match combinator

We should have the super-useful match combinator.

To write this combinator for Text.Parsing.Parser.String, I think we will need some more members of StringLike.

purescript-parsing/src/Text/Parsing/Parser/String.purs

Lines 21 to 25 in e801a0e

    
           class StringLike s where 
        
             drop :: Int -> s -> s 
        
             indexOf :: Pattern -> s -> Maybe Int 
        
             null :: s -> Boolean 
        
             uncons :: s -> Maybe { head :: Char, tail :: s }

Maybe if we have

length :: s -> Int
length = Data.String.length

take :: Int -> s -> s
take = Data.String.take

Then we can write

match :: forall m s a.  ParserT s m a -> ParserT s m (Tuple a s)
match p = do
  ParseState input1 _ _ <- get
  x <- p
  ParseState input2 _ _ <- get
  pure $ Tuple x $ take (length input1 - length input2) input1

Actually we should use the Javascript length, which is in units of “code units”, instead of Data.String.length. which is in units of “code points”.

And likewise for take.

This would be more efficient, and it will be correct as long as we can assume that the ParseState input always begins on a code point boundary.

Here's an implementation of match which works on String instead of StringLike.

https://pursuit.purescript.org/packages/purescript-parsing-replace/1.0.2/docs/Text.Parsing.Replace.String.Combinator#v:match

Done in v7.0.0

	class StringLike s where
	drop :: Int -> s -> s
	indexOf :: Pattern -> s -> Maybe Int
	null :: s -> Boolean
	uncons :: s -> Maybe { head :: Char, tail :: s }