How to get the first index of a given element in Array/List in Elm?

Question

In Javascript, one could use indexOf to get the first instance of a given element in an array.

What's the best way to do this in Elm?

array = Array.fromList ["Stack","Overflow","is","","awesome"]
element = ""

indexOf element array =
  -- should return 3

What if we have a list instead of an array?

Answer 1

There is a third party module elm-list-extra that provides this function. "elemIndex" (works for 0.19).

And this one for < 0.19 elemIndex

Answer 2

Just for fun, here's a hand-rolled version for List:

helper : List a -> a -> Int -> Int
helper lst elem offset = 
  case lst of
    []      -> -1
    x :: xs ->
      if x == elem then offset
      else helper xs elem (offset + 1)

indexOf lst element =
  helper lst element 0

helper recursively searches the list for the element, keeping track of the number of elements we've visited so far. For the empty list, it returns -1. If you don't like magic constants, you could easily change the return type to Maybe Int and return Nothing instead.

And here's a complete program using it (just copy + paste into the online REPL at elm-lang.org:

import Html exposing (text)

lst = ["Stack","Overflow","is","","awesome"]
element = ""

helper : List a -> a -> Int -> Int
helper lst elem offset = 
  case lst of
    []      -> -1
    x :: xs ->
      if x == elem then offset
      else helper xs elem (offset + 1)

indexOf lst element =
  helper lst element 0

main =
  text (toString (indexOf lst ""))