doc-src/IsarImplementation/Thy/Proof.thy
 author wenzelm Thu, 26 Jan 2012 15:28:17 +0100 changeset 46265 b6ab3cdea163 parent 42666 fee67c099d03 permissions -rw-r--r--

theory Proof
imports Base
begin

chapter {* Structured proofs *}

section {* Variables \label{sec:variables} *}

text {*
Any variable that is not explicitly bound by @{text "\<lambda>"}-abstraction
is considered as free''.  Logically, free variables act like
outermost universal quantification at the sequent level: @{text
"A\<^isub>1(x), \<dots>, A\<^isub>n(x) \<turnstile> B(x)"} means that the result
holds \emph{for all} values of @{text "x"}.  Free variables for
terms (not types) can be fully internalized into the logic: @{text
"\<turnstile> B(x)"} and @{text "\<turnstile> \<And>x. B(x)"} are interchangeable, provided
that @{text "x"} does not occur elsewhere in the context.
Inspecting @{text "\<turnstile> \<And>x. B(x)"} more closely, we see that inside the
quantifier, @{text "x"} is essentially arbitrary, but fixed'',
while from outside it appears as a place-holder for instantiation
(thanks to @{text "\<And>"} elimination).

The Pure logic represents the idea of variables being either inside
or outside the current scope by providing separate syntactic
categories for \emph{fixed variables} (e.g.\ @{text "x"}) vs.\
\emph{schematic variables} (e.g.\ @{text "?x"}).  Incidently, a
universal result @{text "\<turnstile> \<And>x. B(x)"} has the HHF normal form @{text
"\<turnstile> B(?x)"}, which represents its generality without requiring an
explicit quantifier.  The same principle works for type variables:
@{text "\<turnstile> B(?\<alpha>)"} represents the idea of @{text "\<turnstile> \<forall>\<alpha>. B(\<alpha>)"}''
without demanding a truly polymorphic framework.

\medskip Additional care is required to treat type variables in a
way that facilitates type-inference.  In principle, term variables
depend on type variables, which means that type variables would have
to be declared first.  For example, a raw type-theoretic framework
would demand the context to be constructed in stages as follows:
@{text "\<Gamma> = \<alpha>: type, x: \<alpha>, a: A(x\<^isub>\<alpha>)"}.

We allow a slightly less formalistic mode of operation: term
variables @{text "x"} are fixed without specifying a type yet
(essentially \emph{all} potential occurrences of some instance
@{text "x\<^isub>\<tau>"} are fixed); the first occurrence of @{text "x"}
within a specific term assigns its most general type, which is then
maintained consistently in the context.  The above example becomes
@{text "\<Gamma> = x: term, \<alpha>: type, A(x\<^isub>\<alpha>)"}, where type @{text
"\<alpha>"} is fixed \emph{after} term @{text "x"}, and the constraint
@{text "x :: \<alpha>"} is an implicit consequence of the occurrence of
@{text "x\<^isub>\<alpha>"} in the subsequent proposition.

This twist of dependencies is also accommodated by the reverse
operation of exporting results from a context: a type variable
@{text "\<alpha>"} is considered fixed as long as it occurs in some fixed
term variable of the context.  For example, exporting @{text "x:
term, \<alpha>: type \<turnstile> x\<^isub>\<alpha> \<equiv> x\<^isub>\<alpha>"} produces in the first step @{text "x: term
\<turnstile> x\<^isub>\<alpha> \<equiv> x\<^isub>\<alpha>"} for fixed @{text "\<alpha>"}, and only in the second step
@{text "\<turnstile> ?x\<^isub>?\<^isub>\<alpha> \<equiv> ?x\<^isub>?\<^isub>\<alpha>"} for schematic @{text "?x"} and @{text "?\<alpha>"}.
The following Isar source text illustrates this scenario.
*}

begin
{
fix x  -- {* all potential occurrences of some @{text "x::\<tau>"} are fixed *}
{
have "x::'a \<equiv> x"  -- {* implicit type assigment by concrete occurrence *}
by (rule reflexive)
}
thm this  -- {* result still with fixed type @{text "'a"} *}
}
thm this  -- {* fully general result for arbitrary @{text "?x::?'a"} *}
end

text {* The Isabelle/Isar proof context manages the details of term
vs.\ type variables, with high-level principles for moving the
frontier between fixed and schematic variables.

The @{text "add_fixes"} operation explictly declares fixed
variables; the @{text "declare_term"} operation absorbs a term into
a context by fixing new type variables and adding syntactic
constraints.

The @{text "export"} operation is able to perform the main work of
generalizing term and type variables as sketched above, assuming
that fixing variables and terms have been declared properly.

There @{text "import"} operation makes a generalized fact a genuine
part of the context, by inventing fixed variables for the schematic
ones.  The effect can be reversed by using @{text "export"} later,
potentially with an extended context; the result is equivalent to
the original modulo renaming of schematic variables.

The @{text "focus"} operation provides a variant of @{text "import"}
for nested propositions (with explicit quantification): @{text
"\<And>x\<^isub>1 \<dots> x\<^isub>n. B(x\<^isub>1, \<dots>, x\<^isub>n)"} is
decomposed by inventing fixed variables @{text "x\<^isub>1, \<dots>,
x\<^isub>n"} for the body.
*}

text %mlref {*
\begin{mldecls}
string list -> Proof.context -> string list * Proof.context"} \\
@{index_ML Variable.variant_fixes: "
string list -> Proof.context -> string list * Proof.context"} \\
@{index_ML Variable.declare_term: "term -> Proof.context -> Proof.context"} \\
@{index_ML Variable.declare_constraints: "term -> Proof.context -> Proof.context"} \\
@{index_ML Variable.export: "Proof.context -> Proof.context -> thm list -> thm list"} \\
@{index_ML Variable.polymorphic: "Proof.context -> term list -> term list"} \\
@{index_ML Variable.import: "bool -> thm list -> Proof.context ->
(((ctyp * ctyp) list * (cterm * cterm) list) * thm list) * Proof.context"} \\
@{index_ML Variable.focus: "term -> Proof.context ->
((string * (string * typ)) list * term) * Proof.context"} \\
\end{mldecls}

\begin{description}

\item @{ML Variable.add_fixes}~@{text "xs ctxt"} fixes term
variables @{text "xs"}, returning the resulting internal names.  By
default, the internal representation coincides with the external
one, which also means that the given variables must not be fixed
already.  There is a different policy within a local proof body: the
given names are just hints for newly invented Skolem variables.

\item @{ML Variable.variant_fixes} is similar to @{ML
Variable.add_fixes}, but always produces fresh variants of the given
names.

\item @{ML Variable.declare_term}~@{text "t ctxt"} declares term
@{text "t"} to belong to the context.  This automatically fixes new
type variables, but not term variables.  Syntactic constraints for
type and term variables are declared uniformly, though.

\item @{ML Variable.declare_constraints}~@{text "t ctxt"} declares
syntactic constraints from term @{text "t"}, without making it part
of the context yet.

\item @{ML Variable.export}~@{text "inner outer thms"} generalizes
fixed type and term variables in @{text "thms"} according to the
difference of the @{text "inner"} and @{text "outer"} context,
following the principles sketched above.

\item @{ML Variable.polymorphic}~@{text "ctxt ts"} generalizes type
variables in @{text "ts"} as far as possible, even those occurring
in fixed term variables.  The default policy of type-inference is to
fix newly introduced type variables, which is essentially reversed
with @{ML Variable.polymorphic}: here the given terms are detached
from the context as far as possible.

\item @{ML Variable.import}~@{text "open thms ctxt"} invents fixed
type and term variables for the schematic ones occurring in @{text
"thms"}.  The @{text "open"} flag indicates whether the fixed names
should be accessible to the user, otherwise newly introduced names
are marked as internal'' (\secref{sec:names}).

\item @{ML Variable.focus}~@{text B} decomposes the outermost @{text
"\<And>"} prefix of proposition @{text "B"}.

\end{description}
*}

text %mlex {* The following example shows how to work with fixed term
and type parameters and with type-inference.  *}

ML {*
(*static compile-time context -- for testing only*)
val ctxt0 = @{context};

(*locally fixed parameters -- no type assignment yet*)
val ([x, y], ctxt1) = ctxt0 |> Variable.add_fixes ["x", "y"];

(*t1: most general fixed type; t1': most general arbitrary type*)
val t1 = Syntax.read_term ctxt1 "x";
val t1' = singleton (Variable.polymorphic ctxt1) t1;

(*term u enforces specific type assignment*)
val u = Syntax.read_term ctxt1 "(x::nat) \<equiv> y";

(*official declaration of u -- propagates constraints etc.*)
val ctxt2 = ctxt1 |> Variable.declare_term u;
val t2 = Syntax.read_term ctxt2 "x";  (*x::nat is enforced*)
*}

text {* In the above example, the starting context is derived from the
toplevel theory, which means that fixed variables are internalized
literally: @{text "x"} is mapped again to @{text "x"}, and
attempting to fix it again in the subsequent context is an error.
Alternatively, fixed parameters can be renamed explicitly as
follows: *}

ML {*
val ctxt0 = @{context};
val ([x1, x2, x3], ctxt1) =
ctxt0 |> Variable.variant_fixes ["x", "x", "x"];
*}

text {* The following ML code can now work with the invented names of
@{text x1}, @{text x2}, @{text x3}, without depending on
the details on the system policy for introducing these variants.
Recall that within a proof body the system always invents fresh
skolem constants'', e.g.\ as follows: *}

begin
ML_prf %"ML" {*
val ctxt0 = @{context};

val ([x1], ctxt1) = ctxt0 |> Variable.add_fixes ["x"];
val ([x2], ctxt2) = ctxt1 |> Variable.add_fixes ["x"];
val ([x3], ctxt3) = ctxt2 |> Variable.add_fixes ["x"];

val ([y1, y2], ctxt4) =
ctxt3 |> Variable.variant_fixes ["y", "y"];
*}
end

text {* In this situation @{ML Variable.add_fixes} and @{ML
Variable.variant_fixes} are very similar, but identical name
proposals given in a row are only accepted by the second version.
*}

section {* Assumptions \label{sec:assumptions} *}

text {*
An \emph{assumption} is a proposition that it is postulated in the
current context.  Local conclusions may use assumptions as
additional facts, but this imposes implicit hypotheses that weaken
the overall statement.

Assumptions are restricted to fixed non-schematic statements, i.e.\
all generality needs to be expressed by explicit quantifiers.
Nevertheless, the result will be in HHF normal form with outermost
quantifiers stripped.  For example, by assuming @{text "\<And>x :: \<alpha>. P
x"} we get @{text "\<And>x :: \<alpha>. P x \<turnstile> P ?x"} for schematic @{text "?x"}
of fixed type @{text "\<alpha>"}.  Local derivations accumulate more and
more explicit references to hypotheses: @{text "A\<^isub>1, \<dots>,
A\<^isub>n \<turnstile> B"} where @{text "A\<^isub>1, \<dots>, A\<^isub>n"} needs to
be covered by the assumptions of the current context.

\medskip The @{text "add_assms"} operation augments the context by
local assumptions, which are parameterized by an arbitrary @{text
"export"} rule (see below).

The @{text "export"} operation moves facts from a (larger) inner
context into a (smaller) outer context, by discharging the
difference of the assumptions as specified by the associated export
rules.  Note that the discharged portion is determined by the
difference of contexts, not the facts being exported!  There is a
separate flag to indicate a goal context, where the result is meant
to refine an enclosing sub-goal of a structured proof state.

\medskip The most basic export rule discharges assumptions directly
by means of the @{text "\<Longrightarrow>"} introduction rule:
$\infer[(@{text "\<Longrightarrow>\<hyphen>intro"})]{@{text "\<Gamma> - A \<turnstile> A \<Longrightarrow> B"}}{@{text "\<Gamma> \<turnstile> B"}}$

The variant for goal refinements marks the newly introduced
premises, which causes the canonical Isar goal refinement scheme to
enforce unification with local premises within the goal:
$\infer[(@{text "#\<Longrightarrow>\<hyphen>intro"})]{@{text "\<Gamma> - A \<turnstile> #A \<Longrightarrow> B"}}{@{text "\<Gamma> \<turnstile> B"}}$

\medskip Alternative versions of assumptions may perform arbitrary
transformations on export, as long as the corresponding portion of
hypotheses is removed from the given facts.  For example, a local
definition works by fixing @{text "x"} and assuming @{text "x \<equiv> t"},
with the following export rule to reverse the effect:
$\infer[(@{text "\<equiv>\<hyphen>expand"})]{@{text "\<Gamma> - (x \<equiv> t) \<turnstile> B t"}}{@{text "\<Gamma> \<turnstile> B x"}}$
This works, because the assumption @{text "x \<equiv> t"} was introduced in
a context with @{text "x"} being fresh, so @{text "x"} does not
occur in @{text "\<Gamma>"} here.
*}

text %mlref {*
\begin{mldecls}
@{index_ML_type Assumption.export} \\
@{index_ML Assumption.assume: "cterm -> thm"} \\
"Assumption.export ->
cterm list -> Proof.context -> thm list * Proof.context"} \\
cterm list -> Proof.context -> thm list * Proof.context"} \\
@{index_ML Assumption.export: "bool -> Proof.context -> Proof.context -> thm -> thm"} \\
\end{mldecls}

\begin{description}

\item Type @{ML_type Assumption.export} represents arbitrary export
rules, which is any function of type @{ML_type "bool -> cterm list
-> thm -> thm"}, where the @{ML_type "bool"} indicates goal mode,
and the @{ML_type "cterm list"} the collection of assumptions to be
discharged simultaneously.

\item @{ML Assumption.assume}~@{text "A"} turns proposition @{text
"A"} into a primitive assumption @{text "A \<turnstile> A'"}, where the
conclusion @{text "A'"} is in HHF normal form.

\item @{ML Assumption.add_assms}~@{text "r As"} augments the context
by assumptions @{text "As"} with export rule @{text "r"}.  The
resulting facts are hypothetical theorems as produced by the raw
@{ML Assumption.assume}.

\item @{ML Assumption.add_assumes}~@{text "As"} is a special case of
@{ML Assumption.add_assms} where the export rule performs @{text
"\<Longrightarrow>\<hyphen>intro"} or @{text "#\<Longrightarrow>\<hyphen>intro"}, depending on goal
mode.

\item @{ML Assumption.export}~@{text "is_goal inner outer thm"}
exports result @{text "thm"} from the the @{text "inner"} context
back into the @{text "outer"} one; @{text "is_goal = true"} means
this is a goal context.  The result is in HHF normal form.  Note
that @{ML "Proof_Context.export"} combines @{ML "Variable.export"}
and @{ML "Assumption.export"} in the canonical way.

\end{description}
*}

text %mlex {* The following example demonstrates how rules can be
derived by building up a context of assumptions first, and exporting
some local fact afterwards.  We refer to @{theory Pure} equality
here for testing purposes.
*}

ML {*
(*static compile-time context -- for testing only*)
val ctxt0 = @{context};

val ([eq], ctxt1) =
ctxt0 |> Assumption.add_assumes [@{cprop "x \<equiv> y"}];
val eq' = Thm.symmetric eq;

(*back to original context -- discharges assumption*)
val r = Assumption.export false ctxt1 ctxt0 eq';
*}

text {* Note that the variables of the resulting rule are not
generalized.  This would have required to fix them properly in the
context beforehand, and export wrt.\ variables afterwards (cf.\ @{ML
Variable.export} or the combined @{ML "Proof_Context.export"}).  *}

section {* Structured goals and results \label{sec:struct-goals} *}

text {*
Local results are established by monotonic reasoning from facts
within a context.  This allows common combinations of theorems,
e.g.\ via @{text "\<And>/\<Longrightarrow>"} elimination, resolution rules, or equational
reasoning, see \secref{sec:thms}.  Unaccounted context manipulations
should be avoided, notably raw @{text "\<And>/\<Longrightarrow>"} introduction or ad-hoc
references to free variables or assumptions not present in the proof
context.

\medskip The @{text "SUBPROOF"} combinator allows to structure a
tactical proof recursively by decomposing a selected sub-goal:
@{text "(\<And>x. A(x) \<Longrightarrow> B(x)) \<Longrightarrow> \<dots>"} is turned into @{text "B(x) \<Longrightarrow> \<dots>"}
after fixing @{text "x"} and assuming @{text "A(x)"}.  This means
the tactic needs to solve the conclusion, but may use the premise as
a local fact, for locally fixed variables.

The family of @{text "FOCUS"} combinators is similar to @{text
"SUBPROOF"}, but allows to retain schematic variables and pending
subgoals in the resulting goal state.

The @{text "prove"} operation provides an interface for structured
backwards reasoning under program control, with some explicit sanity
checks of the result.  The goal context can be augmented by
additional fixed variables (cf.\ \secref{sec:variables}) and
assumptions (cf.\ \secref{sec:assumptions}), which will be available
as local facts during the proof and discharged into implications in
the result.  Type and term variables are generalized as usual,
according to the context.

The @{text "obtain"} operation produces results by eliminating
existing facts by means of a given tactic.  This acts like a dual
conclusion: the proof demonstrates that the context may be augmented
by parameters and assumptions, without affecting any conclusions
\cite{isabelle-isar-ref} for the user-level @{command obtain} and
@{command guess} elements.  Final results, which may not refer to
the parameters in the conclusion, need to exported explicitly into
the original context.  *}

text %mlref {*
\begin{mldecls}
@{index_ML SELECT_GOAL: "tactic -> int -> tactic"} \\
@{index_ML SUBPROOF: "(Subgoal.focus -> tactic) ->
Proof.context -> int -> tactic"} \\
@{index_ML Subgoal.FOCUS: "(Subgoal.focus -> tactic) ->
Proof.context -> int -> tactic"} \\
@{index_ML Subgoal.FOCUS_PREMS: "(Subgoal.focus -> tactic) ->
Proof.context -> int -> tactic"} \\
@{index_ML Subgoal.FOCUS_PARAMS: "(Subgoal.focus -> tactic) ->
Proof.context -> int -> tactic"} \\
@{index_ML Subgoal.focus: "Proof.context -> int -> thm -> Subgoal.focus * thm"} \\
@{index_ML Subgoal.focus_prems: "Proof.context -> int -> thm -> Subgoal.focus * thm"} \\
@{index_ML Subgoal.focus_params: "Proof.context -> int -> thm -> Subgoal.focus * thm"} \\
\end{mldecls}

\begin{mldecls}
@{index_ML Goal.prove: "Proof.context -> string list -> term list -> term ->
({prems: thm list, context: Proof.context} -> tactic) -> thm"} \\
@{index_ML Goal.prove_multi: "Proof.context -> string list -> term list -> term list ->
({prems: thm list, context: Proof.context} -> tactic) -> thm list"} \\
\end{mldecls}
\begin{mldecls}
@{index_ML Obtain.result: "(Proof.context -> tactic) -> thm list ->
Proof.context -> ((string * cterm) list * thm list) * Proof.context"} \\
\end{mldecls}

\begin{description}

\item @{ML SELECT_GOAL}~@{text "tac i"} confines a tactic to the
specified subgoal @{text "i"}.  This introduces a nested goal state,
without decomposing the internal structure of the subgoal yet.

\item @{ML SUBPROOF}~@{text "tac ctxt i"} decomposes the structure
of the specified sub-goal, producing an extended context and a
reduced goal, which needs to be solved by the given tactic.  All
schematic parameters of the goal are imported into the context as
fixed ones, which may not be instantiated in the sub-proof.

\item @{ML Subgoal.FOCUS}, @{ML Subgoal.FOCUS_PREMS}, and @{ML
Subgoal.FOCUS_PARAMS} are similar to @{ML SUBPROOF}, but are
slightly more flexible: only the specified parts of the subgoal are
imported into the context, and the body tactic may introduce new
subgoals and schematic variables.

\item @{ML Subgoal.focus}, @{ML Subgoal.focus_prems}, @{ML
Subgoal.focus_params} extract the focus information from a goal
state in the same way as the corresponding tacticals above.  This is
occasionally useful to experiment without writing actual tactics
yet.

\item @{ML Goal.prove}~@{text "ctxt xs As C tac"} states goal @{text
"C"} in the context augmented by fixed variables @{text "xs"} and
assumptions @{text "As"}, and applies tactic @{text "tac"} to solve
it.  The latter may depend on the local assumptions being presented
as facts.  The result is in HHF normal form.

\item @{ML Goal.prove_multi} is simular to @{ML Goal.prove}, but
states several conclusions simultaneously.  The goal is encoded by
means of Pure conjunction; @{ML Goal.conjunction_tac} will turn this
into a collection of individual subgoals.

\item @{ML Obtain.result}~@{text "tac thms ctxt"} eliminates the
given facts using a tactic, which results in additional fixed
variables and assumptions in the context.  Final results need to be
exported explicitly.

\end{description}
*}

text %mlex {* The following minimal example illustrates how to access
the focus information of a structured goal state. *}

begin
fix A B C :: "'a \<Rightarrow> bool"

have "\<And>x. A x \<Longrightarrow> B x \<Longrightarrow> C x"
ML_val
{*
val {goal, context = goal_ctxt, ...} = @{Isar.goal};
val (focus as {params, asms, concl, ...}, goal') =
Subgoal.focus goal_ctxt 1 goal;
val [A, B] = #prems focus;
val [(_, x)] = #params focus;
*}
oops

text {* \medskip The next example demonstrates forward-elimination in
a local context, using @{ML Obtain.result}.  *}

begin
assume ex: "\<exists>x. B x"

ML_prf %"ML" {*
val ctxt0 = @{context};
val (([(_, x)], [B]), ctxt1) = ctxt0
|> Obtain.result (fn _ => etac @{thm exE} 1) [@{thm ex}];
*}
ML_prf %"ML" {*
singleton (Proof_Context.export ctxt1 ctxt0) @{thm refl};
*}
ML_prf %"ML" {*
Proof_Context.export ctxt1 ctxt0 [Thm.reflexive x]
handle ERROR msg => (warning msg; []);
*}
end

end