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

[Adam Gundry]

On 25/02/16 00:12, Daniel Díaz wrote:
> 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.

Sadly I think transitivity is going to be hard to achieve, without
endless overlapping instance problems, because it's not clear how to
make type inference determine the "in-between" type.

> 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.

It took me a while to understand that the overlap is actually between
the instance being defined, and one of its own superclasses. You can
resolve it by giving a type signature to one of the `fromLabel`
occurrences, thereby fixing the intermediate variable. But I've not been
able to get much further...

All the best,

Adam


> 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
>     <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/