[MLton-user] Stream Fusion
vesa.a.j.k at gmail.com
Wed Sep 19 23:51:04 PDT 2007
On 5/8/07, Stephen Weeks <sweeks at sweeks.com> wrote:
> Another standard way to hide existentials is to use a ref cell hidden
> in a closure.
> datatype 'a step =
> | GIVE of 'a
> | SKIP
> datatype 'a t = T of unit -> 'a step
> val unfold : 's * ('s -> ('a * 's) option) -> 'a t =
> fn (s, f) => let
> val r = ref s
> in T (fn () =>
> case f (!r) of
> NONE => DONE
> | SOME (a, s) => (r := s; GIVE a))
[I meant to sent this note much earlier, but never got around to it.]
Note that this gives a different semantics. A stream using the above
implementation is ephemeral and can be iterated (folded) only once.
Of course, in most cases one only wants to iterate a stream once.
However, it is possible to implement streams using a universal type
(http://mlton.org/UniversalType) and the universal type can be
implemented using either exceptions or refs and closures.
In fact, the Extended Basis library now includes a scaled down
implementation of streams using a universal type:
The stream module implemented in the EB can be compiled with either a
ref+closure or exn based universal type.
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