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"
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@haskell.org] On Behalf Of cheater00 . | Sent: 21 July 2014 18:51 | To: glasgow-haskell-users@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@haskell.org | http://www.haskell.org/mailman/listinfo/glasgow-haskell-users