[Haskell-cafe] Hint and Ambiguous Type issue

[Ryan Ingram]

So, by using the Haskell interpreter, you're using the
not-very-well-supported dynamically-typed subset of Haskell.  You can
tell this from the type signature of "interpret":

> interpret :: Typeable a => String -> a -> Interpreter a

> as :: Typeable a => a
> as = undefined

(from http://hackage.haskell.org/packages/archive/hint/0.2.1/doc/html/src/Language-Haskell-Interpreter-GHC.html)

In particular, the "as" argument to interpret is specifying what type
you want the interpreted result to be typechecked against; the
interpretation fails if it doesn't match that type.  But you need the
result type to be an instance of Typeable; (forall a. Deliverable a =>
Filter a) most certainly is not.


Off the top of my head, you have a couple of directions you can take this.

(1) Make Typeable a superclass of Deliverable, saying that all
deliverable things must be dynamically typeable.  Then derive Typeable
on Filter, and have the result be of type "Filter a" using
ScopedTypeVariables as suggested before. (You can also pass "infer" to
the interpreter and let the compiler try to figure out the result type
instead of passing (as :: SomeType).)

(2) Make a newtype wrapper around Filter and give it an instance of
Typeable, and add a constraint to filterMain that the result type in
the filter is also typeable.  Then unwrap the newtype after the
interpreter completes.

Good luck; I've never tried to use the Haskell interpreter before, so
I'm curious how well it works and what problems you have with it!


  -- ryan

2009/3/5 Joseph Fredette <jfredett at gmail.com>:
> 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
>
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>
>