The incremental-parser library is yet another parser combinator library, providing the usual set of Applicative,
Alternative, and Monad combinators. Apart from this, it has four twists that make it unique.
Parsing incrementally
First, the parser is incremental. Not only can it be fed its input in chunks, but in proper circumstances it can also
provide its output in parsed chunks. For this to be possible the result type must be a Monoid. The complete parsing
result is then a concatenation of the partial results.
In order to make the incremental parsing easier, the combinator set is optimized for monoidal results. Apart from the
usual combinators many and some, for example, there are concatMany and concatSome operators.
many :: Parser s r -> Parser s [r]
concatMany :: (Monoid s, Monoid r) => Parser s r -> Parser s r
Arbitrary monoidal inputs
The second weirdness, this one shared with Picoparsec, is that the the
parser is generic in its input stream type, but this type is parameterized in a holistic way. There is no separate token
type. Primitive parsers that need to peek into the input require its type to be an instance of a monoid subclass, from
the monoid-subclasses package.
In Parsec:
string :: Stream s m Char => String -> ParsecT s u m String
char :: Stream s m Char => Char -> ParsecT s u m Char
anyToken :: (Stream s m t, Show t) => ParsecT s u m t
In Attoparsec:
string :: ByteString -> Parser ByteString
word8 :: Word8 -> Parser Word8
anyWord8 :: Parser Word8
In incremental-parser and Picoparsec:
string :: (LeftCancellativeMonoid s, MonoidNull s) => s -> Parser s s
token :: (Eq s, FactorialMonoid s) => s -> Parser s s
anyToken :: FactorialMonoid s => Parser s s
Two Alternative alternatives
The library being implemented on the basis of Brzozowski derivatives, it can provide both the symmetric and the
left-biased choice, <||> and <<|>. This is the same design choice made by
Text.ParserCombinators.ReadP and
uu-parsinglib. Parsec and its progeny on the other hand provide only
the faster left-biased choice, at some cost to the expressiveness of the combinator language. The standard <|>
operator from the Alternative class acts as one or the other of the above, depending on whether the first type
parameter of Parser is Symmetric or LeftBiasedLocal.
MonadFix and record
Finally, the parser is an instance of the MonadFix class. Beware of its power. In particular, never ever try to
mfix a strict function. This will hang. The argument of mfix takes the value constructed by the argument parser at
the very same input position it’s looking at. The best and probably the only safe and useful argument to mfix is the
record function. See the construct library for examples.
