
are omitted, e.g.\ @{text "\<alpha> list"} instead of @{text "(\<alpha>)list"}.
Further notation is provided for specific constructors, notably the
right-associative infix @{text "\<alpha> \<Rightarrow> \<beta>"} instead of @{text "(\<alpha>,
\<beta>)fun"}.

The logical category \emph{type} is defined inductively over type
variables and type constructors as follows: @{text "\<tau> = \<alpha>\<^sub>s | ?\<alpha>\<^sub>s |
(\<tau>\<^sub>1, \<dots>, \<tau>\<^sub>k)\<kappa>"}.

A \emph{type abbreviation} is a syntactic definition @{text
"(\<^vec>\<alpha>)\<kappa> = \<tau>"} of an arbitrary type expression @{text "\<tau>"} over
variables @{text "\<^vec>\<alpha>"}.  Type abbreviations appear as type
constructors in the syntax, but are expanded before entering the
logical core.

A \emph{type arity} declares the image behavior of a type
constructor wrt.\ the algebra of sorts: @{text "\<kappa> :: (s\<^sub>1, \<dots>,
s\<^sub>k)s"} means that @{text "(\<tau>\<^sub>1, \<dots>, \<tau>\<^sub>k)\<kappa>"} is
of sort @{text "s"} if every argument type @{text "\<tau>\<^sub>i"} is
of sort @{text "s\<^sub>i"}.  Arity declarations are implicitly
completed, i.e.\ @{text "\<kappa> :: (\<^vec>s)c"} entails @{text "\<kappa> ::
(\<^vec>s)c'"} for any @{text "c' \<supseteq> c"}.

\medskip The sort algebra is always maintained as \emph{coregular},
which means that type arities are consistent with the subclass
relation: for any type constructor @{text "\<kappa>"}, and classes @{text
"c\<^sub>1 \<subseteq> c\<^sub>2"}, and arities @{text "\<kappa> ::
(\<^vec>s\<^sub>1)c\<^sub>1"} and @{text "\<kappa> ::
(\<^vec>s\<^sub>2)c\<^sub>2"} holds @{text "\<^vec>s\<^sub>1 \<subseteq>
\<^vec>s\<^sub>2"} component-wise.

The key property of a coregular order-sorted algebra is that sort
constraints can be solved in a most general fashion: for each type
constructor @{text "\<kappa>"} and sort @{text "s"} there is a most general
vector of argument sorts @{text "(s\<^sub>1, \<dots>, s\<^sub>k)"} such
that a type scheme @{text "(\<alpha>\<^bsub>s\<^sub>1\<^esub>, \<dots>,
\<alpha>\<^bsub>s\<^sub>k\<^esub>)\<kappa>"} is of sort @{text "s"}.
Consequently, type unification has most general solutions (modulo
equivalence of sorts), so type-inference produces primary types as
expected \cite{nipkow-prehofer}.
*}

text %mlref {*
\begin{mldecls}
@{index_ML_type class: string} \\
@{index_ML_type sort: "class list"} \\
@{index_ML_type arity: "string * sort list * sort"} \\
@{index_ML_type typ} \\
@{index_ML Term.map_atyps: "(typ -> typ) -> typ -> typ"} \\
@{index_ML Term.fold_atyps: "(typ -> 'a -> 'a) -> typ -> 'a -> 'a"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML Sign.subsort: "theory -> sort * sort -> bool"} \\
@{index_ML Sign.of_sort: "theory -> typ * sort -> bool"} \\
@{index_ML Sign.add_type: "Proof.context -> binding * int * mixfix -> theory -> theory"} \\
binding * string list * typ -> theory -> theory"} \\
@{index_ML Sign.primitive_class: "binding * class list -> theory -> theory"} \\
@{index_ML Sign.primitive_classrel: "class * class -> theory -> theory"} \\
@{index_ML Sign.primitive_arity: "arity -> theory -> theory"} \\
\end{mldecls}

\begin{description}

\item Type @{ML_type class} represents type classes.

\item Type @{ML_type sort} represents sorts, i.e.\ finite
intersections of classes.  The empty list @{ML "[]: sort"} refers to
the empty class intersection, i.e.\ the full sort''.

\item Type @{ML_type arity} represents type arities.  A triple
@{text "(\<kappa>, \<^vec>s, s) : arity"} represents @{text "\<kappa> ::
(\<^vec>s)s"} as described above.

\item Type @{ML_type typ} represents types; this is a datatype with
constructors @{ML TFree}, @{ML TVar}, @{ML Type}.

\item @{ML Term.map_atyps}~@{text "f \<tau>"} applies the mapping @{text
"f"} to all atomic types (@{ML TFree}, @{ML TVar}) occurring in
@{text "\<tau>"}.

\item @{ML Term.fold_atyps}~@{text "f \<tau>"} iterates the operation
@{text "f"} over all occurrences of atomic types (@{ML TFree}, @{ML
TVar}) in @{text "\<tau>"}; the type structure is traversed from left to
right.

\item @{ML Sign.subsort}~@{text "thy (s\<^sub>1, s\<^sub>2)"}
tests the subsort relation @{text "s\<^sub>1 \<subseteq> s\<^sub>2"}.

\item @{ML Sign.of_sort}~@{text "thy (\<tau>, s)"} tests whether type
@{text "\<tau>"} is of sort @{text "s"}.

\item @{ML Sign.add_type}~@{text "ctxt (\<kappa>, k, mx)"} declares a
new type constructors @{text "\<kappa>"} with @{text "k"} arguments and
optional mixfix syntax.

\item @{ML Sign.add_type_abbrev}~@{text "ctxt (\<kappa>, \<^vec>\<alpha>, \<tau>)"}
defines a new type abbreviation @{text "(\<^vec>\<alpha>)\<kappa> = \<tau>"}.

\item @{ML Sign.primitive_class}~@{text "(c, [c\<^sub>1, \<dots>,
c\<^sub>n])"} declares a new class @{text "c"}, together with class
relations @{text "c \<subseteq> c\<^sub>i"}, for @{text "i = 1, \<dots>, n"}.

\item @{ML Sign.primitive_classrel}~@{text "(c\<^sub>1,
c\<^sub>2)"} declares the class relation @{text "c\<^sub>1 \<subseteq>
c\<^sub>2"}.

\item @{ML Sign.primitive_arity}~@{text "(\<kappa>, \<^vec>s, s)"} declares
the arity @{text "\<kappa> :: (\<^vec>s)s"}.

\end{description}
*}

text %mlantiq {*
\begin{matharray}{rcl}
@{ML_antiquotation_def "class"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "sort"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "type_name"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "type_abbrev"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "nonterminal"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "typ"} & : & @{text ML_antiquotation} \\
\end{matharray}

@{rail \<open>
@@{ML_antiquotation class} nameref
;
@@{ML_antiquotation sort} sort
;
(@@{ML_antiquotation type_name} |
@@{ML_antiquotation type_abbrev} |
@@{ML_antiquotation nonterminal}) nameref
;
@@{ML_antiquotation typ} type
\<close>}

\begin{description}

\item @{text "@{class c}"} inlines the internalized class @{text
"c"} --- as @{ML_type string} literal.

\item @{text "@{sort s}"} inlines the internalized sort @{text "s"}
--- as @{ML_type "string list"} literal.

\item @{text "@{type_name c}"} inlines the internalized type
constructor @{text "c"} --- as @{ML_type string} literal.

\item @{text "@{type_abbrev c}"} inlines the internalized type
abbreviation @{text "c"} --- as @{ML_type string} literal.

\item @{text "@{nonterminal c}"} inlines the internalized syntactic
type~/ grammar nonterminal @{text "c"} --- as @{ML_type string}
literal.

\item @{text "@{typ \<tau>}"} inlines the internalized type @{text "\<tau>"}
--- as constructor term for datatype @{ML_type typ}.

\end{description}
*}

section {* Terms \label{sec:terms} *}

text {*
The language of terms is that of simply-typed @{text "\<lambda>"}-calculus
with de-Bruijn indices for bound variables (cf.\ \cite{debruijn72}
or \cite{paulson-ml2}), with the types being determined by the
corresponding binders.  In contrast, free variables and constants
have an explicit name and type in each occurrence.

\medskip A \emph{bound variable} is a natural number @{text "b"},
which accounts for the number of intermediate binders between the
variable occurrence in the body and its binding position.  For
example, the de-Bruijn term @{text "\<lambda>\<^bsub>bool\<^esub>. \<lambda>\<^bsub>bool\<^esub>. 1 \<and> 0"} would
correspond to @{text "\<lambda>x\<^bsub>bool\<^esub>. \<lambda>y\<^bsub>bool\<^esub>. x \<and> y"} in a named
representation.  Note that a bound variable may be represented by
different de-Bruijn indices at different occurrences, depending on
the nesting of abstractions.

A \emph{loose variable} is a bound variable that is outside the
scope of local binders.  The types (and names) for loose variables
can be managed as a separate context, that is maintained as a stack
of hypothetical binders.  The core logic operates on closed terms,
without any loose variables.

A \emph{fixed variable} is a pair of a basic name and a type, e.g.\
@{text "(x, \<tau>)"} which is usually printed @{text "x\<^sub>\<tau>"} here.  A
\emph{schematic variable} is a pair of an indexname and a type,
e.g.\ @{text "((x, 0), \<tau>)"} which is likewise printed as @{text
"?x\<^sub>\<tau>"}.

\medskip A \emph{constant} is a pair of a basic name and a type,
e.g.\ @{text "(c, \<tau>)"} which is usually printed as @{text "c\<^sub>\<tau>"}
here.  Constants are declared in the context as polymorphic families
@{text "c :: \<sigma>"}, meaning that all substitution instances @{text
"c\<^sub>\<tau>"} for @{text "\<tau> = \<sigma>\<vartheta>"} are valid.

The vector of \emph{type arguments} of constant @{text "c\<^sub>\<tau>"} wrt.\
the declaration @{text "c :: \<sigma>"} is defined as the codomain of the
matcher @{text "\<vartheta> = {?\<alpha>\<^sub>1 \<mapsto> \<tau>\<^sub>1, \<dots>, ?\<alpha>\<^sub>n \<mapsto> \<tau>\<^sub>n}"} presented in
canonical order @{text "(\<tau>\<^sub>1, \<dots>, \<tau>\<^sub>n)"}, corresponding to the
left-to-right occurrences of the @{text "\<alpha>\<^sub>i"} in @{text "\<sigma>"}.
Within a given theory context, there is a one-to-one correspondence
between any constant @{text "c\<^sub>\<tau>"} and the application @{text "c(\<tau>\<^sub>1,
\<dots>, \<tau>\<^sub>n)"} of its type arguments.  For example, with @{text "plus :: \<alpha>
\<Rightarrow> \<alpha> \<Rightarrow> \<alpha>"}, the instance @{text "plus\<^bsub>nat \<Rightarrow> nat \<Rightarrow> nat\<^esub>"} corresponds to
@{text "plus(nat)"}.

Constant declarations @{text "c :: \<sigma>"} may contain sort constraints
for type variables in @{text "\<sigma>"}.  These are observed by
type-inference as expected, but \emph{ignored} by the core logic.
This means the primitive logic is able to reason with instances of
polymorphic constants that the user-level type-checker would reject
due to violation of type class restrictions.

\medskip An \emph{atomic term} is either a variable or constant.
The logical category \emph{term} is defined inductively over atomic
terms, with abstraction and application as follows: @{text "t = b |
x\<^sub>\<tau> | ?x\<^sub>\<tau> | c\<^sub>\<tau> | \<lambda>\<^sub>\<tau>. t | t\<^sub>1 t\<^sub>2"}.  Parsing and printing takes care of
converting between an external representation with named bound
variables.  Subsequently, we shall use the latter notation instead
of internal de-Bruijn representation.

The inductive relation @{text "t :: \<tau>"} assigns a (unique) type to a
term according to the structure of atomic terms, abstractions, and
applicatins:
$\infer{@{text "a\<^sub>\<tau> :: \<tau>"}}{} \qquad \infer{@{text "(\<lambda>x\<^sub>\<tau>. t) :: \<tau> \<Rightarrow> \<sigma>"}}{@{text "t :: \<sigma>"}} \qquad \infer{@{text "t u :: \<sigma>"}}{@{text "t :: \<tau> \<Rightarrow> \<sigma>"} & @{text "u :: \<tau>"}}$
A \emph{well-typed term} is a term that can be typed according to these rules.

Typing information can be omitted: type-inference is able to
reconstruct the most general type of a raw term, while assigning
most general types to all of its variables and constants.
Type-inference depends on a context of type constraints for fixed
variables, and declarations for polymorphic constants.

The identity of atomic terms consists both of the name and the type
component.  This means that different variables @{text
"x\<^bsub>\<tau>\<^sub>1\<^esub>"} and @{text "x\<^bsub>\<tau>\<^sub>2\<^esub>"} may become the same after
type instantiation.  Type-inference rejects variables of the same
name, but different types.  In contrast, mixed instances of
polymorphic constants occur routinely.

\medskip The \emph{hidden polymorphism} of a term @{text "t :: \<sigma>"}
is the set of type variables occurring in @{text "t"}, but not in
its type @{text "\<sigma>"}.  This means that the term implicitly depends
on type arguments that are not accounted in the result type, i.e.\
there are different type instances @{text "t\<vartheta> :: \<sigma>"} and
@{text "t\<vartheta>' :: \<sigma>"} with the same type.  This slightly
pathological situation notoriously demands additional care.

\medskip A \emph{term abbreviation} is a syntactic definition @{text
"c\<^sub>\<sigma> \<equiv> t"} of a closed term @{text "t"} of type @{text "\<sigma>"},
without any hidden polymorphism.  A term abbreviation looks like a
constant in the syntax, but is expanded before entering the logical
core.  Abbreviations are usually reverted when printing terms, using
@{text "t \<rightarrow> c\<^sub>\<sigma>"} as rules for higher-order rewriting.

\medskip Canonical operations on @{text "\<lambda>"}-terms include @{text
"\<alpha>\<beta>\<eta>"}-conversion: @{text "\<alpha>"}-conversion refers to capture-free
renaming of bound variables; @{text "\<beta>"}-conversion contracts an
abstraction applied to an argument term, substituting the argument
in the body: @{text "(\<lambda>x. b)a"} becomes @{text "b[a/x]"}; @{text
"\<eta>"}-conversion contracts vacuous application-abstraction: @{text
"\<lambda>x. f x"} becomes @{text "f"}, provided that the bound variable
does not occur in @{text "f"}.

Terms are normally treated modulo @{text "\<alpha>"}-conversion, which is
implicit in the de-Bruijn representation.  Names for bound variables
in abstractions are maintained separately as (meaningless) comments,
mostly for parsing and printing.  Full @{text "\<alpha>\<beta>\<eta>"}-conversion is
commonplace in various standard operations (\secref{sec:obj-rules})
that are based on higher-order unification and matching.
*}

text %mlref {*
\begin{mldecls}
@{index_ML_type term} \\
@{index_ML_op "aconv": "term * term -> bool"} \\
@{index_ML Term.map_types: "(typ -> typ) -> term -> term"} \\
@{index_ML Term.fold_types: "(typ -> 'a -> 'a) -> term -> 'a -> 'a"} \\
@{index_ML Term.map_aterms: "(term -> term) -> term -> term"} \\
@{index_ML Term.fold_aterms: "(term -> 'a -> 'a) -> term -> 'a -> 'a"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML fastype_of: "term -> typ"} \\
@{index_ML lambda: "term -> term -> term"} \\
@{index_ML betapply: "term * term -> term"} \\
@{index_ML incr_boundvars: "int -> term -> term"} \\
@{index_ML Sign.declare_const: "Proof.context ->
(binding * typ) * mixfix -> theory -> term * theory"} \\
@{index_ML Sign.add_abbrev: "string -> binding * term ->
theory -> (term * term) * theory"} \\
@{index_ML Sign.const_typargs: "theory -> string * typ -> typ list"} \\
@{index_ML Sign.const_instance: "theory -> string * typ list -> typ"} \\
\end{mldecls}

\begin{description}

\item Type @{ML_type term} represents de-Bruijn terms, with comments
in abstractions, and explicitly named free variables and constants;
this is a datatype with constructors @{index_ML Bound}, @{index_ML
Free}, @{index_ML Var}, @{index_ML Const}, @{index_ML Abs},
@{index_ML_op "$"}. \item @{text "t"}~@{ML_text aconv}~@{text "u"} checks @{text "\<alpha>"}-equivalence of two terms. This is the basic equality relation on type @{ML_type term}; raw datatype equality should only be used for operations related to parsing or printing! \item @{ML Term.map_types}~@{text "f t"} applies the mapping @{text "f"} to all types occurring in @{text "t"}. \item @{ML Term.fold_types}~@{text "f t"} iterates the operation @{text "f"} over all occurrences of types in @{text "t"}; the term structure is traversed from left to right. \item @{ML Term.map_aterms}~@{text "f t"} applies the mapping @{text "f"} to all atomic terms (@{ML Bound}, @{ML Free}, @{ML Var}, @{ML Const}) occurring in @{text "t"}. \item @{ML Term.fold_aterms}~@{text "f t"} iterates the operation @{text "f"} over all occurrences of atomic terms (@{ML Bound}, @{ML Free}, @{ML Var}, @{ML Const}) in @{text "t"}; the term structure is traversed from left to right. \item @{ML fastype_of}~@{text "t"} determines the type of a well-typed term. This operation is relatively slow, despite the omission of any sanity checks. \item @{ML lambda}~@{text "a b"} produces an abstraction @{text "\<lambda>a. b"}, where occurrences of the atomic term @{text "a"} in the body @{text "b"} are replaced by bound variables. \item @{ML betapply}~@{text "(t, u)"} produces an application @{text "t u"}, with topmost @{text "\<beta>"}-conversion if @{text "t"} is an abstraction. \item @{ML incr_boundvars}~@{text "j"} increments a term's dangling bound variables by the offset @{text "j"}. This is required when moving a subterm into a context where it is enclosed by a different number of abstractions. Bound variables with a matching abstraction are unaffected. \item @{ML Sign.declare_const}~@{text "ctxt ((c, \<sigma>), mx)"} declares a new constant @{text "c :: \<sigma>"} with optional mixfix syntax. \item @{ML Sign.add_abbrev}~@{text "print_mode (c, t)"} introduces a new term abbreviation @{text "c \<equiv> t"}. \item @{ML Sign.const_typargs}~@{text "thy (c, \<tau>)"} and @{ML Sign.const_instance}~@{text "thy (c, [\<tau>\<^sub>1, \<dots>, \<tau>\<^sub>n])"} convert between two representations of polymorphic constants: full type instance vs.\ compact type arguments form. \end{description} *} text %mlantiq {* \begin{matharray}{rcl} @{ML_antiquotation_def "const_name"} & : & @{text ML_antiquotation} \\ @{ML_antiquotation_def "const_abbrev"} & : & @{text ML_antiquotation} \\ @{ML_antiquotation_def "const"} & : & @{text ML_antiquotation} \\ @{ML_antiquotation_def "term"} & : & @{text ML_antiquotation} \\ @{ML_antiquotation_def "prop"} & : & @{text ML_antiquotation} \\ \end{matharray} @{rail \<open> (@@{ML_antiquotation const_name} | @@{ML_antiquotation const_abbrev}) nameref ; @@{ML_antiquotation const} ('(' (type + ',') ')')? ; @@{ML_antiquotation term} term ; @@{ML_antiquotation prop} prop \<close>} \begin{description} \item @{text "@{const_name c}"} inlines the internalized logical constant name @{text "c"} --- as @{ML_type string} literal. \item @{text "@{const_abbrev c}"} inlines the internalized abbreviated constant name @{text "c"} --- as @{ML_type string} literal. \item @{text "@{const c(\<^vec>\<tau>)}"} inlines the internalized constant @{text "c"} with precise type instantiation in the sense of @{ML Sign.const_instance} --- as @{ML Const} constructor term for datatype @{ML_type term}. \item @{text "@{term t}"} inlines the internalized term @{text "t"} --- as constructor term for datatype @{ML_type term}. \item @{text "@{prop \<phi>}"} inlines the internalized proposition @{text "\<phi>"} --- as constructor term for datatype @{ML_type term}. \end{description} *} section {* Theorems \label{sec:thms} *} text {* A \emph{proposition} is a well-typed term of type @{text "prop"}, a \emph{theorem} is a proven proposition (depending on a context of hypotheses and the background theory). Primitive inferences include plain Natural Deduction rules for the primary connectives @{text "\<And>"} and @{text "\<Longrightarrow>"} of the framework. There is also a builtin notion of equality/equivalence @{text "\<equiv>"}. *} subsection {* Primitive connectives and rules \label{sec:prim-rules} *} text {* The theory @{text "Pure"} contains constant declarations for the primitive connectives @{text "\<And>"}, @{text "\<Longrightarrow>"}, and @{text "\<equiv>"} of the logical framework, see \figref{fig:pure-connectives}. The derivability judgment @{text "A\<^sub>1, \<dots>, A\<^sub>n \<turnstile> B"} is defined inductively by the primitive inferences given in \figref{fig:prim-rules}, with the global restriction that the hypotheses must \emph{not} contain any schematic variables. The builtin equality is conceptually axiomatized as shown in \figref{fig:pure-equality}, although the implementation works directly with derived inferences. \begin{figure}[htb] \begin{center} \begin{tabular}{ll} @{text "all :: (\<alpha> \<Rightarrow> prop) \<Rightarrow> prop"} & universal quantification (binder @{text "\<And>"}) \\ @{text "\<Longrightarrow> :: prop \<Rightarrow> prop \<Rightarrow> prop"} & implication (right associative infix) \\ @{text "\<equiv> :: \<alpha> \<Rightarrow> \<alpha> \<Rightarrow> prop"} & equality relation (infix) \\ \end{tabular} \caption{Primitive connectives of Pure}\label{fig:pure-connectives} \end{center} \end{figure} \begin{figure}[htb] \begin{center} $\infer[@{text "(axiom)"}]{@{text "\<turnstile> A"}}{@{text "A \<in> \<Theta>"}} \qquad \infer[@{text "(assume)"}]{@{text "A \<turnstile> A"}}{}$ $\infer[@{text "(\<And>\<hyphen>intro)"}]{@{text "\<Gamma> \<turnstile> \<And>x. B[x]"}}{@{text "\<Gamma> \<turnstile> B[x]"} & @{text "x \<notin> \<Gamma>"}} \qquad \infer[@{text "(\<And>\<hyphen>elim)"}]{@{text "\<Gamma> \<turnstile> B[a]"}}{@{text "\<Gamma> \<turnstile> \<And>x. B[x]"}}$ $\infer[@{text "(\<Longrightarrow>\<hyphen>intro)"}]{@{text "\<Gamma> - A \<turnstile> A \<Longrightarrow> B"}}{@{text "\<Gamma> \<turnstile> B"}} \qquad \infer[@{text "(\<Longrightarrow>\<hyphen>elim)"}]{@{text "\<Gamma>\<^sub>1 \<union> \<Gamma>\<^sub>2 \<turnstile> B"}}{@{text "\<Gamma>\<^sub>1 \<turnstile> A \<Longrightarrow> B"} & @{text "\<Gamma>\<^sub>2 \<turnstile> A"}}$ \caption{Primitive inferences of Pure}\label{fig:prim-rules} \end{center} \end{figure} \begin{figure}[htb] \begin{center} \begin{tabular}{ll} @{text "\<turnstile> (\<lambda>x. b[x]) a \<equiv> b[a]"} & @{text "\<beta>"}-conversion \\ @{text "\<turnstile> x \<equiv> x"} & reflexivity \\ @{text "\<turnstile> x \<equiv> y \<Longrightarrow> P x \<Longrightarrow> P y"} & substitution \\ @{text "\<turnstile> (\<And>x. f x \<equiv> g x) \<Longrightarrow> f \<equiv> g"} & extensionality \\ @{text "\<turnstile> (A \<Longrightarrow> B) \<Longrightarrow> (B \<Longrightarrow> A) \<Longrightarrow> A \<equiv> B"} & logical equivalence \\ \end{tabular} \caption{Conceptual axiomatization of Pure equality}\label{fig:pure-equality} \end{center} \end{figure} The introduction and elimination rules for @{text "\<And>"} and @{text "\<Longrightarrow>"} are analogous to formation of dependently typed @{text "\<lambda>"}-terms representing the underlying proof objects. Proof terms are irrelevant in the Pure logic, though; they cannot occur within propositions. The system provides a runtime option to record explicit proof terms for primitive inferences, see also \secref{sec:proof-terms}. Thus all three levels of @{text "\<lambda>"}-calculus become explicit: @{text "\<Rightarrow>"} for terms, and @{text "\<And>/\<Longrightarrow>"} for proofs (cf.\ \cite{Berghofer-Nipkow:2000:TPHOL}). Observe that locally fixed parameters (as in @{text "\<And>\<hyphen>intro"}) need not be recorded in the hypotheses, because the simple syntactic types of Pure are always inhabitable. Assumptions'' @{text "x :: \<tau>"} for type-membership are only present as long as some @{text "x\<^sub>\<tau>"} occurs in the statement body.\footnote{This is the key difference to @{text "\<lambda>HOL"}'' in the PTS framework \cite{Barendregt-Geuvers:2001}, where hypotheses @{text "x : A"} are treated uniformly for propositions and types.} \medskip The axiomatization of a theory is implicitly closed by forming all instances of type and term variables: @{text "\<turnstile> A\<vartheta>"} holds for any substitution instance of an axiom @{text "\<turnstile> A"}. By pushing substitutions through derivations inductively, we also get admissible @{text "generalize"} and @{text "instantiate"} rules as shown in \figref{fig:subst-rules}. \begin{figure}[htb] \begin{center} $\infer{@{text "\<Gamma> \<turnstile> B[?\<alpha>]"}}{@{text "\<Gamma> \<turnstile> B[\<alpha>]"} & @{text "\<alpha> \<notin> \<Gamma>"}} \quad \infer[\quad@{text "(generalize)"}]{@{text "\<Gamma> \<turnstile> B[?x]"}}{@{text "\<Gamma> \<turnstile> B[x]"} & @{text "x \<notin> \<Gamma>"}}$ $\infer{@{text "\<Gamma> \<turnstile> B[\<tau>]"}}{@{text "\<Gamma> \<turnstile> B[?\<alpha>]"}} \quad \infer[\quad@{text "(instantiate)"}]{@{text "\<Gamma> \<turnstile> B[t]"}}{@{text "\<Gamma> \<turnstile> B[?x]"}}$ \caption{Admissible substitution rules}\label{fig:subst-rules} \end{center} \end{figure} Note that @{text "instantiate"} does not require an explicit side-condition, because @{text "\<Gamma>"} may never contain schematic variables. In principle, variables could be substituted in hypotheses as well, but this would disrupt the monotonicity of reasoning: deriving @{text "\<Gamma>\<vartheta> \<turnstile> B\<vartheta>"} from @{text "\<Gamma> \<turnstile> B"} is correct, but @{text "\<Gamma>\<vartheta> \<supseteq> \<Gamma>"} does not necessarily hold: the result belongs to a different proof context. \medskip An \emph{oracle} is a function that produces axioms on the fly. Logically, this is an instance of the @{text "axiom"} rule (\figref{fig:prim-rules}), but there is an operational difference. The system always records oracle invocations within derivations of theorems by a unique tag. Axiomatizations should be limited to the bare minimum, typically as part of the initial logical basis of an object-logic formalization. Later on, theories are usually developed in a strictly definitional fashion, by stating only certain equalities over new constants. A \emph{simple definition} consists of a constant declaration @{text "c :: \<sigma>"} together with an axiom @{text "\<turnstile> c \<equiv> t"}, where @{text "t :: \<sigma>"} is a closed term without any hidden polymorphism. The RHS may depend on further defined constants, but not @{text "c"} itself. Definitions of functions may be presented as @{text "c \<^vec>x \<equiv> t"} instead of the puristic @{text "c \<equiv> \<lambda>\<^vec>x. t"}. An \emph{overloaded definition} consists of a collection of axioms for the same constant, with zero or one equations @{text "c((\<^vec>\<alpha>)\<kappa>) \<equiv> t"} for each type constructor @{text "\<kappa>"} (for distinct variables @{text "\<^vec>\<alpha>"}). The RHS may mention previously defined constants as above, or arbitrary constants @{text "d(\<alpha>\<^sub>i)"} for some @{text "\<alpha>\<^sub>i"} projected from @{text "\<^vec>\<alpha>"}. Thus overloaded definitions essentially work by primitive recursion over the syntactic structure of a single type argument. See also \cite[\S4.3]{Haftmann-Wenzel:2006:classes}. *} text %mlref {* \begin{mldecls} @{index_ML Logic.all: "term -> term -> term"} \\ @{index_ML Logic.mk_implies: "term * term -> term"} \\ \end{mldecls} \begin{mldecls} @{index_ML_type ctyp} \\ @{index_ML_type cterm} \\ @{index_ML Thm.ctyp_of: "theory -> typ -> ctyp"} \\ @{index_ML Thm.cterm_of: "theory -> term -> cterm"} \\ @{index_ML Thm.apply: "cterm -> cterm -> cterm"} \\ @{index_ML Thm.lambda: "cterm -> cterm -> cterm"} \\ @{index_ML Thm.all: "cterm -> cterm -> cterm"} \\ @{index_ML Drule.mk_implies: "cterm * cterm -> cterm"} \\ \end{mldecls} \begin{mldecls} @{index_ML_type thm} \\ @{index_ML Thm.peek_status: "thm -> {oracle: bool, unfinished: bool, failed: bool}"} \\ @{index_ML Thm.transfer: "theory -> thm -> thm"} \\ @{index_ML Thm.assume: "cterm -> thm"} \\ @{index_ML Thm.forall_intr: "cterm -> thm -> thm"} \\ @{index_ML Thm.forall_elim: "cterm -> thm -> thm"} \\ @{index_ML Thm.implies_intr: "cterm -> thm -> thm"} \\ @{index_ML Thm.implies_elim: "thm -> thm -> thm"} \\ @{index_ML Thm.generalize: "string list * string list -> int -> thm -> thm"} \\ @{index_ML Thm.instantiate: "(ctyp * ctyp) list * (cterm * cterm) list -> thm -> thm"} \\ @{index_ML Thm.add_axiom: "Proof.context -> binding * term -> theory -> (string * thm) * theory"} \\ @{index_ML Thm.add_oracle: "binding * ('a -> cterm) -> theory -> (string * ('a -> thm)) * theory"} \\ @{index_ML Thm.add_def: "Proof.context -> bool -> bool -> binding * term -> theory -> (string * thm) * theory"} \\ \end{mldecls} \begin{mldecls} @{index_ML Theory.add_deps: "Proof.context -> string -> string * typ -> (string * typ) list -> theory -> theory"} \\ \end{mldecls} \begin{description} \item @{ML Thm.peek_status}~@{text "thm"} informs about the current status of the derivation object behind the given theorem. This is a snapshot of a potentially ongoing (parallel) evaluation of proofs. The three Boolean values indicate the following: @{verbatim oracle} if the finished part contains some oracle invocation; @{verbatim unfinished} if some future proofs are still pending; @{verbatim failed} if some future proof has failed, rendering the theorem invalid! \item @{ML Logic.all}~@{text "a B"} produces a Pure quantification @{text "\<And>a. B"}, where occurrences of the atomic term @{text "a"} in the body proposition @{text "B"} are replaced by bound variables. (See also @{ML lambda} on terms.) \item @{ML Logic.mk_implies}~@{text "(A, B)"} produces a Pure implication @{text "A \<Longrightarrow> B"}. \item Types @{ML_type ctyp} and @{ML_type cterm} represent certified types and terms, respectively. These are abstract datatypes that guarantee that its values have passed the full well-formedness (and well-typedness) checks, relative to the declarations of type constructors, constants etc.\ in the background theory. The abstract types @{ML_type ctyp} and @{ML_type cterm} are part of the same inference kernel that is mainly responsible for @{ML_type thm}. Thus syntactic operations on @{ML_type ctyp} and @{ML_type cterm} are located in the @{ML_structure Thm} module, even though theorems are not yet involved at that stage. \item @{ML Thm.ctyp_of}~@{text "thy \<tau>"} and @{ML Thm.cterm_of}~@{text "thy t"} explicitly checks types and terms, respectively. This also involves some basic normalizations, such expansion of type and term abbreviations from the theory context. Full re-certification is relatively slow and should be avoided in tight reasoning loops. \item @{ML Thm.apply}, @{ML Thm.lambda}, @{ML Thm.all}, @{ML Drule.mk_implies} etc.\ compose certified terms (or propositions) incrementally. This is equivalent to @{ML Thm.cterm_of} after unchecked @{ML_op "$"}, @{ML lambda}, @{ML Logic.all}, @{ML
Logic.mk_implies} etc., but there can be a big difference in
performance when large existing entities are composed by a few extra
constructions on top.  There are separate operations to decompose
certified terms and theorems to produce certified terms again.

\item Type @{ML_type thm} represents proven propositions.  This is
an abstract datatype that guarantees that its values have been
constructed by basic principles of the @{ML_structure Thm} module.
Every @{ML_type thm} value refers its background theory,
cf.\ \secref{sec:context-theory}.

\item @{ML Thm.transfer}~@{text "thy thm"} transfers the given
This formal adjustment of the background context has no logical
significance, but is occasionally required for formal reasons, e.g.\
when theorems that are imported from more basic theories are used in
the current situation.

\item @{ML Thm.assume}, @{ML Thm.forall_intr}, @{ML
Thm.forall_elim}, @{ML Thm.implies_intr}, and @{ML Thm.implies_elim}
correspond to the primitive inferences of \figref{fig:prim-rules}.

\item @{ML Thm.generalize}~@{text "(\<^vec>\<alpha>, \<^vec>x)"}
corresponds to the @{text "generalize"} rules of
\figref{fig:subst-rules}.  Here collections of type and term
variables are generalized simultaneously, specified by the given
basic names.

\item @{ML Thm.instantiate}~@{text "(\<^vec>\<alpha>\<^sub>s,
\<^vec>x\<^sub>\<tau>)"} corresponds to the @{text "instantiate"} rules
of \figref{fig:subst-rules}.  Type variables are substituted before
term variables.  Note that the types in @{text "\<^vec>x\<^sub>\<tau>"}
refer to the instantiated versions.

\item @{ML Thm.add_axiom}~@{text "ctxt (name, A)"} declares an
arbitrary proposition as axiom, and retrieves it as a theorem from
the resulting theory, cf.\ @{text "axiom"} in
\figref{fig:prim-rules}.  Note that the low-level representation in
the axiom table may differ slightly from the returned theorem.

\item @{ML Thm.add_oracle}~@{text "(binding, oracle)"} produces a named
oracle rule, essentially generating arbitrary axioms on the fly,
cf.\ @{text "axiom"} in \figref{fig:prim-rules}.

\<^vec>x \<equiv> t)"} states a definitional axiom for an existing constant
@{text "c"}.  Dependencies are recorded via @{ML Theory.add_deps},
unless the @{text "unchecked"} option is set.  Note that the
low-level representation in the axiom table may differ slightly from
the returned theorem.

\item @{ML Theory.add_deps}~@{text "ctxt name c\<^sub>\<tau> \<^vec>d\<^sub>\<sigma>"}
declares dependencies of a named specification for constant @{text
"c\<^sub>\<tau>"}, relative to existing specifications for constants @{text
"\<^vec>d\<^sub>\<sigma>"}.

\end{description}
*}

text %mlantiq {*
\begin{matharray}{rcl}
@{ML_antiquotation_def "ctyp"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "cterm"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "cprop"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "thm"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "thms"} & : & @{text ML_antiquotation} \\
@{ML_antiquotation_def "lemma"} & : & @{text ML_antiquotation} \\
\end{matharray}

@{rail \<open>
@@{ML_antiquotation ctyp} typ
;
@@{ML_antiquotation cterm} term
;
@@{ML_antiquotation cprop} prop
;
@@{ML_antiquotation thm} thmref
;
@@{ML_antiquotation thms} thmrefs
;
@@{ML_antiquotation lemma} ('(' @'open' ')')? ((prop +) + @'and') \<newline>
@'by' method method?
\<close>}

\begin{description}

\item @{text "@{ctyp \<tau>}"} produces a certified type wrt.\ the
current background theory --- as abstract value of type @{ML_type
ctyp}.

\item @{text "@{cterm t}"} and @{text "@{cprop \<phi>}"} produce a
certified term wrt.\ the current background theory --- as abstract
value of type @{ML_type cterm}.

\item @{text "@{thm a}"} produces a singleton fact --- as abstract
value of type @{ML_type thm}.

\item @{text "@{thms a}"} produces a general fact --- as abstract
value of type @{ML_type "thm list"}.

\item @{text "@{lemma \<phi> by meth}"} produces a fact that is proven on
the spot according to the minimal proof, which imitates a terminal
Isar proof.  The result is an abstract value of type @{ML_type thm}
or @{ML_type "thm list"}, depending on the number of propositions
given here.

The internal derivation object lacks a proper theorem name, but it
is formally closed, unless the @{text "(open)"} option is specified
(this may impact performance of applications with proof terms).

Since ML antiquotations are always evaluated at compile-time, there
is no run-time overhead even for non-trivial proofs.  Nonetheless,
the justification is syntactically limited to a single @{command
"by"} step.  More complex Isar proofs should be done in regular
theory source, before compiling the corresponding ML text that uses
the result.

\end{description}

*}

subsection {* Auxiliary connectives \label{sec:logic-aux} *}

text {* Theory @{text "Pure"} provides a few auxiliary connectives
that are defined on top of the primitive ones, see
\figref{fig:pure-aux}.  These special constants are useful in
certain internal encodings, and are normally not directly exposed to
the user.

\begin{figure}[htb]
\begin{center}
\begin{tabular}{ll}
@{text "conjunction :: prop \<Rightarrow> prop \<Rightarrow> prop"} & (infix @{text "&&&"}) \\
@{text "\<turnstile> A &&& B \<equiv> (\<And>C. (A \<Longrightarrow> B \<Longrightarrow> C) \<Longrightarrow> C)"} \$1ex] @{text "prop :: prop \<Rightarrow> prop"} & (prefix @{text "#"}, suppressed) \\ @{text "#A \<equiv> A"} \\[1ex] @{text "term :: \<alpha> \<Rightarrow> prop"} & (prefix @{text "TERM"}) \\ @{text "term x \<equiv> (\<And>A. A \<Longrightarrow> A)"} \\[1ex] @{text "type :: \<alpha> itself"} & (prefix @{text "TYPE"}) \\ @{text "(unspecified)"} \\ \end{tabular} \caption{Definitions of auxiliary connectives}\label{fig:pure-aux} \end{center} \end{figure} The introduction @{text "A \<Longrightarrow> B \<Longrightarrow> A &&& B"}, and eliminations (projections) @{text "A &&& B \<Longrightarrow> A"} and @{text "A &&& B \<Longrightarrow> B"} are available as derived rules. Conjunction allows to treat simultaneous assumptions and conclusions uniformly, e.g.\ consider @{text "A \<Longrightarrow> B \<Longrightarrow> C &&& D"}. In particular, the goal mechanism represents multiple claims as explicit conjunction internally, but this is refined (via backwards introduction) into separate sub-goals before the user commences the proof; the final result is projected into a list of theorems using eliminations (cf.\ \secref{sec:tactical-goals}). The @{text "prop"} marker (@{text "#"}) makes arbitrarily complex propositions appear as atomic, without changing the meaning: @{text "\<Gamma> \<turnstile> A"} and @{text "\<Gamma> \<turnstile> #A"} are interchangeable. See \secref{sec:tactical-goals} for specific operations. The @{text "term"} marker turns any well-typed term into a derivable proposition: @{text "\<turnstile> TERM t"} holds unconditionally. Although this is logically vacuous, it allows to treat terms and proofs uniformly, similar to a type-theoretic framework. The @{text "TYPE"} constructor is the canonical representative of the unspecified type @{text "\<alpha> itself"}; it essentially injects the language of types into that of terms. There is specific notation @{text "TYPE(\<tau>)"} for @{text "TYPE\<^bsub>\<tau> itself\<^esub>"}. Although being devoid of any particular meaning, the term @{text "TYPE(\<tau>)"} accounts for the type @{text "\<tau>"} within the term language. In particular, @{text "TYPE(\<alpha>)"} may be used as formal argument in primitive definitions, in order to circumvent hidden polymorphism (cf.\ \secref{sec:terms}). For example, @{text "c TYPE(\<alpha>) \<equiv> A[\<alpha>]"} defines @{text "c :: \<alpha> itself \<Rightarrow> prop"} in terms of a proposition @{text "A"} that depends on an additional type argument, which is essentially a predicate on types. *} text %mlref {* \begin{mldecls} @{index_ML Conjunction.intr: "thm -> thm -> thm"} \\ @{index_ML Conjunction.elim: "thm -> thm * thm"} \\ @{index_ML Drule.mk_term: "cterm -> thm"} \\ @{index_ML Drule.dest_term: "thm -> cterm"} \\ @{index_ML Logic.mk_type: "typ -> term"} \\ @{index_ML Logic.dest_type: "term -> typ"} \\ \end{mldecls} \begin{description} \item @{ML Conjunction.intr} derives @{text "A &&& B"} from @{text "A"} and @{text "B"}. \item @{ML Conjunction.elim} derives @{text "A"} and @{text "B"} from @{text "A &&& B"}. \item @{ML Drule.mk_term} derives @{text "TERM t"}. \item @{ML Drule.dest_term} recovers term @{text "t"} from @{text "TERM t"}. \item @{ML Logic.mk_type}~@{text "\<tau>"} produces the term @{text "TYPE(\<tau>)"}. \item @{ML Logic.dest_type}~@{text "TYPE(\<tau>)"} recovers the type @{text "\<tau>"}. \end{description} *} subsection {* Sort hypotheses *} text {* Type variables are decorated with sorts, as explained in \secref{sec:types}. This constrains type instantiation to certain ranges of types: variable @{text "\<alpha>\<^sub>s"} may only be assigned to types @{text "\<tau>"} that belong to sort @{text "s"}. Within the logic, sort constraints act like implicit preconditions on the result @{text "\<lparr>\<alpha>\<^sub>1 : s\<^sub>1\<rparr>, \<dots>, \<lparr>\<alpha>\<^sub>n : s\<^sub>n\<rparr>, \<Gamma> \<turnstile> \<phi>"} where the type variables @{text "\<alpha>\<^sub>1, \<dots>, \<alpha>\<^sub>n"} cover the propositions @{text "\<Gamma>"}, @{text "\<phi>"}, as well as the proof of @{text "\<Gamma> \<turnstile> \<phi>"}. These \emph{sort hypothesis} of a theorem are passed monotonically through further derivations. They are redundant, as long as the statement of a theorem still contains the type variables that are accounted here. The logical significance of sort hypotheses is limited to the boundary case where type variables disappear from the proposition, e.g.\ @{text "\<lparr>\<alpha>\<^sub>s : s\<rparr> \<turnstile> \<phi>"}. Since such dangling type variables can be renamed arbitrarily without changing the proposition @{text "\<phi>"}, the inference kernel maintains sort hypotheses in anonymous form @{text "s \<turnstile> \<phi>"}. In most practical situations, such extra sort hypotheses may be stripped in a final bookkeeping step, e.g.\ at the end of a proof: they are typically left over from intermediate reasoning with type classes that can be satisfied by some concrete type @{text "\<tau>"} of sort @{text "s"} to replace the hypothetical type variable @{text "\<alpha>\<^sub>s"}. *} text %mlref {* \begin{mldecls} @{index_ML Thm.extra_shyps: "thm -> sort list"} \\ @{index_ML Thm.strip_shyps: "thm -> thm"} \\ \end{mldecls} \begin{description} \item @{ML Thm.extra_shyps}~@{text "thm"} determines the extraneous sort hypotheses of the given theorem, i.e.\ the sorts that are not present within type variables of the statement. \item @{ML Thm.strip_shyps}~@{text "thm"} removes any extraneous sort hypotheses that can be witnessed from the type signature. \end{description} *} text %mlex {* The following artificial example demonstrates the derivation of @{prop False} with a pending sort hypothesis involving a logically empty sort. *} class empty = assumes bad: "\<And>(x::'a) y. x \<noteq> y" theorem (in empty) false: False using bad by blast ML {* @{assert} (Thm.extra_shyps @{thm false} = [@{sort empty}]) *} text {* Thanks to the inference kernel managing sort hypothesis according to their logical significance, this example is merely an instance of \emph{ex falso quodlibet consequitur} --- not a collapse of the logical framework! *} section {* Object-level rules \label{sec:obj-rules} *} text {* The primitive inferences covered so far mostly serve foundational purposes. User-level reasoning usually works via object-level rules that are represented as theorems of Pure. Composition of rules involves \emph{backchaining}, \emph{higher-order unification} modulo @{text "\<alpha>\<beta>\<eta>"}-conversion of @{text "\<lambda>"}-terms, and so-called \emph{lifting} of rules into a context of @{text "\<And>"} and @{text "\<Longrightarrow>"} connectives. Thus the full power of higher-order Natural Deduction in Isabelle/Pure becomes readily available. *} subsection {* Hereditary Harrop Formulae *} text {* The idea of object-level rules is to model Natural Deduction inferences in the style of Gentzen \cite{Gentzen:1935}, but we allow arbitrary nesting similar to \cite{extensions91}. The most basic rule format is that of a \emph{Horn Clause}: \[ \infer{@{text "A"}}{@{text "A\<^sub>1"} & @{text "\<dots>"} & @{text "A\<^sub>n"}}$
where @{text "A, A\<^sub>1, \<dots>, A\<^sub>n"} are atomic propositions
of the framework, usually of the form @{text "Trueprop B"}, where
@{text "B"} is a (compound) object-level statement.  This
object-level inference corresponds to an iterated implication in
Pure like this:
$@{text "A\<^sub>1 \<Longrightarrow> \<dots> A\<^sub>n \<Longrightarrow> A"}$
As an example consider conjunction introduction: @{text "A \<Longrightarrow> B \<Longrightarrow> A \<and>
B"}.  Any parameters occurring in such rule statements are
conceptionally treated as arbitrary:
$@{text "\<And>x\<^sub>1 \<dots> x\<^sub>m. A\<^sub>1 x\<^sub>1 \<dots> x\<^sub>m \<Longrightarrow> \<dots> A\<^sub>n x\<^sub>1 \<dots> x\<^sub>m \<Longrightarrow> A x\<^sub>1 \<dots> x\<^sub>m"}$

Nesting of rules means that the positions of @{text "A\<^sub>i"} may
again hold compound rules, not just atomic propositions.
Propositions of this format are called \emph{Hereditary Harrop
Formulae} in the literature \cite{Miller:1991}.  Here we give an
inductive characterization as follows:

\medskip
\begin{tabular}{ll}
@{text "\<^bold>x"} & set of variables \\
@{text "\<^bold>A"} & set of atomic propositions \\
@{text "\<^bold>H  =  \<And>\<^bold>x\<^sup>*. \<^bold>H\<^sup>* \<Longrightarrow> \<^bold>A"} & set of Hereditary Harrop Formulas \\
\end{tabular}
\medskip

Thus we essentially impose nesting levels on propositions formed
from @{text "\<And>"} and @{text "\<Longrightarrow>"}.  At each level there is a prefix
of parameters and compound premises, concluding an atomic
proposition.  Typical examples are @{text "\<longrightarrow>"}-introduction @{text
"(A \<Longrightarrow> B) \<Longrightarrow> A \<longrightarrow> B"} or mathematical induction @{text "P 0 \<Longrightarrow> (\<And>n. P n
\<Longrightarrow> P (Suc n)) \<Longrightarrow> P n"}.  Even deeper nesting occurs in well-founded
induction @{text "(\<And>x. (\<And>y. y \<prec> x \<Longrightarrow> P y) \<Longrightarrow> P x) \<Longrightarrow> P x"}, but this
already marks the limit of rule complexity that is usually seen in
practice.

\medskip Regular user-level inferences in Isabelle/Pure always
maintain the following canonical form of results:

\begin{itemize}

\item Normalization by @{text "(A \<Longrightarrow> (\<And>x. B x)) \<equiv> (\<And>x. A \<Longrightarrow> B x)"},
which is a theorem of Pure, means that quantifiers are pushed in
front of implication at each level of nesting.  The normal form is a
Hereditary Harrop Formula.

\item The outermost prefix of parameters is represented via
schematic variables: instead of @{text "\<And>\<^vec>x. \<^vec>H \<^vec>x
\<Longrightarrow> A \<^vec>x"} we have @{text "\<^vec>H ?\<^vec>x \<Longrightarrow> A ?\<^vec>x"}.
Note that this representation looses information about the order of
parameters, and vacuous quantifiers vanish automatically.

\end{itemize}
*}

text %mlref {*
\begin{mldecls}
@{index_ML Simplifier.norm_hhf: "Proof.context -> thm -> thm"} \\
\end{mldecls}

\begin{description}

\item @{ML Simplifier.norm_hhf}~@{text "ctxt thm"} normalizes the given
theorem according to the canonical form specified above.  This is
occasionally helpful to repair some low-level tools that do not
handle Hereditary Harrop Formulae properly.

\end{description}
*}

subsection {* Rule composition *}

text {*
The rule calculus of Isabelle/Pure provides two main inferences:
@{inference resolution} (i.e.\ back-chaining of rules) and
@{inference assumption} (i.e.\ closing a branch), both modulo
higher-order unification.  There are also combined variants, notably
@{inference elim_resolution} and @{inference dest_resolution}.

To understand the all-important @{inference resolution} principle,
we first consider raw @{inference_def composition} (modulo
higher-order unification with substitution @{text "\<vartheta>"}):
$\infer[(@{inference_def composition})]{@{text "\<^vec>A\<vartheta> \<Longrightarrow> C\<vartheta>"}} {@{text "\<^vec>A \<Longrightarrow> B"} & @{text "B' \<Longrightarrow> C"} & @{text "B\<vartheta> = B'\<vartheta>"}}$
Here the conclusion of the first rule is unified with the premise of
the second; the resulting rule instance inherits the premises of the
first and conclusion of the second.  Note that @{text "C"} can again
consist of iterated implications.  We can also permute the premises
of the second rule back-and-forth in order to compose with @{text
"B'"} in any position (subsequently we shall always refer to
position 1 w.l.o.g.).

In @{inference composition} the internal structure of the common
part @{text "B"} and @{text "B'"} is not taken into account.  For
proper @{inference resolution} we require @{text "B"} to be atomic,
and explicitly observe the structure @{text "\<And>\<^vec>x. \<^vec>H
\<^vec>x \<Longrightarrow> B' \<^vec>x"} of the premise of the second rule.  The
idea is to adapt the first rule by lifting'' it into this context,
by means of iterated application of the following inferences:
$\infer[(@{inference_def imp_lift})]{@{text "(\<^vec>H \<Longrightarrow> \<^vec>A) \<Longrightarrow> (\<^vec>H \<Longrightarrow> B)"}}{@{text "\<^vec>A \<Longrightarrow> B"}}$
$\infer[(@{inference_def all_lift})]{@{text "(\<And>\<^vec>x. \<^vec>A (?\<^vec>a \<^vec>x)) \<Longrightarrow> (\<And>\<^vec>x. B (?\<^vec>a \<^vec>x))"}}{@{text "\<^vec>A ?\<^vec>a \<Longrightarrow> B ?\<^vec>a"}}$
By combining raw composition with lifting, we get full @{inference
resolution} as follows:
$\infer[(@{inference_def resolution})] {@{text "(\<And>\<^vec>x. \<^vec>H \<^vec>x \<Longrightarrow> \<^vec>A (?\<^vec>a \<^vec>x))\<vartheta> \<Longrightarrow> C\<vartheta>"}} {\begin{tabular}{l} @{text "\<^vec>A ?\<^vec>a \<Longrightarrow> B ?\<^vec>a"} \\ @{text "(\<And>\<^vec>x. \<^vec>H \<^vec>x \<Longrightarrow> B' \<^vec>x) \<Longrightarrow> C"} \\ @{text "(\<lambda>\<^vec>x. B (?\<^vec>a \<^vec>x))\<vartheta> = B'\<vartheta>"} \\ \end{tabular}}$

Continued resolution of rules allows to back-chain a problem towards
more and sub-problems.  Branches are closed either by resolving with
a rule of 0 premises, or by producing a short-circuit'' within a
solved situation (again modulo unification):
$\infer[(@{inference_def assumption})]{@{text "C\<vartheta>"}} {@{text "(\<And>\<^vec>x. \<^vec>H \<^vec>x \<Longrightarrow> A \<^vec>x) \<Longrightarrow> C"} & @{text "A\<vartheta> = H\<^sub>i\<vartheta>"}~~\text{(for some~@{text i})}}$

%FIXME @{inference_def elim_resolution}, @{inference_def dest_resolution}
*}

text %mlref {*
\begin{mldecls}
@{index_ML_op "RSN": "thm * (int * thm) -> thm"} \\
@{index_ML_op "RS": "thm * thm -> thm"} \\

@{index_ML_op "RLN": "thm list * (int * thm list) -> thm list"} \\
@{index_ML_op "RL": "thm list * thm list -> thm list"} \\

@{index_ML_op "MRS": "thm list * thm -> thm"} \\
@{index_ML_op "OF": "thm * thm list -> thm"} \\
\end{mldecls}

\begin{description}

\item @{text "rule\<^sub>1 RSN (i, rule\<^sub>2)"} resolves the conclusion of
@{text "rule\<^sub>1"} with the @{text i}-th premise of @{text "rule\<^sub>2"},
according to the @{inference resolution} principle explained above.
Unless there is precisely one resolvent it raises exception @{ML
THM}.

This corresponds to the rule attribute @{attribute THEN} in Isar
source language.

\item @{text "rule\<^sub>1 RS rule\<^sub>2"} abbreviates @{text "rule\<^sub>1 RSN (1,
rule\<^sub>2)"}.

\item @{text "rules\<^sub>1 RLN (i, rules\<^sub>2)"} joins lists of rules.  For
every @{text "rule\<^sub>1"} in @{text "rules\<^sub>1"} and @{text "rule\<^sub>2"} in
@{text "rules\<^sub>2"}, it resolves the conclusion of @{text "rule\<^sub>1"} with
the @{text "i"}-th premise of @{text "rule\<^sub>2"}, accumulating multiple
results in one big list.  Note that such strict enumerations of
higher-order unifications can be inefficient compared to the lazy
variant seen in elementary tactics like @{ML resolve_tac}.

\item @{text "rules\<^sub>1 RL rules\<^sub>2"} abbreviates @{text "rules\<^sub>1 RLN (1,
rules\<^sub>2)"}.

\item @{text "[rule\<^sub>1, \<dots>, rule\<^sub>n] MRS rule"} resolves @{text "rule\<^sub>i"}
against premise @{text "i"} of @{text "rule"}, for @{text "i = n, \<dots>,
1"}.  By working from right to left, newly emerging premises are
concatenated in the result, without interfering.

\item @{text "rule OF rules"} is an alternative notation for @{text
"rules MRS rule"}, which makes rule composition look more like
function application.  Note that the argument @{text "rules"} need
not be atomic.

This corresponds to the rule attribute @{attribute OF} in Isar
source language.

\end{description}
*}

section {* Proof terms \label{sec:proof-terms} *}

text {* The Isabelle/Pure inference kernel can record the proof of
each theorem as a proof term that contains all logical inferences in
detail.  Rule composition by resolution (\secref{sec:obj-rules}) and
type-class reasoning is broken down to primitive rules of the
logical framework.  The proof term can be inspected by a separate
proof-checker, for example.

According to the well-known \emph{Curry-Howard isomorphism}, a proof
can be viewed as a @{text "\<lambda>"}-term. Following this idea, proofs in
Isabelle are internally represented by a datatype similar to the one
for terms described in \secref{sec:terms}.  On top of these
syntactic terms, two more layers of @{text "\<lambda>"}-calculus are added,
which correspond to @{text "\<And>x :: \<alpha>. B x"} and @{text "A \<Longrightarrow> B"}
according to the propositions-as-types principle.  The resulting
3-level @{text "\<lambda>"}-calculus resembles @{text "\<lambda>HOL"}'' in the
more abstract setting of Pure Type Systems (PTS)
\cite{Barendregt-Geuvers:2001}, if some fine points like schematic
polymorphism and type classes are ignored.

\medskip\emph{Proof abstractions} of the form @{text "\<^bold>\<lambda>x :: \<alpha>. prf"}
or @{text "\<^bold>\<lambda>p : A. prf"} correspond to introduction of @{text
"\<And>"}/@{text "\<Longrightarrow>"}, and \emph{proof applications} of the form @{text
"p \<cdot> t"} or @{text "p \<bullet> q"} correspond to elimination of @{text
"\<And>"}/@{text "\<Longrightarrow>"}.  Actual types @{text "\<alpha>"}, propositions @{text
"A"}, and terms @{text "t"} might be suppressed and reconstructed
from the overall proof term.

\medskip Various atomic proofs indicate special situations within
the proof construction as follows.

A \emph{bound proof variable} is a natural number @{text "b"} that
acts as de-Bruijn index for proof term abstractions.

A \emph{minimal proof} @{text "?"}'' is a dummy proof term.  This
indicates some unrecorded part of the proof.

@{text "Hyp A"} refers to some pending hypothesis by giving its
proposition.  This indicates an open context of implicit hypotheses,
similar to loose bound variables or free variables within a term
(\secref{sec:terms}).

An \emph{axiom} or \emph{oracle} @{text "a : A[\<^vec>\<tau>]"} refers
some postulated @{text "proof constant"}, which is subject to
schematic polymorphism of theory content, and the particular type
instantiation may be given explicitly.  The vector of types @{text
"\<^vec>\<tau>"} refers to the schematic type variables in the generic
proposition @{text "A"} in canonical order.

A \emph{proof promise} @{text "a : A[\<^vec>\<tau>]"} is a placeholder
for some proof of polymorphic proposition @{text "A"}, with explicit
type instantiation as given by the vector @{text "\<^vec>\<tau>"}, as
above.  Unlike axioms or oracles, proof promises may be
\emph{fulfilled} eventually, by substituting @{text "a"} by some
particular proof @{text "q"} at the corresponding type instance.
This acts like Hindley-Milner @{text "let"}-polymorphism: a generic
local proof definition may get used at different type instances, and
is replaced by the concrete instance eventually.

A \emph{named theorem} wraps up some concrete proof as a closed
formal entity, in the manner of constant definitions for proof
terms.  The \emph{proof body} of such boxed theorems involves some
digest about oracles and promises occurring in the original proof.
This allows the inference kernel to manage this critical information
without the full overhead of explicit proof terms.
*}

subsection {* Reconstructing and checking proof terms *}

text {* Fully explicit proof terms can be large, but most of this
information is redundant and can be reconstructed from the context.
Therefore, the Isabelle/Pure inference kernel records only
\emph{implicit} proof terms, by omitting all typing information in
terms, all term and type labels of proof abstractions, and some
argument terms of applications @{text "p \<cdot> t"} (if possible).

There are separate operations to reconstruct the full proof term
later on, using \emph{higher-order pattern unification}
\cite{nipkow-patterns,Berghofer-Nipkow:2000:TPHOL}.

The \emph{proof checker} expects a fully reconstructed proof term,
and can turn it into a theorem by replaying its primitive inferences
within the kernel.  *}

subsection {* Concrete syntax of proof terms *}

text {* The concrete syntax of proof terms is a slight extension of
the regular inner syntax of Isabelle/Pure \cite{isabelle-isar-ref}.
Its main syntactic category @{syntax (inner) proof} is defined as
follows:

\begin{center}
\begin{supertabular}{rclr}

@{syntax_def (inner) proof} & = & @{verbatim Lam} @{text params} @{verbatim "."} @{text proof} \\
& @{text "|"} & @{text "\<^bold>\<lambda>"} @{text "params"} @{verbatim "."} @{text proof} \\
& @{text "|"} & @{text proof} @{verbatim "%"} @{text any} \\
& @{text "|"} & @{text proof} @{text "\<cdot>"} @{text any} \\
& @{text "|"} & @{text proof} @{verbatim "%%"} @{text proof} \\
& @{text "|"} & @{text proof} @{text "\<bullet>"} @{text proof} \\
& @{text "|"} & @{text "id  |  longid"} \\
\\

@{text param} & = & @{text idt} \\
& @{text "|"} & @{text idt} @{verbatim ":"} @{text prop} \\
& @{text "|"} & @{verbatim "("} @{text param} @{verbatim ")"} \\
\\

@{text params} & = & @{text param} \\
& @{text "|"} & @{text param} @{text params} \\

\end{supertabular}
\end{center}

Implicit term arguments in partial proofs are indicated by @{text
"_"}''.  Type arguments for theorems and axioms may be specified
using @{text "p \<cdot> TYPE(type)"} (they must appear before any other
term argument of a theorem or axiom, but may be omitted altogether).

\medskip There are separate read and print operations for proof
terms, in order to avoid conflicts with the regular term language.
*}

text %mlref {*
\begin{mldecls}
@{index_ML_type proof} \\
@{index_ML_type proof_body} \\
@{index_ML proofs: "int Unsynchronized.ref"} \\
@{index_ML Reconstruct.reconstruct_proof:
"theory -> term -> proof -> proof"} \\
@{index_ML Reconstruct.expand_proof: "theory ->
(string * term option) list -> proof -> proof"} \\
@{index_ML Proof_Checker.thm_of_proof: "theory -> proof -> thm"} \\
@{index_ML Proof_Syntax.read_proof: "theory -> bool -> bool -> string -> proof"} \\
@{index_ML Proof_Syntax.pretty_proof: "Proof.context -> proof -> Pretty.T"} \\
\end{mldecls}

\begin{description}

\item Type @{ML_type proof} represents proof terms; this is a
datatype with constructors @{index_ML Abst}, @{index_ML AbsP},
@{index_ML_op "%"}, @{index_ML_op "%%"}, @{index_ML PBound},
@{index_ML MinProof}, @{index_ML Hyp}, @{index_ML PAxm}, @{index_ML
Oracle}, @{index_ML Promise}, @{index_ML PThm} as explained above.
%FIXME OfClass (!?)

\item Type @{ML_type proof_body} represents the nested proof
information of a named theorem, consisting of a digest of oracles
and named theorem over some proof term.  The digest only covers the
directly visible part of the proof: in order to get the full
information, the implicit graph of nested theorems needs to be
traversed (e.g.\ using @{ML Proofterm.fold_body_thms}).

\item @{ML Thm.proof_of}~@{text "thm"} and @{ML
Thm.proof_body_of}~@{text "thm"} produce the proof term or proof
body (with digest of oracles and theorems) from a given theorem.
Note that this involves a full join of internal futures that fulfill
pending proof promises, and thus disrupts the natural bottom-up
construction of proofs by introducing dynamic ad-hoc dependencies.
Parallel performance may suffer by inspecting proof terms at
run-time.

\item @{ML proofs} specifies the detail of proof recording within
@{ML_type thm} values produced by the inference kernel: @{ML 0}
records only the names of oracles, @{ML 1} records oracle names and
propositions, @{ML 2} additionally records full proof terms.
Officially named theorems that contribute to a result are recorded
in any case.

\item @{ML Reconstruct.reconstruct_proof}~@{text "thy prop prf"}
turns the implicit proof term @{text "prf"} into a full proof of the
given proposition.

Reconstruction may fail if @{text "prf"} is not a proof of @{text
"prop"}, or if it does not contain sufficient information for
reconstruction.  Failure may only happen for proofs that are
constructed manually, but not for those produced automatically by
the inference kernel.

\item @{ML Reconstruct.expand_proof}~@{text "thy [thm\<^sub>1, \<dots>, thm\<^sub>n]
prf"} expands and reconstructs the proofs of all specified theorems,
with the given (full) proof.  Theorems that are not unique specified
via their name may be disambiguated by giving their proposition.

\item @{ML Proof_Checker.thm_of_proof}~@{text "thy prf"} turns the
given (full) proof into a theorem, by replaying it using only
primitive rules of the inference kernel.

proof term. The Boolean flags indicate the use of sort and type
information.  Usually, typing information is left implicit and is
inferred during proof reconstruction.  %FIXME eliminate flags!?

\item @{ML Proof_Syntax.pretty_proof}~@{text "ctxt prf"}
pretty-prints the given proof term.

\end{description}
*}

text %mlex {* Detailed proof information of a theorem may be retrieved
as follows: *}

lemma ex: "A \<and> B \<longrightarrow> B \<and> A"
proof
assume "A \<and> B"
then obtain B and A ..
then show "B \<and> A" ..
qed

ML_val {*
(*proof body with digest*)
val body = Proofterm.strip_thm (Thm.proof_body_of @{thm ex});

(*proof term only*)
val prf = Proofterm.proof_of body;
Pretty.writeln (Proof_Syntax.pretty_proof @{context} prf);

(*all theorems used in the graph of nested proofs*)
val all_thms =
Proofterm.fold_body_thms
(fn (name, _, _) => insert (op =) name) [body] [];
*}

text {* The result refers to various basic facts of Isabelle/HOL:
@{thm [source] HOL.impI}, @{thm [source] HOL.conjE}, @{thm [source]
HOL.conjI} etc.  The combinator @{ML Proofterm.fold_body_thms}
recursively explores the graph of the proofs of all theorems being
used here.

\medskip Alternatively, we may produce a proof term manually, and
turn it into a theorem as follows: *}

ML_val {*
val thy = @{theory};
val prf =
"impI \<cdot> _ \<cdot> _ \<bullet> \
\   (\<^bold>\<lambda>H: _. \
\     conjE \<cdot> _ \<cdot> _ \<cdot> _ \<bullet> H \<bullet> \
\       (\<^bold>\<lambda>(H: _) Ha: _. conjI \<cdot> _ \<cdot> _ \<bullet> Ha \<bullet> H))";
val thm =
prf
|> Reconstruct.reconstruct_proof thy @{prop "A \<and> B \<longrightarrow> B \<and> A"}
|> Proof_Checker.thm_of_proof thy
|> Drule.export_without_context;
*}