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 . 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" 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@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

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 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 ." 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 . 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" 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@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

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