Andrew Wagner wrote:
Wadler posted a blog entry the other day about a paper on pattern-matching in Haskell (http://wadler.blogspot.com/). I've taken a first stab at turning it into actual code for hackage (http://hpaste.org/13215). There are two commented-out definitions that don't type-check, though, and the types are too wild for me to grok. Anybody have any suggestions for 1.) How to fix it and/or 2.) How to use data/type/newtype to simplify the types and make it more manageable? Thanks! Both errors are because you are using "any" instead of "any'"; you might wish to put import Prelude hiding any at the top of your code, just to avoid such confusion.
To make the types more readable (but not necessarily more manageable), I have made some changes to my version of this code. For example, instead of () as the "empty stack", I use data Void void = undefined :: Void in the definition of 'zero' (and thus also in p .>. k). I also use data FUnit = FUnit -- Unit just for fail Lastly, instead of having a matcher be a pair (which obscures the use of pairs as a stack in other places, as well as matching on pairs), I defined data Matcher a b = Matcher a b and use that everywhere. This all makes the types larger to type, but at least there is a cleaner separation of concerns, which makes the errors easier to figure out. The principal weakness of these pattern-matching combinators is that there is no support for algebraic types, i.e. things like data Tree a = Leaf | Branch (Tree a) (Tree a) I can see how to use Typeable to deal with that, but is there a simpler way? Jacques