494 lines
19 KiB
Coq
494 lines
19 KiB
Coq
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(** Definition of Fsub (System F with subtyping).
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Authors: Brian Aydemir and Arthur Charguéraud, with help from
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Aaron Bohannon, Jeffrey Vaughan, and Dimitrios Vytiniotis.
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Table of contents:
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- #<a href="##syntax">Syntax (pre-terms)</a>#
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- #<a href="##open">Opening</a>#
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- #<a href="##lc">Local closure</a>#
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- #<a href="##env">Environments</a>#
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- #<a href="##wf">Well-formedness</a>#
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- #<a href="##sub">Subtyping</a>#
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- #<a href="##typing_doc">Typing</a>#
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- #<a href="##values">Values</a>#
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- #<a href="##reduction">Reduction</a>#
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- #<a href="##auto">Automation</a>#
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*)
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Require Export Metatheory.
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(* ********************************************************************** *)
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(** * #<a name="syntax"></a># Syntax (pre-terms) *)
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(** We use a locally nameless representation for Fsub, where bound
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variables are represented as natural numbers (de Bruijn indices)
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and free variables are represented as [atom]s. The type [atom],
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defined in the [Atom] library, represents names: there are
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infinitely many atoms, equality is decidable on atoms, and it is
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possible to generate an atom fresh for any given finite set of
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atoms.
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We say that the definitions below define pre-types ([typ]) and
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pre-expressions ([exp]), collectively pre-terms, since the
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datatypes admit terms, such as [(typ_all typ_top (typ_bvar 3))],
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where indices are unbound. A term is locally closed when it
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contains no unbound indices.
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Note that indices for bound type variables are distinct from
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indices for bound expression variables. We make this explicit in
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the definitions below of the opening operations. *)
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Inductive typ : Set :=
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| typ_top : typ
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| typ_bvar : nat -> typ
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| typ_fvar : atom -> typ
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| typ_arrow : typ -> typ -> typ
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| typ_all : typ -> typ -> typ
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.
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Inductive exp : Set :=
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| exp_bvar : nat -> exp
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| exp_fvar : atom -> exp
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| exp_abs : typ -> exp -> exp
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| exp_app : exp -> exp -> exp
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| exp_tabs : typ -> exp -> exp
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| exp_tapp : exp -> typ -> exp
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.
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(** We declare the constructors for indices and variables to be
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coercions. For example, if Coq sees a [nat] where it expects an
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[exp], it will implicitly insert an application of [exp_bvar];
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similar behavior happens for [atom]s. Thus, we may write
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[(exp_abs typ_top (exp_app 0 x))] instead of [(exp_abs typ_top
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(exp_app (exp_bvar 0) (exp_fvar x)))]. *)
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Coercion typ_bvar : nat >-> typ.
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Coercion typ_fvar : atom >-> typ.
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Coercion exp_bvar : nat >-> exp.
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Coercion exp_fvar : atom >-> exp.
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(* ********************************************************************** *)
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(** * #<a name="open"></a># Opening terms *)
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(** Opening replaces an index with a term. This operation is required
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if we wish to work only with locally closed terms when going under
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binders (e.g., the typing rule for [exp_abs]). It also
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corresponds to informal substitution for a bound variable, which
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occurs in the rule for beta reduction.
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We need to define three functions for opening due the syntax of
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Fsub, and we name them according to the following convention.
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- [tt]: Denotes an operation involving types appearing in types.
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- [te]: Denotes an operation involving types appearing in
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expressions.
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- [ee]: Denotes an operation involving expressions appearing in
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expressions.
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The notation used below for decidable equality on atoms and
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natural numbers (e.g., [K === J]) is defined in the [Metatheory]
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library. The order of arguments to each "open" function is the
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same. For example, [(open_tt_rec K U T)] can be read as
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"substitute [U] for index [K] in [T]."
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Note that we assume that [U] is locally closed (and similarly for
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the other opening functions). This assumption simplifies the
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implementations of opening by letting us avoid shifting. Since
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bound variables are indices, there is no need to rename variables
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to avoid capture. Finally, we assume that these functions are
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initially called with index zero and that zero is the only unbound
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index in the term. This eliminates the need to possibly subtract
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one in the case of indices. *)
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Fixpoint open_tt_rec (K : nat) (U : typ) (T : typ) {struct T} : typ :=
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match T with
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| typ_top => typ_top
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| typ_bvar J => if K === J then U else (typ_bvar J)
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| typ_fvar X => typ_fvar X
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| typ_arrow T1 T2 => typ_arrow (open_tt_rec K U T1) (open_tt_rec K U T2)
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| typ_all T1 T2 => typ_all (open_tt_rec K U T1) (open_tt_rec (S K) U T2)
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end.
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Fixpoint open_te_rec (K : nat) (U : typ) (e : exp) {struct e} : exp :=
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match e with
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| exp_bvar i => exp_bvar i
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| exp_fvar x => exp_fvar x
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| exp_abs V e1 => exp_abs (open_tt_rec K U V) (open_te_rec K U e1)
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| exp_app e1 e2 => exp_app (open_te_rec K U e1) (open_te_rec K U e2)
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| exp_tabs V e1 => exp_tabs (open_tt_rec K U V) (open_te_rec (S K) U e1)
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| exp_tapp e1 V => exp_tapp (open_te_rec K U e1) (open_tt_rec K U V)
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end.
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Fixpoint open_ee_rec (k : nat) (f : exp) (e : exp) {struct e} : exp :=
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match e with
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| exp_bvar i => if k === i then f else (exp_bvar i)
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| exp_fvar x => exp_fvar x
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| exp_abs V e1 => exp_abs V (open_ee_rec (S k) f e1)
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| exp_app e1 e2 => exp_app (open_ee_rec k f e1) (open_ee_rec k f e2)
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| exp_tabs V e1 => exp_tabs V (open_ee_rec k f e1)
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| exp_tapp e1 V => exp_tapp (open_ee_rec k f e1) V
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end.
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(** Many common applications of opening replace index zero with an
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expression or variable. The following definitions provide
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convenient shorthands for such uses. Note that the order of
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arguments is switched relative to the definitions above. For
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example, [(open_tt T X)] can be read as "substitute the variable
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[X] for index [0] in [T]" and "open [T] with the variable [X]."
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Recall that the coercions above let us write [X] in place of
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[(typ_fvar X)], assuming that [X] is an [atom]. *)
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Definition open_tt T U := open_tt_rec 0 U T.
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Definition open_te e U := open_te_rec 0 U e.
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Definition open_ee e1 e2 := open_ee_rec 0 e2 e1.
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(* ********************************************************************** *)
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(** * #<a name="lc"></a># Local closure *)
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(** Recall that [typ] and [exp] define pre-terms; these datatypes
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admit terms that contain unbound indices. A term is locally
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closed, or syntactically well-formed, when no indices appearing in
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it are unbound. The proposition [(type T)] holds when a type [T]
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is locally closed, and [(expr e)] holds when an expression [e] is
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locally closed.
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The inductive definitions below formalize local closure such that
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the resulting induction principles serve as structural induction
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principles over (locally closed) types and (locally closed)
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expressions. In particular, unlike the situation with pre-terms,
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there are no cases for indices. Thus, these induction principles
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correspond more closely to informal practice than the ones arising
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from the definitions of pre-terms.
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The interesting cases in the inductive definitions below are those
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that involve binding constructs, e.g., [typ_all]. Intuitively, to
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check if the pre-term [(typ_all T1 T2)] is locally closed we much
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check that [T1] is locally closed, and that [T2] is locally closed
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when opened with a variable. However, there is a choice as to how
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many variables to quantify over. One possibility is to quantify
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over only one variable ("existential" quantification), as in
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<<
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type_all : forall X T1 T2,
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type T1 ->
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type (open_tt T2 X) ->
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type (typ_all T1 T2)
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>> or we could quantify over as many variables as possible ("universal"
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quantification), as in
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<<
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type_all : forall T1 T2,
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type T1 ->
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(forall X : atom, type (open_tt T2 X)) ->
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type (typ_all T1 T2)
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>> It is possible to show that the resulting relations are equivalent.
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The former makes it easy to build derivations, while the latter
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provides a strong induction principle. McKinna and Pollack used
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both forms of this relation in their work on formalizing Pure Type
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Systems.
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We take a different approach here and use "cofinite
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quantification": we quantify over all but finitely many variables.
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This approach provides a convenient middle ground: we can build
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derivations reasonably easily and get reasonably strong induction
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principles. With some work, one can show that the definitions
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below are equivalent to ones that use existential, and hence also
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universal, quantification. *)
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Inductive type : typ -> Prop :=
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| type_top :
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type typ_top
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| type_var : forall X,
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type (typ_fvar X)
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| type_arrow : forall T1 T2,
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type T1 ->
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type T2 ->
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type (typ_arrow T1 T2)
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| type_all : forall L T1 T2,
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type T1 ->
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(forall X : atom, X `notin` L -> type (open_tt T2 X)) ->
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type (typ_all T1 T2)
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.
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Inductive expr : exp -> Prop :=
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| expr_var : forall x,
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expr (exp_fvar x)
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| expr_abs : forall L T e1,
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type T ->
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(forall x : atom, x `notin` L -> expr (open_ee e1 x)) ->
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expr (exp_abs T e1)
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| expr_app : forall e1 e2,
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expr e1 ->
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expr e2 ->
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expr (exp_app e1 e2)
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| expr_tabs : forall L T e1,
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type T ->
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(forall X : atom, X `notin` L -> expr (open_te e1 X)) ->
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expr (exp_tabs T e1)
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| expr_tapp : forall e1 V,
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expr e1 ->
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type V ->
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expr (exp_tapp e1 V)
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.
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(* ********************************************************************** *)
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(** * #<a name="env"></a># Environments *)
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(** In our presentation of System F with subtyping, we use a single
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environment for both typing and subtyping assumptions. We
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formalize environments by representing them as association lists
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(lists of pairs of keys and values) whose keys are atoms.
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The [Metatheory] and [Environment] libraries provide functions,
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predicates, tactics, notations and lemmas that simplify working
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with environments. The [Environment] library treats environments
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as lists of type [list (atom * A)].
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Since environments map [atom]s, the type [A] should encode whether
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a particular binding is a typing or subtyping assumption. Thus,
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we instantiate [A] with the type [binding], defined below. *)
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Inductive binding : Set :=
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| bind_sub : typ -> binding
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| bind_typ : typ -> binding.
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(** A binding [(X, bind_sub T)] records that a type variable [X] is a
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subtype of [T], and a binding [(x, bind_typ U)] records that an
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expression variable [x] has type [U].
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We define an abbreviation [env] for the type of environments, and
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an abbreviation [empty] for the empty environment.
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Note: Each instance of [Notation] below defines an abbreviation
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since the left-hand side consists of a single identifier that is
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not in quotes. These abbreviations are used for both parsing (the
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left-hand side is equivalent to the right-hand side in all
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contexts) and printing (the right-hand side is pretty-printed as
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the left-hand side). Since [nil] is normally a polymorphic
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constructor whose type argument is implicit, we prefix the name
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with "[@]" to signal to Coq that we are going to supply arguments
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to [nil] explicitly. *)
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Notation env := (list (atom * binding)).
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Notation empty := (@nil (atom * binding)).
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(** We also define a notation that makes it convenient to write one
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element lists. This notation is useful because of our convention
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for building environments; see the examples below. *)
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Notation "[ x ]" := (x :: nil).
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(** #<b>#Examples:#</b># We use a convention where environments are
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never built using a cons operation [((x, a) :: E)] where [E] is
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non-[nil]. This makes the shape of environments more uniform and
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saves us from excessive fiddling with the shapes of environments.
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For example, Coq's tactics sometimes distinguish between consing
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on a new binding and prepending a one element list, even though
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the two operations are convertible with each other.
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Consider the following environments written in informal notation.
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<<
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1. (empty environment)
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2. x : T
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3. x : T, Y <: S
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4. E, x : T, F
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>> In the third example, we have an environment that binds an
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expression variable [x] to [T] and a type variable [Y] to [S].
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In Coq, we would write these environments as follows.
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<<
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1. empty
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2. [(x, bind_typ T)]
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3. [(Y, bind_sub S)] ++ [(x, bind_typ T)]
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4. F ++ [(x, bind_typ T)] ++ E
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>> The symbol "[++]" denotes list concatenation and associates to the
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right. (That notation is defined in Coq's [List] library.) Note
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that in Coq, environments grow on the left, since that is where
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the head of a list is. *)
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(* ********************************************************************** *)
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(** * #<a name="wf"></a># Well-formedness *)
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(** A type [T] is well-formed with respect to an environment [E],
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denoted [(wf_typ E T)], when [T] is locally-closed and its free
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variables are bound in [E]. We need this relation in order to
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restrict the subtyping and typing relations, defined below, to
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contain only well-formed types. (This relation is missing in the
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original statement of the POPLmark Challenge.)
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Note: It is tempting to define the premise of [wf_typ_var] as [(X
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`in` dom E)], since that makes the rule easier to apply (no need
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to guess an instantiation for [U]). Unfortunately, this is
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incorrect. We need to check that [X] is bound as a type-variable,
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not an expression-variable; [(dom E)] does not distinguish between
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the two kinds of bindings. *)
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Inductive wf_typ : env -> typ -> Prop :=
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| wf_typ_top : forall E,
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wf_typ E typ_top
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| wf_typ_var : forall U E (X : atom),
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binds X (bind_sub U) E ->
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wf_typ E (typ_fvar X)
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| wf_typ_arrow : forall E T1 T2,
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wf_typ E T1 ->
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wf_typ E T2 ->
|
||
|
|
wf_typ E (typ_arrow T1 T2)
|
||
|
|
| wf_typ_all : forall L E T1 T2,
|
||
|
|
wf_typ E T1 ->
|
||
|
|
(forall X : atom, X `notin` L ->
|
||
|
|
wf_typ ([(X, bind_sub T1)] ++ E) (open_tt T2 X)) ->
|
||
|
|
wf_typ E (typ_all T1 T2)
|
||
|
|
.
|
||
|
|
|
||
|
|
(** An environment E is well-formed, denoted [(wf_env E)], if each
|
||
|
|
atom is bound at most at once and if each binding is to a
|
||
|
|
well-formed type. This is a stronger relation than the [ok]
|
||
|
|
relation defined in the [Environment] library. We need this
|
||
|
|
relation in order to restrict the subtyping and typing relations,
|
||
|
|
defined below, to contain only well-formed environments. (This
|
||
|
|
relation is missing in the original statement of the POPLmark
|
||
|
|
Challenge.) *)
|
||
|
|
|
||
|
|
Inductive wf_env : env -> Prop :=
|
||
|
|
| wf_env_empty :
|
||
|
|
wf_env empty
|
||
|
|
| wf_env_sub : forall (E : env) (X : atom) (T : typ),
|
||
|
|
wf_env E ->
|
||
|
|
wf_typ E T ->
|
||
|
|
X `notin` dom E ->
|
||
|
|
wf_env ([(X, bind_sub T)] ++ E)
|
||
|
|
| wf_env_typ : forall (E : env) (x : atom) (T : typ),
|
||
|
|
wf_env E ->
|
||
|
|
wf_typ E T ->
|
||
|
|
x `notin` dom E ->
|
||
|
|
wf_env ([(x, bind_typ T)] ++ E).
|
||
|
|
|
||
|
|
|
||
|
|
(* ********************************************************************** *)
|
||
|
|
(** * #<a name="sub"></a># Subtyping *)
|
||
|
|
|
||
|
|
(** The definition of subtyping is straightforward. It uses the
|
||
|
|
[binds] relation from the [Environment] library (in the
|
||
|
|
[sub_trans_tvar] case) and cofinite quantification (in the
|
||
|
|
[sub_all] case). *)
|
||
|
|
|
||
|
|
Inductive sub : env -> typ -> typ -> Prop :=
|
||
|
|
| sub_top : forall E S,
|
||
|
|
wf_env E ->
|
||
|
|
wf_typ E S ->
|
||
|
|
sub E S typ_top
|
||
|
|
| sub_refl_tvar : forall E X,
|
||
|
|
wf_env E ->
|
||
|
|
wf_typ E (typ_fvar X) ->
|
||
|
|
sub E (typ_fvar X) (typ_fvar X)
|
||
|
|
| sub_trans_tvar : forall U E T X,
|
||
|
|
binds X (bind_sub U) E ->
|
||
|
|
sub E U T ->
|
||
|
|
sub E (typ_fvar X) T
|
||
|
|
| sub_arrow : forall E S1 S2 T1 T2,
|
||
|
|
sub E T1 S1 ->
|
||
|
|
sub E S2 T2 ->
|
||
|
|
sub E (typ_arrow S1 S2) (typ_arrow T1 T2)
|
||
|
|
| sub_all : forall L E S1 S2 T1 T2,
|
||
|
|
sub E T1 S1 ->
|
||
|
|
(forall X : atom, X `notin` L ->
|
||
|
|
sub ([(X, bind_sub T1)] ++ E) (open_tt S2 X) (open_tt T2 X)) ->
|
||
|
|
sub E (typ_all S1 S2) (typ_all T1 T2)
|
||
|
|
.
|
||
|
|
|
||
|
|
|
||
|
|
(* ********************************************************************** *)
|
||
|
|
(** * #<a name="typing_doc"></a># Typing *)
|
||
|
|
|
||
|
|
(** The definition of typing is straightforward. It uses the [binds]
|
||
|
|
relation from the [Environment] library (in the [typing_var] case)
|
||
|
|
and cofinite quantification in the cases involving binders (e.g.,
|
||
|
|
[typing_abs] and [typing_tabs]). *)
|
||
|
|
|
||
|
|
Inductive typing : env -> exp -> typ -> Prop :=
|
||
|
|
| typing_var : forall E x T,
|
||
|
|
wf_env E ->
|
||
|
|
binds x (bind_typ T) E ->
|
||
|
|
typing E (exp_fvar x) T
|
||
|
|
| typing_abs : forall L E V e1 T1,
|
||
|
|
(forall x : atom, x `notin` L ->
|
||
|
|
typing ([(x, bind_typ V)] ++ E) (open_ee e1 x) T1) ->
|
||
|
|
typing E (exp_abs V e1) (typ_arrow V T1)
|
||
|
|
| typing_app : forall T1 E e1 e2 T2,
|
||
|
|
typing E e1 (typ_arrow T1 T2) ->
|
||
|
|
typing E e2 T1 ->
|
||
|
|
typing E (exp_app e1 e2) T2
|
||
|
|
| typing_tabs : forall L E V e1 T1,
|
||
|
|
(forall X : atom, X `notin` L ->
|
||
|
|
typing ([(X, bind_sub V)] ++ E) (open_te e1 X) (open_tt T1 X)) ->
|
||
|
|
typing E (exp_tabs V e1) (typ_all V T1)
|
||
|
|
| typing_tapp : forall T1 E e1 T T2,
|
||
|
|
typing E e1 (typ_all T1 T2) ->
|
||
|
|
sub E T T1 ->
|
||
|
|
typing E (exp_tapp e1 T) (open_tt T2 T)
|
||
|
|
| typing_sub : forall S E e T,
|
||
|
|
typing E e S ->
|
||
|
|
sub E S T ->
|
||
|
|
typing E e T
|
||
|
|
.
|
||
|
|
|
||
|
|
|
||
|
|
(* ********************************************************************** *)
|
||
|
|
(** * #<a name="values"></a># Values *)
|
||
|
|
|
||
|
|
Inductive value : exp -> Prop :=
|
||
|
|
| value_abs : forall T e1,
|
||
|
|
expr (exp_abs T e1) ->
|
||
|
|
value (exp_abs T e1)
|
||
|
|
| value_tabs : forall T e1,
|
||
|
|
expr (exp_tabs T e1) ->
|
||
|
|
value (exp_tabs T e1)
|
||
|
|
.
|
||
|
|
|
||
|
|
|
||
|
|
(* ********************************************************************** *)
|
||
|
|
(** * #<a name="reduction"></a># Reduction *)
|
||
|
|
|
||
|
|
Inductive red : exp -> exp -> Prop :=
|
||
|
|
| red_app_1 : forall e1 e1' e2,
|
||
|
|
expr e2 ->
|
||
|
|
red e1 e1' ->
|
||
|
|
red (exp_app e1 e2) (exp_app e1' e2)
|
||
|
|
| red_app_2 : forall e1 e2 e2',
|
||
|
|
value e1 ->
|
||
|
|
red e2 e2' ->
|
||
|
|
red (exp_app e1 e2) (exp_app e1 e2')
|
||
|
|
| red_tapp : forall e1 e1' V,
|
||
|
|
type V ->
|
||
|
|
red e1 e1' ->
|
||
|
|
red (exp_tapp e1 V) (exp_tapp e1' V)
|
||
|
|
| red_abs : forall T e1 v2,
|
||
|
|
expr (exp_abs T e1) ->
|
||
|
|
value v2 ->
|
||
|
|
red (exp_app (exp_abs T e1) v2) (open_ee e1 v2)
|
||
|
|
| red_tabs : forall T1 e1 T2,
|
||
|
|
expr (exp_tabs T1 e1) ->
|
||
|
|
type T2 ->
|
||
|
|
red (exp_tapp (exp_tabs T1 e1) T2) (open_te e1 T2)
|
||
|
|
.
|
||
|
|
|
||
|
|
|
||
|
|
(* ********************************************************************** *)
|
||
|
|
(** * #<a name="auto"></a># Automation *)
|
||
|
|
|
||
|
|
(** We declare most constructors as [Hint]s to be used by the [auto]
|
||
|
|
and [eauto] tactics. We exclude constructors from the subtyping
|
||
|
|
and typing relations that use cofinite quantification. It is
|
||
|
|
unlikely that [eauto] will find an instantiation for the finite
|
||
|
|
set [L], and in those cases, [eauto] can take some time to fail.
|
||
|
|
(A priori, this is not obvious. In practice, one adds as hints
|
||
|
|
all constructors and then later removes some constructors when
|
||
|
|
they cause proof search to take too long.) *)
|
||
|
|
|
||
|
|
Hint Constructors type expr wf_typ wf_env value red.
|
||
|
|
Hint Resolve sub_top sub_refl_tvar sub_arrow.
|
||
|
|
Hint Resolve typing_var typing_app typing_tapp typing_sub.
|