Hi Daniel,
One way to get around your problem is to make a list of all paths through
the given records, then filter that list to contain only the fields that
match with the last label and result type:
https://gist.github.com/aavogt/c206c45362ed2115f392
I'm not sure that "filtering by result type" is a good idea, at least at
this point, because it doesn't work well when you have type variables in
the record or result type. Part of the problem seems to be that
Data.Type.Equality.== gets stuck when it sees type variables: I think what
is really needed in that case is a way to ask if (a ~ b) would be a type
error.
Regards,
Adam
On Wed, Feb 24, 2016 at 7:12 PM, 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...@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
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