[MLton-user] Extended Basis Library: iterating thunks

[Geoffrey Alan Washburn]

Vesa Karvonen wrote:
> I don't like the type of this.  A thunk is a delayed computation that you
> invoke for the computed value (unit thunk is a sign of a mistake).  The
> type of iterate seems error prone to me.  For example, you can easily 
> call
> it accidentally with a partially applied function inside an explicit
> thunk:
>
>  iterate (fn () => curried a b (* Oops! Forgot the last argument: c. 
> *)) n
>
> I think that when a non-unit return value is always ignored, it should be
> explicitly visible at the call site.  For the same reason, I don't
> particularly like the type of app and even less the type of appWith in 
> the
> MONAD_EX signature, but those are more arguable, because it may be costly
> (performance wise) to ignore the result (an extra bind may be required).
> In this case, however, the cost is always negligible or zero (function
> composition is optimized by all serious ML compilers and in most cases
> with MLton the cost of composing with ignore is literally zero).
>
> To me, a more natural function to provide would be either
>
>  val repeat : Unit.t Effect.t -> Int.t Effect.t
>
> or
>
>  val repeat : 'a Effect.t -> Int.t -> 'a Effect.t
>
> (or some permutation of the arguments of either) in the Effect : EFFECT
> structure.
>
    Fair enough, this certainly sounds much better than my proposal that 
evolved out of a function I was using in my code.

> Instead of writing
>
>  Thunk.iterate th n
>
> you would write (using the former repeat)
>
>  Effect.repeat (ignore o th) n
>
> or (using the latter repeat)
>
>  Effect.repeat (ignore o th) n ()
    The latter seems slightly more general, so I'll go with that when I 
implement it. Though I suppose there is also the question of whether

    val repeat : 'a Effect.t -> Int.t -> (Int.t -> 'a) Effect.t

would be even better.  This would make the the above example

     Effect.repeat (ignore o th) n |< Fn.const ()

Alternately, this proposal could be called repeatWith. 

-- 
[Geoff Washburn|geoffw at cis.upenn.edu|http://www.cis.upenn.edu/~geoffw/]

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