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

[Daniel Díaz]

I wonder if is there is some way of making the labels "transitive". For 
example, if I have a record embedded in another record, it would be nice if 
the labels of the embedded record worked for the enclosing record as well, 
assuming there's no ambiguity.

Here's an attempt. Consider this "strengthened" version of IsLabel that 
uses functional dependencies. Only certain fields will be able to have 
instances:

class IsLabel symbol (a -> b) => IsUnambiguousLabel symbol a b | symbol -> 
a b, a b -> symbol, symbol a -> b, symbol b -> a


and then this

instance (IsUnambiguousLabel symbol1 a b, IsUnambiguousLabel symbol2 b c) 
=> IsLabel symbol2 (a -> c) where
    fromLabel _ = fromLabel (proxy# :: (Proxy# symbol2)) . fromLabel 
(proxy# :: (Proxy# symbol1))


But it doesn't work. GHC complains angrily about overlapping instances.

On Tuesday, February 23, 2016 at 10:16:21 AM UTC+1, Adam Gundry wrote:
>
> 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/ 
> _______________________________________________ 
> Haskell-Cafe mailing list 
> Haskel... at haskell.org <javascript:> 
> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe 
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20160224/71fa4998/attachment-0001.html>