I've been working on a little project, and one of the things I need to
do is dynamically compile and import a Haskell Source file containing
filtering definitions. I've written a small monad called Filter which is
simply:
type Filter a = Reader (Config, Email) a
To encompass all the email filtering. The method I need to import,
filterMain, has type:
filterMain :: Deliverable a => Filter a
where Deliverable is a type class which abstracts over delivery to a
path in the file system. The notion is that I can write a type like:
data DEmail = {email :: Email, path :: FilePath}
newtype Maildir = MD DEmail
instance Deliverable Maildir where
{- ... omitted -}
However, Filter a should not be restricted to Deliverable types- it also
encompasses the results of regular expression matching, etc, which are
not -- in general -- Deliverable instances.
My question is this, when importing the file containing the definitions
of filterMain, I have the following code to grab filterMain and return
it as a function.
getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter
a)
getFilterMain fMainLoc =
do
loadModules [fMainLoc]; setTopLevelModules [(takeWhile
(/='.') fMainLoc)]
fMain <- (interpret "(filterMain)" (as :: Deliverable a =>
Filter a))
return
(fMain)
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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email;internet:[email protected]
tel;home:1-508-966-9889
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