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@mcmillhj mcmillhj/example.sml
Created Aug 24, 2017

What would you like to do?
(* foldl type
fn: ('a * 'b -> 'b) -> 'b -> 'a list -> 'b
fun fromBase base = let
fun f (x, NONE ) = NONE
| f (x, SOME n) =
if x >= 0 andalso x < base then SOME(n * base + x)
else NONE
foldl f (SOME 0)

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commented Aug 24, 2017

Conversation from #sml that helped get to this solution:

[20:56] <mcmillhj> I am having some trouble getting foldl to work with option types, I am sure I am overlooking something obvious. Would someone mind taking a look?
[20:56] <mcmillhj> thanks
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[22:11] <ski> mcmillhj : first match on `acc' to determine whether it's `NONE' or `SOME n' for some `n' of type `int'
[22:12] ski [] requested CTCP PING from mcmillhj: 1503540730 103116
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[22:23] <mcmillhj> ski: thanks, that was exactly what I needed
[22:23] <mcmillhj> I was hoping it would look a little cleaner though, I was trying to model it after this Haskell solution
[22:23] <ski> you can define `f' like :
[22:25] <ski>   fun f (x,NONE  ) = NONE
[22:25] <ski>     | f (x,SOME n) = if x >= 0 andalso x < base
[22:25] <ski>                 then SOME (n * base + x)
[22:25] <ski>                 else NONE
[22:27] <mcmillhj> oh wow, that looks much cleaner. Thank you ski
[22:27] <ski> oh, that paste uses `foldM', not `foldl'
[22:28] <ski> you could define a `foldM' (for `option') if you wanted to. then your `f' would look more similar to that Haskell one
[22:29] <ski> (an alternative would be to use an exception instead of `NONE')
[22:30] <ski> oh .. and if you want to, you can also avoid giving `f' a name :
[22:30] <ski>   fun fromBase base = foldl
[22:30] <ski>         (fn f (x,NONE  ) => NONE
[22:31] <ski>           | f (x,SOME n) => ...)
[22:31] <ski>         (SOME 0)
[22:31] <ski> er, sorry, there shouldn't be `f's after `fn' and `|' there, scrap those
[22:32] <mcmillhj> ski: sorry to ask so many questions, but how does foldM differ from foldl?
[22:32] <mcmillhj> does it do some sort of implicit unwrapping of option types?
[22:32] <ski>   val foldl : ('a * 'b -> 'b) -> 'b -> 'a list -> 'b
[22:32] <ski>   val foldM : ('a * 'b -> 'b option) -> 'b -> 'a list -> 'b option  (* in this case *)
[22:33] <ski> (i suppose you could call it `fold_option' or something)
[22:34] <ski> then the function passed to `foldM' / `fold_option' would not need to check whether it's argument is `NONE' or `SOME n' (since `foldM' / `fold_option' itself would handle that)
[22:35] <mcmillhj> ah I see
[22:35] <mcmillhj> thanks so much
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