[Haskell-cafe] Playing with OverloadedLabels in GHC 8 RC2, how to do this?
Daniel Díaz
diaz.carrete at gmail.com
Tue Feb 23 08:29:37 UTC 2016
Hi all,
I'm playing with the OverloadedLabels extension in GHC 8 RC2. I have been
able to define simple record accessors, like in this
gist: https://gist.github.com/danidiaz/3b9a6865686c777f328c
After realizing than with OverloadedLabels a single symbol can be used to
extract two different types from the same record, I tried to define an
instance that says: "if a symbol can be used to extract an string from my
record, then it can also be used to extract that a Text value".
Here's my attempt (using a dummy Text type):
{-# LANGUAGE OverloadedLabels #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE MagicHash #-}
module Main where
import GHC.OverloadedLabels
import GHC.Prim
newtype Text = Text { getText :: String } deriving Show
data Person = Person { _id :: Int , _name :: String }
instance IsLabel "name" (Person -> String) where
fromLabel _ = _name
instance IsLabel symbol (Person -> String) => IsLabel symbol (Person ->
Text) where
fromLabel _ = Text . fromLabel (proxy# :: (Proxy# (Person -> String)))
person :: Person
person = Person 123 "Horace"
main :: IO ()
main = do
print (#name person :: String)
print (#name person :: Text)
Bu this doesn't work. The error I get is puzzling:
• Expected kind ‘Proxy# ((->) Person String)’,
but ‘proxy# :: Proxy# (Person -> String)’ has kind ‘Proxy# (Person
-> String)’
• In the first argument of ‘fromLabel’, namely
‘(proxy# :: Proxy# (Person -> String))’
Is this a bug? What is going on here?
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