Re: [Haskell-cafe] Playing with OverloadedLabels in GHC 8 RC2, how to do this?

23 Feb 2016

      Hi,

The type of `fromLabel` is

    forall (x :: Symbol) a . IsLabel x a => Proxy# x -> a

where `x` represents the text of the label, so rather than applying it to

    (proxy# :: (Proxy# (Person -> String)))

you need to apply it to

    (proxy# :: Proxy# symbol)

and you will need to turn on the ScopedTypeVariables extension (so that
`symbol` refers to the variable bound in the class instance). With that
change, your program works.

That's a truly atrocious error message though. It's marginally better if
you enable -fprint-explicit-kinds:

    • Expected kind ‘Proxy# GHC.Types.Symbol ((->) Person String)’,
        but ‘proxy# :: Proxy# (Person -> String)’ has kind
          ‘Proxy# * (Person -> String)’

This shows the real problem, namely that you have `Proxy# *` instead of
`Proxy# Symbol`. However, `Proxy# Symbol ((->) Person String)` is
blatantly ill-kinded, so the error message doesn't make much sense. I
suggest you file a GHC ticket, if there isn't a suitable one already.

Hope this helps,

Adam

On 23/02/16 08:29, Daniel Díaz wrote:
...
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?
-- 
Adam Gundry, Haskell Consultant
Well-Typed LLP, http://www.well-typed.com/