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signature ABSTRACT_MACHINE =
sig
datatype term = Var of int | Const of int | App of term * term | Abs of term | Computed of term
datatype pattern = PVar | PConst of int * (pattern list)
datatype guard = Guard of term * term
type program
exception Compile of string;
(* The de-Bruijn index 0 occurring on the right hand side refers to the LAST pattern variable, when traversing the pattern from left to right,
1 to the second last, and so on. *)
val compile : (guard list * pattern * term) list -> program
exception Run of string;
val run : program -> term -> term
(* Utilities *)
val check_freevars : int -> term -> bool
val forall_consts : (int -> bool) -> term -> bool
val closed : term -> bool
val erase_Computed : term -> term
end
structure AbstractMachine : ABSTRACT_MACHINE =
struct
datatype term = Var of int | Const of int | App of term * term | Abs of term | Computed of term
datatype pattern = PVar | PConst of int * (pattern list)
datatype guard = Guard of term * term
type program = unit
exception Compile of string;
fun erase_Computed (Computed t) = erase_Computed t
| erase_Computed (App (t1, t2)) = App (erase_Computed t1, erase_Computed t2)
| erase_Computed (Abs t) = Abs (erase_Computed t)
| erase_Computed t = t
(*Returns true iff at most 0 .. (free-1) occur unbound. therefore
check_freevars 0 t iff t is closed*)
fun check_freevars free (Var x) = x < free
| check_freevars free (Const _) = true
| check_freevars free (App (u, v)) = check_freevars free u andalso check_freevars free v
| check_freevars free (Abs m) = check_freevars (free+1) m
| check_freevars free (Computed t) = check_freevars free t
fun forall_consts pred (Const c) = pred c
| forall_consts pred (Var _) = true
| forall_consts pred (App (u,v)) = forall_consts pred u
andalso forall_consts pred v
| forall_consts pred (Abs m) = forall_consts pred m
| forall_consts pred (Computed t) = forall_consts pred t
fun closed t = check_freevars 0 t
fun compile _ = raise Compile "abstract machine stub"
exception Run of string;
fun run _ _ = raise Run "abstract machine stub"
end
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