This doesn't seem to do it, same type error... Maybe I need to use some
kind of witness type -- to inform the compiler
of the type of a @ runtime?
Daniel Fischer wrote:
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
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