Opened 2 years ago

Closed 2 years ago

Last modified 2 years ago

#13774 closed bug (invalid)

Singletons code fails to typecheck when type signature involving type family is added

Reported by: RyanGlScott Owned by:
Priority: normal Milestone:
Component: Compiler (Type checker) Version: 8.0.1
Keywords: FunDeps, TypeFamilies Cc: goldfire
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: GHC rejects valid program Test Case:
Blocked By: Blocking:
Related Tickets: Differential Rev(s):
Wiki Page:


Yes, I know "singletons" is in the title... but the code isn't that scary, I promise. Here's some code which does typecheck:

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
module Bug where

data family Sing (a :: k)

data Nat = Zero | Succ Nat

data instance Sing (b :: Bool) where
  SFalse :: Sing 'False
  STrue  :: Sing 'True

data instance Sing (n :: Nat) where
  SZero :: Sing 'Zero
  SSucc :: Sing n -> Sing ('Succ n)

type family Not (x :: Bool) :: Bool where
  Not 'True = 'False
  Not 'False = 'True

sNot :: Sing b -> Sing (Not b)
sNot STrue = SFalse
sNot SFalse = STrue

class PFD a b | a -> b where
  type L2r (x :: a) :: b

instance PFD Bool Nat where
  type L2r 'False = 'Zero
  type L2r 'True = 'Succ 'Zero

type T2 = L2r 'False

class SFD a b | a -> b where
  sL2r :: forall (x :: a). Sing x -> Sing (L2r x :: b)

instance SFD Bool Nat where
  sL2r SFalse = SZero
  sL2r STrue = SSucc SZero

sT2 = sL2r SFalse

It also typechecks if you give sT2 this particular type signature:

sT2 :: Sing 'Zero
sT2 = sL2r SFalse

However, if you give it either of these two type signatures:

sT2 :: Sing T2
sT2 :: Sing (L2r 'False)

Then GHC 8.0.1, 8.0.2, 8.2.1, and HEAD will choke:

GHCi, version  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:46:7: error:
    • No instance for (SFD Bool k) arising from a use of ‘sL2r’
    • In the expression: sL2r SFalse
      In an equation for ‘sT2’: sT2 = sL2r SFalse
46 | sT2 = sL2r SFalse
   |       ^^^^^^^^^^^

At this point, I get the urge to yell obscenities at GHC, because there definitely is an instance of the form SFD Bool k in scope (and moreover, SFD's functional dependency should make sure that k ~ Nat). Shouldn't it be using that?

Change History (5)

comment:1 Changed 2 years ago by RyanGlScott

Cc: goldfire added
Keywords: FunDeps added

Some possible wisdom from the singletons README:

Inference dependent on functional dependencies is unpredictably bad. The problem is that a use of an associated type family tied to a class with fundeps doesn't provoke the fundep to kick in. This is GHC's problem, in the end.

And this comment:

If only a type family could have functional dependencies, we could get somewhere, but alas, no. (Sidenote: of course, a type family can be declared within a class with functional dependencies, but GHC doesn't apply the fundeps when examining the type family.)

I was unable to dig up anything which explained these comments. Richard, can you elaborate more on this interaction between type families and functional dependencies? Is there an existing ticket which describes the root issue?

comment:2 Changed 2 years ago by goldfire

I don't think this is related to those comments. To explain those comments:

class C a b | a -> b where
  f :: a -> b

class PC a b | a -> b where
  type F (x :: a) :: b

A use of f will trigger a need for a C instance. But a use of F won't. So, an inference that succeeds on terms fails on types. But that's not what's going on here.

comment:3 Changed 2 years ago by simonpj

Resolution: invalid
Status: newclosed

Your type signature

sT2 :: Sing (L2r 'False)


sT2 :: forall k. Sing k (L2r Bool k 'False)

And, since k is universally quantified, we can't unify it with Nat. So the instance doesn't match.

You can fix it thus

sT2 :: Sing (L2r 'False :: Nat)

which indeed compiles fine.

I'm not sure what other error message would be better. I'll close as invalid for now.

comment:4 Changed 2 years ago by goldfire

Well spotted -- thanks.

comment:5 Changed 2 years ago by RyanGlScott

Urgh. I'd like to feign ignorance and claim that I would have never thought in a million years to put in a kind ascription, but this isn't even the first time that this bug has bit me (#11275). So I should probably know better by now... but thanks for patiently explaining to me again anyways :)

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