Am Donnerstag, 5. März 2009 19:48 schrieb Joseph Fredette:
getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter a) getFilterMain fMainLoc = do loadModules [fMainLoc]; setTopLevelModules [(takeWhile (/='.') fMainLoc)] fMain <- (interpret "(filterMain)" (as :: Deliverable a => Filter a))
Without looking at more code, the type variable a here is a fresh type variable, not the one from getFilterMain's signature.
return (fMain)
Maybe bringing the type variable a into scope in the function body by {-# LANGUAGE ScopedTypeVariables #-} getFilterMain :: forall a. Deliverable a => FilePath -> Interpreter (Filter, a) would suffice.
However, when I try to compile this, I get the type error:
Hackmain.hs:70:43: Ambiguous type variable `a' in the constraint: `Deliverable a' arising from a use of `getFilterMainStuff' at Hackmain.hs:70:43-60 Probable fix: add a type signature that fixes these type variable(s)
My understanding is that a type like "Foo a => Bar a" (where Foo is a class and Bar is a datatype) would simply restrict the values of a to only those implementing Foo. But evidently I'm wrong. Is there a good (read, easy... :) ) fix to this?
Any help would be greatly appreciated.
/Joe
PS. All the actual code is on patch-tag, here http://patch-tag.com/repo/Hackmail/home -- if anyone prefers to look at that directly, the relevant files are in Src, namely, Hackmain.hs, Filter.hs, and Deliverable.hs