Combinators should be more strict

[tibbe]

I found and fixed lots of small space leaks in cassava but rewriting several of the combinators like so:

-- | Specialized version of 'sepBy1'' which is faster due to not
-- accepting an arbitrary separator.
sepByDelim1' :: AL.Parser a
             -> Word8  -- ^ Field delimiter
             -> AL.Parser [a]
sepByDelim1' p !delim = liftM2' (:) p loop
  where
    loop = do
        mb <- A.peekWord8
        case mb of
            Just b | b == delim -> liftM2' (:) (A.anyWord8 *> p) loop
            _                   -> pure []
{-# INLINE sepByDelim1' #-}

(Ignore the use of peekWord8 instead of <|>, that's an optimization specific to my case.)

Note the use of liftM2' instead of (:) <$> (A.anyWord8 *> p) <*> loop. Without liftM2' we're building up a thunk in the list. This thunk will always be evaluated when we eventually run the parser so creating it here is unnecessary and wastes space. I would guess that (almost) all uses of <$> and <*> should be replaced by strict versions.

[bgamari]

One notable example of this is in the Alternative instance for Parser, which defines some with <$>. I don't see too many others though.