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@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@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@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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