lib/mlyacc-lib/stream.sml

   1 (* ML-Yacc Parser Generator (c) 1989 Andrew W. Appel, David R. Tarditi *)
   2
   3 (* Stream: a structure implementing a lazy stream.  The signature STREAM
   4    is found in base.sig *)
   5
   6 structure Stream :> STREAM =
   7 struct
   8    datatype 'a str = EVAL of 'a * 'a str ref | UNEVAL of (unit->'a)
   9
  10    type 'a stream = 'a str ref
  11
  12    fun get(ref(EVAL t)) = t
  13      | get(s as ref(UNEVAL f)) =
  14             let val t = (f(), ref(UNEVAL f)) in s := EVAL t; t end
  15
  16    fun streamify f = ref(UNEVAL f)
  17    fun cons(a,s) = ref(EVAL(a,s))
  18
  19 end;