Question

I’m having a little trouble rewriting a method to use a higher-order function. Here’s the attempt (it compiles but doesn’t run it works fine):

fun lookup2 key hash = 
  List.foldr (fn ((k, v), r) => if k = key then SOME v else r) NONE hash;

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Background:

The idea is that there is a hash like so and I’m lookup up an int option by string key:

- val h = [("a", 5), ("b", 4)];
- lookup "a" h;
val it = SOME 5 : int option

I wrote one that works just fine:

fun lookup _ nil = NONE
  | lookup key ((k, v)::xs) = if k = key then SOME v else lookup key xs;

However I wanted to rewrite it to help get familiar with sml. I have a sneaking suspicion I don’t quite understand foldr…

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My mistake – I was accidentally overwriting it with a different lookup2. It works as expected.

Answer 1

What you are looking for is not List.foldr, but List.find:

find f l

applies f to each element x of the list l, from left to right, until x evaluates to true. It returns SOME(x) if such an x exists; otherwise it returns NONE.

(source)

Now you can do the following:

fun lookup key hash = Option.map (fn (_,v) => v)
  ( List.find (fn (k,_) => k = key) hash );

Where List.find returns the matching tuple (key, val) or NONE, and Option.map returns either val from the tuple or also NONE.