Type family stopped compiling on upgrade from GHC 7.6.3 to 7.8.3

[Simon Peyton Jones]

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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