[Haskell-cafe] Hint and Ambiguous Type issue

[Daniel Gorín]

Hi

I've downloaded Hackmain from patch-tag, but I'm getting a different  
error. The error I get is:

Hackmain.hs:63:10:
     No instance for (Data.Typeable.Typeable2
                        Control.Monad.Reader.Reader)
       arising from a use of `interpret' at Hackmain.hs:63:10-67

Hint requires the interpreted values to be an instance of Typeable in  
order to check, in runtime, that the interpreted value matches the  
type declared at compile. Therefore, you need to make  sure that  
(Filter a) is indeed an instance of Typeable.

Since you have Filter a = Reader (Config, Email) a, you probably need to

- Derive Config and Email instances for Filter,

- Manually provide Typeable instances for Reader a b, something along  
the lines of:

instance (Typeable a, Typeable b) => Typeable (Reader a b) where...

(I don't know why this isn't done in the mtl....)

- Change the signature to:

getFilterMain :: (Typeable a, Deliverable a) => FilePath ->  
Interpreter (Filter a)

Also, you can try using "infer" instead of "as :: ...."

Hope that helps

Daniel

On Mar 5, 2009, at 8:47 PM, Joseph Fredette wrote:

> So, I tried both of those things, both each alone and together. No  
> dice. Same error, so I reverted back to the
> original.  :(
> However, I was, after some random type signature insertions, able to  
> convert the problem into a different one, via:
>
> getFilterMain :: Deliverable a => FilePath -> Interpreter (Filter  
> a)                          getFilterMain MainLoc = do
>       loadModules [fMainLoc]; setTopLevelModules [(takeWhile (/='.')  
> fMainLoc)]                               fMain  <- (interpret  
> "(filterMain)" infer)
>       return (fMain :: Deliverable a => Filter a)
>
>  Inferred type is less polymorphic than expected
>     Quantified type variable `a' is mentioned in the environment:
>       fMain :: Filter a (bound at Hackmain.hs:77:1)
>   In the first argument of `return', namely
>       `(fMain :: (Deliverable a) => Filter a)'
>   In the expression: return (fMain :: (Deliverable a) => Filter a)
>   In the expression:
>       do loadModules [fMainLoc]
>          setTopLevelModules [(takeWhile (/= '.') fMainLoc)]
>          fMain <- (interpret "(filterMain)" infer)
>          return (fMain :: (Deliverable a) => Filter a)
>                                                                                               I'm 
>  thinking that this might be more easily solved -- I do think I  
> understand the issue. somehow, I need to tell the compiler
> that the 'a' used in the return statement (return (fMain :: ...)) is  
> the same as the 'a' in the type sig for the whole function.
>
> While I ponder this, and hopefully receive some more help -- thanks  
> again Dan, Ryan -- Are there any other options besides Hint that  
> might -- at least in the short term -- make this easier? I'd really  
> like to finish this up. I'm _so_ close to getting it done.
>
> Thanks,
>
> /Joe
>
> Ryan Ingram wrote:
>> 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
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>>>
>>>
>>>
>>
>>
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