[Haskell-cafe] returning a polymorphic function

[Paolino]

Thanks, I'd never have guessed.
I have a GADT Get a where I define a protocol to query a database where 'a'
is the return type a.
Then prepare :: IO (Get a -> ErrorAndWriterMonad a) is opening the database
and return the query function.
I suspect I have to insert the constraint in the wrapper to let transform
'a' to JSON or String, which I don't like. I'll have to dig it.

Regards

paolino


2015-06-15 16:18 GMT+02:00 Patrick Chilton <chpatrick at gmail.com>:

> Yes, but you have to wrap it up:
>
> {-# LANGUAGE RankNTypes #-}
>
> -- obviously not to useful for this example :)
> newtype Id = Id (forall a. a -> a)
>
> f :: IO Id
> f = return (Id id)
>
> main = do
>   Id g <- f
>   print $ g 1
>   print $ g "ciao"
>
> Can I ask what you're trying to do with this?
>
> Patrick
>
> On Mon, Jun 15, 2015 at 3:11 PM, Paolino <paolo.veronelli at gmail.com>
> wrote:
>
>> Hello list, I'm trying to accomplish something along this line
>>
>> {-# LANGUAGE ScopedTypeVariables #-}
>>
>> f :: IO (a -> a)
>> f = return id
>>
>> main = do
>>   (g :: a -> a) <- f
>>   print $ g 1
>>   print $ g "ciao"
>>
>>
>>
>> Is it possible or I have to call f more than once to get different g's
>>
>>
>> Thanks
>>
>> paolino
>>
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>>
>>
>
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