It works, thanks! I was wondering: if I define a bunch of records in a module, how to make this the behaviour for all records in the module, without much boilerplate and without affecting any records elsewhere? One possible solution would be to define a empty type class that will not be exported: class Marker r and the following instance: instance (Marker r,IsLabel symbol (r -> String)) => IsLabel symbol (r -> Text) where fromLabel _ = Text . fromLabel (proxy# :: (Proxy# symbol)) And make every record in the module an instance of Marker: instance Marker Person I'm not sure if there's a simpler way. Even if we don't export the fields directly, another way to employ OverloadedLabels (OverloadedRecordFields, once it arrives) is for giving default implementations of public interfaces, in combination with DefaultSignatures. A not very useful example: class Named r where name :: r -> String default name :: IsLabel "name" (r -> String) => r -> String name = #name instance Named Person 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...@haskell.org javascript: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe