On Wednesday 04 July 2007, Andrew Coppin wrote:
Well, I eventually got it to work correctly... (!)
My goal is to be able to stack multiple parsers one on top of the other - but be able to *change* the stack half way through parsing if needed. This I eventually succeeded in doing. The external interface is fairly simple, but the type signatures are NOT. (!!)
My basic idea was to abstract the data source that a parser gets its data from:
class Source s where empty :: s x -> Bool fetch :: s x -> (x, s x)
instance Source [] where -- Nice syntax... :-S empty = null fetch xs = (head xs, tail xs)
Now I can define a parser type. But... uh... there's a slight glitch. What I *want* to say is
Parser state in out = ...
But what I ended up with is
newtype Parser state src x y = Parser ((state, src x) -> (state, src x, y))
<snip>
One problem remains... That pesky source type. Every time I mention a parser, I have to say what kind of course object it reads from - even though all parsers work with *any* source object! (That's the whole point of the Source class.) I really want to get rid of this. (See, for example, the type signature for "stacked". Yuck!) Also, every time I write a simple parser, I get a compile-time error saying something about a "monomorphism restriction" or something... If I add an explicit type it goes away, but it's very annoying to keep typing things like
test7 :: (Source src) => Parser state src Int Int
and so forth. And I can't help thinking if I could just get *rid* of that stupid source type in the signature, there wouldn't be a problem...
Anybody have a solution to this?
newtype Parser state x y = Parser (forall src. Source src => (state, src x) -> (state, src x, y)) Definition of monad functions, etc, works exactly as for your version, but this way all your parsers have polymorphic implementation types, but none has a type that trips the monomorphism restriction. There's some kind of argument here in the debate about the monomorphism restriction, but I'm not sure if it's for or against . . . [1] Jonathan Cast http://sourceforge.net/projects/fid-core http://sourceforge.net/projects/fid-emacs [1] http://www.lysator.liu.se/c/duffs-device.html