Add replacement for String.repeat

[jfmengels]

This is a replacement for String.repeat

Original

repeat : Int -> String -> String
repeat n chunk =
    repeatHelp n chunk ""

repeatHelp : Int -> String -> String -> String
repeatHelp n chunk result =
    if n <= 0 then
        result

    else
        repeatHelp (Bitwise.shiftRightBy 1 n) (chunk ++ chunk) <|
            if Bitwise.and n 1 == 0 then
                result

            else
                result ++ chunk

Replacement

The replacement is equivalent to the following Elm code:

repeat : Int -> String -> String
repeat n chunk =
    repeatHelp n chunk ""

repeatHelp : Int -> String -> String -> String
repeatHelp n chunk result =
    if n <= 0 then
        result

    else
        repeatHelp (Bitwise.shiftRightBy 1 n)
            (chunk ++ chunk ++ "")
            (if Bitwise.and n 1 == 0 then
                result

             else
                result ++ chunk ++ ""
            )

The main improvements are:

• the function is made tailcall recursive (by removing the <|)
• ++ "" has been added when concatenating strings, forcing the compiler to use + instead of the slower _Utils_ap. This was the biggest gain in performance.

For this PR, I have slightly altered the replacement to

• not have a repeatHelp function (its replacement is empty to remove it) by inlining repeatHelp inside repeat
• not have the unnecessary + '' (that could be removed by the compiler IMO)
• remove the unnecessary temporary variables

These don't seem to have a noticeable effect as far as I can tell, but it reduces the bundle size ever so slightly maybe. Let me know if you want me to use the "original version".

Results

Benchmark is run with elm-optimize-level-2.

Chrome:

Firefox:

Notes

String replacements seem to be disabled in all configurations for the tool and I'm not sure why. That also needs to change otherwise this replacement will not have any effect.