Add List.Extra.insertAt

[Janiczek]

insertAt 3 "X" ["a","b","c","d","e"]
---------------0---1---2---^3
-->            ["a","b","c","X","d","e"]

The behaviour with out-of-bounds indexes mirrors Array.Extra.insertAt.

Benchmarks:

I chose the splitAt approach.

• intended "happy" case: the splitAt approach is 2x faster
• edge cases:
  • negative index: All approaches do the same thing - any differences are basically a measurement error
  • large positive index: the takeDrop approach is faster, but this is still an edge case so I wouldn't give this too much weight

[lue-bird]

Cool!
When I change the recursive definition to use List.foldl (::) instead of List.reverse ++ I get these results:

with similar results in firefox.
(I also increased the benchmarked list to 100 elements and moved the let helper to the top level but the foldl is the biggest influence)

[lue-bird]

Also, here's a nice test to add to verify the final order is correct:

fuzz2
    (Fuzz.intRange 0 4)
    (Fuzz.listOfLengthBetween 4 10 (Fuzz.intRange 0 10))
    "index in bounds (0 <= index <= length) inserts in the right place in the list keeping the elements in the right order"
  <|
    \goodIndex list ->
        List.Extra.insertAt goodIndex -1 list
            |> List.Extra.removeAt goodIndex
            |> Expect.equal list

[Janiczek]

Also, here's a nice test

ah yes, that one's much better than mine, thanks!

I've incorporated your notes in 587293c. Here's the benchmark on a second machine:

[lue-bird]

Oh I forgot to mention that in my benchmarks this instead counts down from index and checks when it goes to 0. This avoids the extra i argument but should not be that much faster (not sure if comparisons with a literal are only optimized in eol2 or elm make)

[lydell]

It’s an Elm compiler optimization – comparisons with literals are faster.

https://github.com/elm/compiler/blob/cce7a8bbd8fe690fc83fa795f8d7e02505d1f25f/compiler/src/Generate/JavaScript/Expression.hs#L550-L555

[Janiczek]

@lue-bird 2x speedup for such a small change, that's awesome!