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What's an elegant way to write the classic take function in CameLIGO? take takes the first num elements of a list.

This is how I'm writing it currently, and it's ugly. Here, message is a type I defined:

let rcons((xs, x): message list * message): message list =
  x :: xs

let reverse (xs: message list): message list =
  List.fold rcons xs ([]: message list)

let takeReverse ((num, xs): nat * message list): message list =
  let accumulate ((taken, x): message list * message) =
    if List.length taken < num then
      x :: taken
    else
      taken
    in
  List.fold accumulate xs ([]: message list)

// This is such an ugly construction.
let take ((num, xs): nat * message list): message list =
  let reversed = takeReverse (num, xs) in
  reverse reversed

Basically, to write take I'm reversing the list twice. I could write it more succinctly if I had non-tail recursion, but alas that's not offered by LIGO.

Thanks!

2

Accumulating the results to a linked list, and reversing the result would be more concise (and efficient, because your implementation traverses the entire input list, while this only looks at the prefix):

let take (count: int) (input: message list): message list =
  let rec go (acc, n, xs: message list * int * message list): message list =
    if n <= 0
    then acc
    else match xs with
         | [] -> acc
         | y::ys -> go (y::acc, n-1, ys) in
  reverse (go (([]: message list), count, input))

It's essentially same as the recursive implementation, where the accumulator is the stack and the last reverse is unrolling the stack inside-out.

1
  • Thanks! This is indeed nicer, and it's also good to know I can destructure lists inside match clauses. – emchristiansen Mar 5 at 18:22

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