Type Family Shenanigans in GHCi

问题

Here's my code:

{-# LANGUAGE TypeFamilies, TypeOperators, DataKinds, PolyKinds, FlexibleContexts, UndecidableInstances #-}

module Foo where

import Data.Singletons.Prelude
import Data.Type.Equality

data TP a b

-- foldl (\(bool, r) x -> (bool && (r == x), r)) (True, head xs) xs
type family Same (b :: Bool) (r :: k) (rq :: [k]) :: k where
  Same bool r (x ': xs) = Same (bool :&& (r == x)) r xs
  Same bool r '[] = TP bool r

data NotEqualFailure
-- converts a True result to a type
type family EqResult tp where
  EqResult (TP 'True r) = r
  EqResult (TP 'False r) = NotEqualFailure



data Zq q i
type family Foo rq
type instance Foo (Zq q i) = i
type R = 
  EqResult 
    (Same 'True (Foo (Zq Double Int)) 
      (Map (TyCon1 Foo) 
        '[Zq Double Int, Zq Float Int]))

f :: R
f = 3

This doesn't compile in GHC 7.8.3:

No instance for (Num NotEqualFailure)
      arising from a use of ‘f’
    In the expression: f
    In an equation for ‘it’: it = f

but if I comment out f and start GHCi:

> :kind! R
R :: *
= Int

So GHC can't seem to decide if the elements in my list are equal or not. If they are equal, EqResult should return the common type (Int here), otherwise it returns NotEqualFailure. Can anyone explain what's going on here? Let me know if you also think this looks like a bug, and I'll file it on the GHC trac.

If you can figure out a way to write "EqResult (Same ...)" in a different way, it might get around this issue.

EDIT I rewrote the type family, but unfortunately I'm having essentially the same problem. The code is smaller and simpler now.

{-# LANGUAGE TypeFamilies, TypeOperators, DataKinds, FlexibleContexts, UndecidableInstances #-}

module Foo where

import Data.Singletons.Prelude
import Data.Singletons.Prelude.List
import Data.Type.Equality

-- foldl (\(bool, r) x -> (bool && (r == x), r)) (True, head xs) xs
type family Same rq where
  Same (x ': xs) = 
    EqResult (And (Map ((TyCon2 (==)) $ x) xs)) x

data NotEqualFailure
-- converts a True result to a type
type family EqResult b v where
  EqResult 'True r = r
  EqResult 'False r = NotEqualFailure

type R = Same '[Int, Int]

f :: R
f = 3

GHCi can still resolve R to Int, but GHC can't resolve the type family for EqResult at all now (before it incorrectly resolved it to NotEqualFailure). Note that this example works if I change the size of the list to one, i.e. '[Int].

EDIT 2 I removed all external libraries, and got everything to work. This solution is probably the smallest, but I still want to know why the larger examples appear to break GHC.

{-# LANGUAGE TypeFamilies, DataKinds,
             UndecidableInstances #-}

module Foo where

type family Same (rq :: [*]) :: * where
  Same (x ': xs) = EqTo x xs

--tests if x==y for all x\in xs
type family EqTo y xs where
  EqTo y '[] = y
  EqTo y (y ': xs) = EqTo y xs
  EqTo y (x ': xs) = NotEqualFailure

data NotEqualFailure

type R = Same '[Int, Int]
f :: R
f = 3

回答1:

There is in fact a bug, and it will be fixed in the next release of GHC.

来源：https://stackoverflow.com/questions/27490352/type-family-shenanigans-in-ghci

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