--- a/doc-src/IsarImplementation/Thy/proof.thy Thu Feb 26 06:39:06 2009 -0800
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-
-(* $Id$ *)
-
-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 nicely 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> = x\<^isub>\<alpha>"} produces in the first step
- @{text "x: term \<turnstile> x\<^isub>\<alpha> = x\<^isub>\<alpha>"} for fixed @{text "\<alpha>"},
- and only in the second step @{text "\<turnstile> ?x\<^isub>?\<^isub>\<alpha> =
- ?x\<^isub>?\<^isub>\<alpha>"} for schematic @{text "?x"} and @{text "?\<alpha>"}.
-
- \medskip The Isabelle/Isar proof context manages the gory 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}
- @{index_ML Variable.add_fixes: "
- 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_thms: "bool -> thm list -> Proof.context ->
- ((ctyp list * cterm list) * thm list) * Proof.context"} \\
- @{index_ML Variable.focus: "cterm -> Proof.context -> (cterm list * cterm) * 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_thms}~@{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}
-*}
-
-
-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 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 (cf.\
- \secref{sec:isar-proof-state}).
-
- \medskip The most basic export rule discharges assumptions directly
- by means of the @{text "\<Longrightarrow>"} introduction rule:
- \[
- \infer[(@{text "\<Longrightarrow>_intro"})]{@{text "\<Gamma> \\ A \<turnstile> A \<Longrightarrow> B"}}{@{text "\<Gamma> \<turnstile> B"}}
- \]
-
- 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:
- \[
- \infer[(@{text "#\<Longrightarrow>_intro"})]{@{text "\<Gamma> \\ A \<turnstile> #A \<Longrightarrow> B"}}{@{text "\<Gamma> \<turnstile> B"}}
- \]
-
- \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:
- \[
- \infer[(@{text "\<equiv>-expand"})]{@{text "\<Gamma> \\ x \<equiv> t \<turnstile> B t"}}{@{text "\<Gamma> \<turnstile> B x"}}
- \]
- 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"} \\
- @{index_ML Assumption.add_assms:
- "Assumption.export ->
- cterm list -> Proof.context -> thm list * Proof.context"} \\
- @{index_ML Assumption.add_assumes: "
- cterm list -> Proof.context -> thm list * Proof.context"} \\
- @{index_ML Assumption.export: "bool -> Proof.context -> Proof.context -> thm -> thm"} \\
- \end{mldecls}
-
- \begin{description}
-
- \item @{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 raw 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>_intro"} or @{text "#\<Longrightarrow>_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 "ProofContext.export"} combines @{ML "Variable.export"}
- and @{ML "Assumption.export"} in the canonical way.
-
- \end{description}
-*}
-
-
-section {* Results \label{sec:results} *}
-
-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 @{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 certain fixed variables and assumptions. See also
- \cite{isabelle-isar-ref} for the user-level @{text "\<OBTAIN>"} and
- @{text "\<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 SUBPROOF:
- "({context: Proof.context, schematics: ctyp list * cterm list,
- params: cterm list, asms: cterm list, concl: cterm,
- prems: thm list} -> tactic) -> Proof.context -> int -> tactic"} \\
- \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 -> (cterm list * thm list) * Proof.context"} \\
- \end{mldecls}
-
- \begin{description}
-
- \item @{ML SUBPROOF}~@{text "tac"} decomposes the structure of a
- particular 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 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}
-*}
-
-end