Type family stopped compiling on upgrade from GHC 7.6.3 to 7.8.3

[Richard Eisenberg]

This seems to be a bug in GHC. I can write the Show instance manually:

instance (Show c, Show d) => Show (CmpInterval (V c) (V d)) where
  show (c `Interval` d) = show c ++ " `Interval` " ++ show d

Perhaps you should file a bug report -- your code looks sensible to me.

Richard

On Jul 23, 2014, at 10:49 AM, "cheater00 ." <cheater00 at gmail.com> wrote:

> Dear all,
> while still not understanding kinds and type families well enough, my
> random explorations have led me to finding syntax which currently is
> accepted in 7.8.3 but seems to be surprising as well. This is to mean
> the code is probably bogus, but GHC somehow manages not to notice.
> 
> If I write:
> 
> data Cmp a where
>  Inf ::      Cmp a
>  Sup ::      Cmp a
>  V   :: a -> Cmp a
>  deriving (Show, Eq)
> 
> data family   CmpInterval (a :: Cmp k) (b :: Cmp k) :: *
> data instance CmpInterval  Inf          Sup          = Always
> data instance CmpInterval (V c)         Sup          = Starting c
> data instance CmpInterval  Inf         (V d)         = Ending d
> data instance CmpInterval (V c)        (V d)         = c `Interval` d
> 
> that compiles without complaint. However, if I add deriving (Show) to
> any instance but the first one:
> 
> data family   CmpInterval (a :: Cmp k) (b :: Cmp k) :: *
> data instance CmpInterval  Inf          Sup          = Always
> data instance CmpInterval (V c)         Sup          = Starting c
> data instance CmpInterval  Inf         (V d)         = Ending d
> data instance CmpInterval (V c)        (V d)         = c `Interval` d
>  deriving (Show)
> 
> then I get:
> 
> src/Parser.hs:864:13:
>    Can't make a derived instance of
>      ‘Show (CmpInterval ('V c) ('V d))’:
>      No family instance for ‘CmpInterval ('V c) ('V d)’
>    In the data instance declaration for ‘CmpInterval’
> 
> Which is surprising, because the instance gets accepted without error,
> whereas if we actually try to use it then it turns out not to be
> there.
> 
> I was wondering if I again did something wrong (I'm still negotiating
> with type families whether they'll let me understand them) and if so,
> whether GHC would normally be expected to tell me of that - or do I
> need to populate the type families with types and/or values in order
> to let GHC finally figure out the code I'm writing is bogus?
> 
> Thanks!
> 
> On Tue, Jul 22, 2014 at 11:20 AM, cheater00 . <cheater00 at gmail.com> wrote:
>> Indeed, I hadn't come to use that at the type level; the original code used
>> my own types which ended up holding LocalTime; I used Float as a
>> simplification as it displayed the same weird behaviour.
>> 
>> I guess in the act of randomly walking parseable type family code I have
>> inadvertently unearthed a bug, which someone else inadvertently fixed,
>> making me a sort of human QuickCheck.
>> 
>> On 22 Jul 2014 10:57, "Simon Peyton Jones" <simonpj at microsoft.com> wrote:
>>> 
>>> I don't know why 7.6.3 accepts it.  'Float' is a valid type but not a
>>> valid kind.  For it to be a useful kind we'd need float literal at the type
>>> level, and we have no such thing.  You can use Nat instead, which does exist
>>> at the type level.
>>> 
>>> Simon
>>> 
>>> | -----Original Message-----
>>> | From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
>>> | bounces at haskell.org] On Behalf Of cheater00 .
>>> | Sent: 21 July 2014 18:51
>>> | To: glasgow-haskell-users at haskell.org
>>> | Subject: Type family stopped compiling on upgrade from GHC 7.6.3 to
>>> | 7.8.3
>>> |
>>> | Hi, I was experimenting a bit with type families recently and ran into
>>> | a bit of an issue. Given that I don't know type families that well yet,
>>> | I was wondering if I made an error somewhere. One thing is that I can't
>>> | find any relevant changes in the GHC release notes for 7.8.1, .2 or .3.
>>> |
>>> | Maybe this code contains an error which 7.6.3 simply wasn't able to
>>> | find?
>>> |
>>> | Thanks.
>>> |
>>> | --------
>>> |
>>> | -- this code compiles in 7.6.3, but breaks in 7.8.3 with the following
>>> | message:
>>> | -- TypeFamilies.hs:14:31:
>>> | --     ‘End’ of kind ‘*’ is not promotable
>>> | --     In the kind ‘End’
>>> | -- In 7.6.3, using :kind!, I can see that the type synonyms contained
>>> | in the family do work the way I intend them to.
>>> |
>>> |
>>> | {-# Language
>>> |     GADTs
>>> |   , TypeFamilies
>>> |   , DataKinds
>>> |    #-}
>>> | module TypeFamilies where
>>> |
>>> | data End = Least | Spot Float | Most
>>> |   deriving (Eq, Show)
>>> |
>>> | data Interval = IntervalCons { left :: End, right :: End }
>>> |   deriving (Eq, Show)
>>> |
>>> | type family   Interval2 (a :: End) (b :: End) :: Interval
>>> | type instance Interval2  Least      Most      =  IntervalCons  Least
>>> | Most
>>> | type instance Interval2 (Spot l)    Most      =  IntervalCons (Spot l)
>>> | Most
>>> | type instance Interval2  Least     (Spot r)   =  IntervalCons  Least
>>> | (Spot r)
>>> | type instance Interval2 (Spot l)   (Spot r)   =  IntervalCons (Spot l)
>>> | (Spot r)
>>> | _______________________________________________
>>> | Glasgow-haskell-users mailing list
>>> | Glasgow-haskell-users at haskell.org
>>> | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
> _______________________________________________
> Glasgow-haskell-users mailing list
> Glasgow-haskell-users at haskell.org
> http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
>