86 lines
3.1 KiB
OCaml
86 lines
3.1 KiB
OCaml
(* The source calculus. *)
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module S = Lambda
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(* The target calculus. *)
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module T = Tail
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exception NotValue (** <- Raised when trying to use a non-value term as such *)
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let freshId =
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(** Generates a fresh variable name string *)
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let cId = ref 0 in
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(fun () ->
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incr cId ;
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(string_of_int !cId))
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let freshWithPrefix pre =
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Atom.fresh (pre ^ (freshId ()) ^ "_")
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let prefixHint prefix hint = match hint with
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| Some h -> prefix ^ h
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| None -> prefix
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let freshBlockVarHinted hint = freshWithPrefix (prefixHint "bl_" hint)
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let freshBlockVar () = freshBlockVarHinted None
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let freshVarHinted hint = freshWithPrefix (prefixHint "v_" hint)
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let freshVar () = freshVarHinted None
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let letCont name varName body next =
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(** Allocates a block for a continuation, then runs [next] *)
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T.LetBlo(name, T.Lam(T.NoSelf, [varName], body), next)
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let rec cps_value (t: S.term) : T.value = match t with
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| S.Var v -> T.VVar v
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| S.Lit v -> T.VLit v
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| S.BinOp (l, op, r) -> T.VBinOp (cps_value l, op, cps_value r)
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| S.Let _ | S.Lam _ | S.App _ | S.Print _ | S.IfZero _ -> raise NotValue
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let cps_value_as_term (t: S.term) (cont: T.variable): T.term =
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T.TailCall(T.vvar cont, [cps_value t])
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let rec cps_term_inner (t: S.term) (cont: T.variable) (nameHint: string option)
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: T.term = match t with
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| S.Var _ -> cps_value_as_term t cont
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| S.Lit _ -> cps_value_as_term t cont
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| S.BinOp _ -> cps_value_as_term t cont
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| S.Lam (self, var, term) ->
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let fName = freshBlockVarHinted nameHint
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and innerCont = freshBlockVar () in
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T.LetBlo(fName,
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T.Lam(self, [var; innerCont], cps_term_inner term innerCont None),
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T.TailCall(T.vvar cont, T.vvars [fName]))
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| S.App (f, x) ->
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let xCont = freshBlockVar ()
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and fCont = freshBlockVar () in
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let xVal = freshVarHinted nameHint
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and fVal = freshVar () in
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letCont xCont xVal (
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letCont fCont fVal
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(T.TailCall(T.vvar fVal, T.vvars [xVal; cont])) @@
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(cps_term_inner f fCont None)) @@
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cps_term_inner x xCont None
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| S.Print term ->
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let curCont = freshBlockVar ()
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and termVal = freshVar () in
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letCont curCont termVal (
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T.Print(T.vvar termVal,
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T.TailCall(T.vvar cont, T.vvars [termVal])))
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(cps_term_inner term curCont None)
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| S.Let (var, value, next) ->
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let curCont = freshBlockVar () in
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letCont curCont var (cps_term_inner next cont None) @@
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cps_term_inner value curCont (Some (Atom.hint var))
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| S.IfZero (expr, tIf, tElse) ->
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let curCont = freshBlockVar ()
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and exprVal = freshVar () in
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letCont curCont exprVal (T.IfZero(T.vvar exprVal,
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cps_term_inner tIf cont None,
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cps_term_inner tElse cont None)) @@
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cps_term_inner expr curCont None
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let cps_term (t: S.term): T.term =
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(** Entry point. Transforms a [Lambda] term into a [Tail] term, applying a
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* continuation-passing-style transformation. *)
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let exitBlock = freshBlockVarHinted (Some "exit") in
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letCont exitBlock (freshVar ()) T.Exit @@
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cps_term_inner t exitBlock (Some "main_entry")
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