On Wed, 25 Feb 1998, Jerzy Karczmarczuk wrote:
: PS. Could somebody inform me what is the current status of
: multi-parametric classes? Concretely (for example) I would
: like to construct a generic Universal Parser
:
: type Uparser a c = [c] -> [(a,[c])]
:
: which consumes any stream and produces any result. The classical
: combinators do not care about [c] being character strings, but
: with two parameters it is not possible to apply directly the magic
: word Monad. Of course, my problem is not veeery dramatic, but
: those multiparametric classes bother me already some time.
Well, it is actually possible to solve with Haskell 1.4 without multi
parameter type classes.
Instead of making a specific parser data type, I've implemented a general
state transformer type, which can be directly used as a parser.
it has the type
> newtype StateM s m a = SM (s -> m (a,s))
> unStateM (SM f) = f
and takes three arguments, a state type, a container monad and the result
type.
a typical Monad instantiation looks like this
> instance Monad m => Monad (StateM s m) where
> (SM stm) >>= f = SM $ \s -> stm s >>= \(v,s') -> unStateM (f v) s'
> return v = SM $ \s -> return (v, s)
so your Uparser would be
> type Uparser a c = StateM c [] a
I have actually implemented the Hugs Parselib using these types.
If you are interested, you can mail me for the source.
n.
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