(* Define the abstract syntax of the calculus
 * by inductive definition of type-terms
 * and expression-terms.
 *)

From Coq Require Import Strings.String.
Module Terms.

(* types *)
Inductive type_term : Type :=
  | type_id : string -> type_term
  | type_var : string -> type_term
  | type_fun : type_term -> type_term -> type_term
  | type_univ : string -> type_term -> type_term
  | type_spec : type_term -> type_term -> type_term
  | type_morph : type_term -> type_term -> type_term
  | type_ladder : type_term -> type_term -> type_term
.

(* expressions *)
Inductive expr_term : Type :=
  | expr_var : string -> expr_term
  | expr_ty_abs : string -> expr_term -> expr_term
  | expr_ty_app : expr_term -> type_term -> expr_term
  | expr_abs   : string -> type_term -> expr_term -> expr_term
  | expr_morph : string -> type_term -> expr_term -> expr_term
  | expr_app : expr_term -> expr_term -> expr_term
  | expr_let : string -> type_term -> expr_term -> expr_term -> expr_term
  | expr_ascend : type_term -> expr_term -> expr_term
  | expr_descend : type_term -> expr_term -> expr_term
.

(* values *)
Inductive is_value : expr_term -> Prop :=
  | V_Abs : forall x τ e,
    (is_value (expr_abs x τ e))

  | V_TypAbs : forall τ e,
    (is_value (expr_ty_abs τ e))

  | V_Ascend : forall τ e,
    (is_value e) ->
    (is_value (expr_ascend τ e))
.



Declare Scope ladder_type_scope.
Declare Scope ladder_expr_scope.
Declare Custom Entry ladder_type.
Declare Custom Entry ladder_expr.

Notation "[< t >]" := t
  (t custom ladder_type at level 80) : ladder_type_scope.
Notation "'∀' x ',' t" := (type_univ x t)
  (t custom ladder_type at level 80, in custom ladder_type at level 80, x constr).
Notation "'<' σ τ '>'" := (type_spec σ τ)
  (in custom ladder_type at level 80, left associativity) : ladder_type_scope.
Notation "'(' τ ')'" := τ
  (in custom ladder_type at level 70) : ladder_type_scope.
Notation "σ '->' τ" := (type_fun σ τ)
  (in custom ladder_type at level 75, right associativity) : ladder_type_scope.
Notation "σ '->morph' τ" := (type_morph σ τ)
  (in custom ladder_type at level 75, right associativity, τ at level 80) : ladder_type_scope.
Notation "σ '~' τ" := (type_ladder σ τ)
  (in custom ladder_type at level 70, right associativity) : ladder_type_scope.
Notation "'$' x '$'" := (type_id x%string)
  (in custom ladder_type at level 0, x constr) : ladder_type_scope.
Notation "'%' x '%'" := (type_var x%string)
  (in custom ladder_type at level 0, x constr) : ladder_type_scope.

Notation "[{ e }]" := e
  (e custom ladder_expr at level 80) : ladder_expr_scope.
Notation "'%' x '%'" := (expr_var x%string)
  (in custom ladder_expr at level 0, x constr) : ladder_expr_scope.
Notation "'λ' x τ '↦' e" := (expr_abs x τ e) (in custom ladder_expr at level 0, x constr, τ custom ladder_type at level 99, e custom ladder_expr at level 99).
Notation "'Λ' t '↦' e" := (expr_ty_abs t e)
  (in custom ladder_expr at level 0, t constr, e custom ladder_expr at level 80).



(* EXAMPLES *)

Open Scope ladder_type_scope.
Open Scope ladder_expr_scope.

Check [< ∀"α", (< $"Seq"$ %"α"% > ~ < $"List"$ %"α"% >) >].

Definition polymorphic_identity1 : expr_term := [{ Λ"T" ↦ λ"x"%"T"% ↦ %"x"% }].
Definition polymorphic_identity2 : expr_term := [{ Λ"T" ↦ λ"y"%"T"% ↦ %"y"% }].

Compute polymorphic_identity1.

Close Scope ladder_type_scope.
Close Scope ladder_expr_scope.

End Terms.