[Haskell-cafe] Hint and Ambiguous Type issue

[Joseph Fredette]

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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