Write You a Haskell Stephen Diehl

instance Monad Parser where
return = unit
(>>=) = bind
Of particular importance is that this particular monad has a zero value (failure), namely the function
which halts reading the stream and returns the empty stream. Together this forms a monoidal structure
with a secondary operation (combine) which applies two parser functions over the same stream and
concatenates the result. Together these give rise to both the Alternative and MonadPlus class instances
which encode the logic for trying multiple parse functions over the same stream and handling failure
and rollover.
e core operator introduced here is the () operator for combining two optional paths of parser logic,
switching to the second path if the first fails with the zero value.
instance MonadPlus Parser where
mzero = failure
mplus = combine
instance Alternative Parser where
empty = mzero
() = option
combine :: Parser a -> Parser a -> Parser a
combine p q = Parser (\s -> parse p s ++ parse q s)
failure :: Parser a
failure = Parser (\cs -> [])
option :: Parser a -> Parser a -> Parser a
option p q = Parser $ \s ->
case parse p s of
[] -> parse q s
res -> res
Derived automatically from the Alternative typeclass definition are the many and some functions. Many
takes a single function argument and repeatedly applies it until the function fails and then yields the
collected results up to that point. e some function behaves similar except that it will fail itself if there
is not at least a single match.
-- | One or more.
some :: f a -> f [a]
some v = some_v
where
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