[Haskell-cafe] hint and type synonyms

[Claude Heiland-Allen]

Hi Daniel, cafe,

On 31/03/12 17:47, Daniel Gorín wrote:
> Could you provide a short example of the code you'd like to write but gives you problems? I'm not able to infer it from your workaround alone...

This problem originally came up on #haskell, where Rc43 had a problem 
making a library with a common module that re-exports several other modules:

http://hpaste.org/66281

My personal interest is somewhat secondary, having not yet used hint in 
a real project, but code I would like to write at some point in the 
future is much like the 'failure' below, unrolled it looks like:

main = (print =<<) . runInterpreter $ do
   setImports ["Prelude"]
   interpret "1/5" (as :: Rational)
-- fails

Having to remember that Rational is defined as type Rational = Ratio 
Integer and that Ratio is defined in Data.Ratio and then to add that to 
the setImports list is a little inconvenient:

main = (print =<<) . runInterpreter $ do
   setImports ["Prelude", "Data.Ratio" ]
   interpret "1/5" (as :: Rational)
-- works

But for my own purposes this is probably much saner in the long run than 
my newtype wrapping approach below.

However, this is not always possible: supposing Ratio itself was defined 
as a type synonym of Ratio2, and Ratio2 was not exported.  Perhaps this 
is what Rc43 was experiencing, but I shouldn't speculate, as this is all 
getting a bit theoretical - I should try out hint in the real world to 
see if this problem makes things impractical for me - sorry for the noise!

Thanks,


Claude

> Thanks,
> Daniel
>
>
> On Mar 31, 2012, at 6:19 PM, Claude Heiland-Allen wrote:
>
>> Hi all,
>>
>> What's the recommended way to get hint[0] to play nice with type synonyms[1]?
>>
>> A problem occurs with in scope type synonyms involving types not in scope.
>>
>> I came up with this after looking at the source[2], but it makes me feel ill:
>>
>> --8<--
>> -- hint and type synonyms don't play nice
>> module Main where
>>
>> import Language.Haskell.Interpreter
>>
>> import Data.Typeable as T
>> import Data.Typeable.Internal
>> import GHC.Fingerprint.Type
>>
>> main = failure>>  success
>>
>> test t = (print =<<) . runInterpreter $ do
>>   setImports [ "Prelude" ]
>>   interpret "1/5" t
>>
>> failure = test (as :: Rational)
>> -- Left (WontCompile [GhcError {errMsg = "Not in scope: type constructor or class `Ratio'"}])
>>
>> success = test (as :: Q)
>> -- Right (1 % 5)
>>
>> newtype Q = Q Rational
>>
>> instance Show Q where
>>   show (Q a) = show a
>>   showsPrec n (Q a) = showsPrec n a
>>
>> instance Typeable Q where
>>   typeOf _ = TypeRep (Fingerprint 0 0) (T.mkTyCon "Rational") []
>> --8<--
>>
>> Thanks,
>>
>>
>> Claude
>>
>> [0] http://hackage.haskell.org/package/hint
>> [1] http://www.haskell.org/onlinereport/decls.html#type-synonym-decls
>> [2] http://hackage.haskell.org/packages/archive/hint/0.3.3.4/doc/html/src/Hint-Eval.html#interpret
>>
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>