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 :

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 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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I've done something similar recently for an emulator I'm writing. It essentially turns GHCI into a debugger for free: newtype Run = Run (forall a. Emu a -> IO a) start :: FilePath -> IO Run breakpoint :: Word16 -> Emu () readMem :: Word16 -> Emu Word8 step :: Emu () Run emu <- start "foo.rom" emu $ breakpoint 0x4567 emu $ readMem 0xdead emu step etc On Mon, Jun 15, 2015 at 4:57 PM, Paolino wrote:

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 :

...

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 :

...

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 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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