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added map2 solution #19

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added map2 solution #19

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9 changes: 9 additions & 0 deletions review-problems-sol.ml
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Original file line number Diff line number Diff line change
Expand Up @@ -207,6 +207,15 @@ let rec combine l1 l2 = match l1, l2 with
let map2 f l1 l2 =
let tup = combine l1 l2 in
map (fun (x,y) -> f x y) tup

let map2 f l1 l2 =
let partial lst1 = match lst1 with
| Nil -> Nil
| xs -> map (fun x -> f x) xs (* use map to partially apply f to each item of l1 *)
in let rec final combinedlst = match combinedlst with
| Nil -> Nil
| Cons ((prtl, x), pxs) -> Cons (prtl x, final pxs) (* now finish applying the partially applied f with each item (x) of l2 *)
in final (combine (partial l1) (l2))

let map2' f l1 l2 =
fold_right (fun x1 acc -> (fun l2' -> begin
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