[Haskell-cafe] returning a polymorphic function

[Paolino]

It worked without adding JSON constraint in the newtype.
I can do whatever I want with 'a'.

A simplified example where the database module is not polluted with future
use of the query results.

http://lpaste.net/134541#a134548

 I tried refactoring common code ...... not easy

Thanks again

paolino

2015-06-15 16:26 GMT+02:00 Paolino <paolo.veronelli at gmail.com>:

> 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
>>>
>>> _______________________________________________
>>> Haskell-Cafe mailing list
>>> Haskell-Cafe at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20150615/40102708/attachment.html>