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Annotation of researchv10no/cmd/sml/src/parse/stream.sml, revision 1.1.1.1

1.1       root        1: (* Copyright 1989 by AT&T Bell Laboratories *)
                      2: signature STREAM =
                      3:  sig type 'a stream
                      4:      val streamify : (unit -> '1a) -> '1a stream
                      5:      val cons : '1a * '1a stream -> '1a stream
                      6:      val get : '1a stream -> '1a * '1a stream
                      7:  end
                      8: 
                      9: abstraction Stream : STREAM =
                     10: struct
                     11:    datatype 'a str = EVAL of 'a * 'a str ref | UNEVAL of (unit->'a)
                     12: 
                     13:    type 'a stream = 'a str ref
                     14: 
                     15:    fun get(ref(EVAL t)) = t
                     16:      | get(s as ref(UNEVAL f)) = 
                     17:            let val t = (f(), ref(UNEVAL f)) in s := EVAL t; t end
                     18: 
                     19:    fun streamify f = ref(UNEVAL f)
                     20:    fun cons(a,s) = ref(EVAL(a,s))
                     21: 
                     22: end

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