When making a fast parser, you want to avoid allocation as much as possible. That means you never want to mess with the source string, only keep track of an offset into that string.

You use isSubString like this:

isSubString "let" offset row col "let x = 4 in x"
    --==> ( newOffset, newRow, newCol )

You are looking for "let" at a given offset. On failure, the newOffset is -1. On success, the newOffset is the new offset. With our "let" example, it would be offset + 3.

You also provide the current row and col which do not align with offset in a clean way. For example, when you see a \n you are at row = row + 1 and col = 1. Furthermore, some UTF16 characters are two words wide, so even if there are no newlines, offset and col may not be equal.

Again, when parsing, you want to allocate as little as possible. So this function lets you say:

isSubChar isSpace offset "this is the source string"
    --==> newOffset

The (Char -> Bool) argument is called a predicate. The newOffset value can be a few different things:

• -1 means that the predicate failed
• -2 means the predicate succeeded with a \n
• otherwise you will get offset + 1 or offset + 2 depending on whether the UTF16 character is one or two words wide.

It is better to use union types in general, but it is worth the danger within parsing libraries to get the benefit outside.

So you can write a chomp function like this:

chomp : (Char -> Bool) -> Int -> Int -> Int -> String -> (Int, Int, Int)
chomp isGood offset row col source =
  let
    newOffset =
      Prim.isSubChar isGood offset source
  in
    -- no match
    if newOffset == -1 then
      (offset, row, col)


    -- newline match
    else if newOffset == -2 then
      chomp isGood (offset + 1) (row + 1) 1 source


    -- normal match
    else
      chomp isGood newOffset row (col + 1) source

Notice that chomp can be tail-call optimized, so this turns into a while loop under the hood.

Find a substring after a given offset.

findSubString before "42" offset row col "Is 42 the answer?"
    --==> (newOffset, newRow, newCol)

If offset = 0 and before = True we would get (3, 1, 4) If offset = 0 and before = False we would get (5, 1, 6)

If offset = 7 we would get (-1, 1, 18)