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/