On Fri, Feb 21, 2003 at 12:14:04AM -0500, Mike T. Machenry wrote: > Hmm, that does seem like alot of code to say such a little thing. Is it > possible to come at the problem from the other direction? By this I mean I > am trying to have two sets of symbols be enumerated together. This solution > I asked for tries to impose the enumeration over the data. Can I define > > data Player = Red | Green | Blue | MrX deriving (Enum) > > and then use type classes or something else to say that MrX is a fugitive and > the others are detectives such that I can pattern match on them? I have > functions that recur over the Player argument and terminate at MrX, but I also > want to be able to formally say that some functions cannot take a player that > is Mrx, or can only take Mrx.
I think type classes would be overkill here. What's wrong with
isDetective :: Player -> Bool
isDetective MrX = False
isDetective _ = True
? (The pattern matching doesn't work quite the same way, but you can
use guards to acheive the same effect, especially with ghc's pattern
guards extension.)
Best,
Dylan Thurston
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