Type family stopped compiling on upgrade from GHC 7.6.3 to 7.8.3

[cheater00 .]

Thanks all for the conversation, it seems that Simon fixed the bug
before I could even report it, displaying his expectably excellent
programming abilities.

On Wed, Jul 23, 2014 at 5:22 PM, Richard Eisenberg <eir at cis.upenn.edu> wrote:
> 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)
>>>> | _______________________________________________
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>>>> | Glasgow-haskell-users at haskell.org
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