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

[Adam Gundry]

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/