#5499: Tagging constructors with record/product phantom type
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Reporter: basvandijk | Owner:
Type: feature request | Status: new
Priority: normal | Component: libraries (other)
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by dreixel):
I see the advantage in doing this statically. But I would hope this can
already be done. And indeed it seems like it can, unfortunately not with
type families, but with functional dependencies...:
{{{
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE DeriveGeneric #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE OverlappingInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FunctionalDependencies #-}
module Test where
import GHC.Generics
data Pair a = Pair a a deriving Generic
data Sum a b = SL { recm :: a } | SR b deriving Generic
data True
data False
type family IsRec (f :: * -> *) :: *
type instance IsRec (M1 D c f) = IsRec f
-- Simplified: we should instead use a type-level Or
type instance IsRec (f :+: g) = IsRec f
type instance IsRec (M1 C c f) = IsRec f
type instance IsRec (f :*: g) = IsRec f
type instance IsRec (M1 S NoSelector f) = False
-- Unfortunately we cannot give the following type instance, as they
overlap
--type instance IsRec (M1 S c f) = IsRec f
-- With fundeps it works, though...
class IsRecord (f :: * -> *) b | f -> b
instance (IsRecord f b) => IsRecord (M1 D c f) b
-- Simplified: we should instead use a type-level Or
instance (IsRecord f b) => IsRecord (f :+: g) b
instance (IsRecord f b) => IsRecord (M1 C c f) b
-- For products we don't need an Or, either branch will do
instance (IsRecord f b) => IsRecord (f :*: g) b
instance IsRecord (M1 S NoSelector f) False
-- We cannot set b directly to True here...
instance (IsRecord f b) => IsRecord (M1 S c f) b
-- ... but we can do it in what comes after the M1 S, namely the K1:
instance IsRecord (K1 i c) True
-- Testing if a type uses record syntax, statically
class Test a where test :: a -> Bool
instance (IsRecord (Rep (Sum x y)) True ) => Test (Sum x y) where
test _ = True
instance (IsRecord (Rep (Pair x) ) False) => Test (Pair x) where
test _ = False
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5499#comment:1>
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