GHC issueshttps://gitlab.haskell.org/ghc/ghc/-/issues2019-07-07T18:49:28Zhttps://gitlab.haskell.org/ghc/ghc/-/issues/7503Bug with PolyKinds, type synonyms & GADTs2019-07-07T18:49:28ZAshley YakeleyBug with PolyKinds, type synonyms & GADTsGHC incorrectly rejects this program:
```
{-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds, KindSignatures, GADTs #-}
module TestConstraintKinds where
import GHC.Exts hiding (Any)
data WrappedType = forall a. WrapTy...GHC incorrectly rejects this program:
```
{-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds, KindSignatures, GADTs #-}
module TestConstraintKinds where
import GHC.Exts hiding (Any)
data WrappedType = forall a. WrapType a
data A :: WrappedType -> * where
MkA :: forall (a :: *). AW a -> A (WrapType a)
type AW (a :: k) = A (WrapType a)
type AW' (a :: k) = A (WrapType a)
class C (a :: k) where
aw :: AW a -- workaround: AW'
instance C [] where
aw = aw
```
GHC accepts the program when AW is replaced with AW' on that line.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 7.6.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Bug with PolyKinds, type synonyms & GADTs","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"7.6.1","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"GHC incorrectly rejects this program:\r\n{{{\r\n{-# LANGUAGE ExistentialQuantification, DataKinds, PolyKinds, KindSignatures, GADTs #-}\r\nmodule TestConstraintKinds where\r\n import GHC.Exts hiding (Any)\r\n\r\n data WrappedType = forall a. WrapType a\r\n\r\n data A :: WrappedType -> * where\r\n MkA :: forall (a :: *). AW a -> A (WrapType a)\r\n\r\n type AW (a :: k) = A (WrapType a)\r\n type AW' (a :: k) = A (WrapType a)\r\n\r\n class C (a :: k) where\r\n aw :: AW a -- workaround: AW'\r\n\r\n instance C [] where\r\n aw = aw\r\n}}}\r\n\r\nGHC accepts the program when AW is replaced with AW' on that line.","type_of_failure":"OtherFailure","blocking":[]} -->8.0.1Simon Peyton JonesSimon Peyton Joneshttps://gitlab.haskell.org/ghc/ghc/-/issues/15589Always promoting metavariables during type inference may be wrong2019-07-07T18:04:00ZRichard Eisenbergrae@richarde.devAlways promoting metavariables during type inference may be wrongCurrently, when checking a type signature, GHC promotes all the metavariables that arise during checking as soon as it's done checking the signature. This may be incorrect sometimes.
Consider
```hs
{-# LANGUAGE ScopedTypeVariables, Typ...Currently, when checking a type signature, GHC promotes all the metavariables that arise during checking as soon as it's done checking the signature. This may be incorrect sometimes.
Consider
```hs
{-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies,
AllowAmbiguousTypes #-}
import Data.Proxy
import Data.Type.Equality
import Data.Type.Bool
import Data.Kind
data SameKind :: forall k. k -> k -> Type
type family IfK (e :: Proxy (j :: Bool)) (f :: m) (g :: n) :: If j m n where
IfK (_ :: Proxy True) f _ = f
IfK (_ :: Proxy False) _ g = g
y :: forall (cb :: Bool) (c :: Proxy cb). cb :~: True -> ()
y Refl = let x :: forall a b (d :: a). SameKind (IfK c b d) d
x = undefined
in ()
```
This panics currently (#15588), but I'm pretty sure it will erroneously be rejected even after the panic is fixed. Let's walk through it.
- We can derive `IfK :: forall (j :: Bool) (m :: Type) (n :: Type). Proxy j -> m -> n -> If j m n`, where `If :: forall k. Bool -> k -> k -> k` is imported from `Data.Type.Bool` and is a straightforward conditional choice operator.
- In the type of `x`, we see that we need the kind of `IfK c b d` to match the kind of `d`. That is, if `b :: kappa[3]`, we have `[W] If cb kappa[3] a ~ a`. Here, the `forall` in `x`'s type is at level 3; the RHS of `y` is at level 2.
- If we could reduce `If cb kappa[3] a` to `kappa[3]`, then we would solve `kappa[3] := a`, but we can't make this reduction, because `cb` is a skolem.
- Instead, we finish checking the type of `x` and promote `kappa[3]` to `kappa[2]`.
- Later, we'll make an implication constraint with `[G] cb ~ True`. When solving that implication constraint, we'll get `[W] If True kappa[2] a ~ a` and simplify to `[W] kappa[2] ~ a`, but that will be insoluble because we'll be solving at level 3, and now `kappa[2]` is at level 2. We're too late.
Yet, I claim that this program should be accepted, and it would be if GHC tracked a set of ambient givens and used them in local calls to the solver. With these "ambient givens" (instead of putting them only in implication constraints), we would know `cb ~ True` the first time we try to solve, and then we'll succeed.
An alternative story is to change how levels are used with variables. Currently, levels are, essentially, the number of type variables available from an outer scope. Accordingly, we must make sure that the level of a variable is never higher than the ambient level. (If it were, we wouldn't know what extra variable(s) were in scope.) Instead, we could just store the list of variables that were in scope. We wouldn't then need to promote in this case -- promotion would happen only during floating. But tracking these lists is a real pain. (If we decide to pursue this further, I can add more details, but it's all in Chapter 6 in [my thesis](https://repository.brynmawr.edu/cgi/viewcontent.cgi?article=1074&context=compsci_pubs) -- section 6.5 to be specific.)
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 8.4.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Always promoting metavariables during type inference may be wrong","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"8.6.1","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.4.3","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"Currently, when checking a type signature, GHC promotes all the metavariables that arise during checking as soon as it's done checking the signature. This may be incorrect sometimes.\r\n\r\nConsider\r\n\r\n{{{#!hs\r\n{-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies,\r\n AllowAmbiguousTypes #-}\r\n\r\nimport Data.Proxy\r\nimport Data.Type.Equality\r\nimport Data.Type.Bool\r\nimport Data.Kind\r\n\r\ndata SameKind :: forall k. k -> k -> Type\r\ntype family IfK (e :: Proxy (j :: Bool)) (f :: m) (g :: n) :: If j m n where\r\n IfK (_ :: Proxy True) f _ = f\r\n IfK (_ :: Proxy False) _ g = g\r\n\r\ny :: forall (cb :: Bool) (c :: Proxy cb). cb :~: True -> ()\r\ny Refl = let x :: forall a b (d :: a). SameKind (IfK c b d) d\r\n x = undefined\r\n in ()\r\n}}}\r\n\r\nThis panics currently (#15588), but I'm pretty sure it will erroneously be rejected even after the panic is fixed. Let's walk through it.\r\n\r\n* We can derive `IfK :: forall (j :: Bool) (m :: Type) (n :: Type). Proxy j -> m -> n -> If j m n`, where `If :: forall k. Bool -> k -> k -> k` is imported from `Data.Type.Bool` and is a straightforward conditional choice operator.\r\n\r\n* In the type of `x`, we see that we need the kind of `IfK c b d` to match the kind of `d`. That is, if `b :: kappa[3]`, we have `[W] If cb kappa[3] a ~ a`. Here, the `forall` in `x`'s type is at level 3; the RHS of `y` is at level 2.\r\n\r\n* If we could reduce `If cb kappa[3] a` to `kappa[3]`, then we would solve `kappa[3] := a`, but we can't make this reduction, because `cb` is a skolem.\r\n\r\n* Instead, we finish checking the type of `x` and promote `kappa[3]` to `kappa[2]`.\r\n\r\n* Later, we'll make an implication constraint with `[G] cb ~ True`. When solving that implication constraint, we'll get `[W] If True kappa[2] a ~ a` and simplify to `[W] kappa[2] ~ a`, but that will be insoluble because we'll be solving at level 3, and now `kappa[2]` is at level 2. We're too late.\r\n\r\nYet, I claim that this program should be accepted, and it would be if GHC tracked a set of ambient givens and used them in local calls to the solver. With these \"ambient givens\" (instead of putting them only in implication constraints), we would know `cb ~ True` the first time we try to solve, and then we'll succeed.\r\n\r\nAn alternative story is to change how levels are used with variables. Currently, levels are, essentially, the number of type variables available from an outer scope. Accordingly, we must make sure that the level of a variable is never higher than the ambient level. (If it were, we wouldn't know what extra variable(s) were in scope.) Instead, we could just store the list of variables that were in scope. We wouldn't then need to promote in this case -- promotion would happen only during floating. But tracking these lists is a real pain. (If we decide to pursue this further, I can add more details, but it's all in Chapter 6 in [https://repository.brynmawr.edu/cgi/viewcontent.cgi?article=1074&context=compsci_pubs my thesis] -- section 6.5 to be specific.)","type_of_failure":"OtherFailure","blocking":[]} -->8.6.1https://gitlab.haskell.org/ghc/ghc/-/issues/15588Panic when abusing kind inference2019-07-07T18:04:01ZRichard Eisenbergrae@richarde.devPanic when abusing kind inferenceWhen I say
```hs
{-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies,
AllowAmbiguousTypes #-}
import Data.Proxy
import Data.Type.Equality
import Data.Type.Bool
import Data.Kind
data SameKind :: foral...When I say
```hs
{-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies,
AllowAmbiguousTypes #-}
import Data.Proxy
import Data.Type.Equality
import Data.Type.Bool
import Data.Kind
data SameKind :: forall k. k -> k -> Type
type family IfK (e :: Proxy (j :: Bool)) (f :: m) (g :: n) :: If j m n where
IfK (_ :: Proxy True) f _ = f
IfK (_ :: Proxy False) _ g = g
y :: forall ck (c :: ck). ck :~: Proxy True -> ()
y Refl = let x :: forall a b (d :: a). SameKind (IfK c b d) d
x = undefined
in ()
```
HEAD says
```
ghc-stage2: panic! (the 'impossible' happened)
(GHC version 8.7.20180827 for x86_64-apple-darwin):
ASSERT failed!
Bad coercion hole co_a3iZ: If
j_a3j0[tau:2] m_a3j1[tau:2] a_a3gV[sk:3]
a_a3gV[sk:3]
nominal
If j_a3j0[tau:2] m_a3j1[tau:2] a_a3jj[sk:3] ~# a_a3jj[sk:3]
Call stack:
CallStack (from HasCallStack):
callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable
pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable
assertPprPanic, called at compiler/typecheck/TcMType.hs:316:25 in ghc:TcMType
Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug
```
It's as yet unclear whether the program should be accepted. My best guess is that it should, but that (even with this panic fixed) GHC isn't up to the task.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 8.5 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Panic when abusing kind inference","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"8.6.1","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.5","keywords":["TypeInType"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"When I say\r\n\r\n{{{#!hs\r\n{-# LANGUAGE ScopedTypeVariables, TypeInType, TypeOperators, TypeFamilies,\r\n AllowAmbiguousTypes #-}\r\n\r\nimport Data.Proxy\r\nimport Data.Type.Equality\r\nimport Data.Type.Bool\r\nimport Data.Kind\r\n\r\ndata SameKind :: forall k. k -> k -> Type\r\ntype family IfK (e :: Proxy (j :: Bool)) (f :: m) (g :: n) :: If j m n where\r\n IfK (_ :: Proxy True) f _ = f\r\n IfK (_ :: Proxy False) _ g = g\r\n\r\ny :: forall ck (c :: ck). ck :~: Proxy True -> ()\r\ny Refl = let x :: forall a b (d :: a). SameKind (IfK c b d) d\r\n x = undefined\r\n in ()\r\n}}}\r\n\r\nHEAD says\r\n\r\n{{{\r\nghc-stage2: panic! (the 'impossible' happened)\r\n (GHC version 8.7.20180827 for x86_64-apple-darwin):\r\n\tASSERT failed!\r\n Bad coercion hole co_a3iZ: If\r\n j_a3j0[tau:2] m_a3j1[tau:2] a_a3gV[sk:3]\r\n a_a3gV[sk:3]\r\n nominal\r\n If j_a3j0[tau:2] m_a3j1[tau:2] a_a3jj[sk:3] ~# a_a3jj[sk:3]\r\n Call stack:\r\n CallStack (from HasCallStack):\r\n callStackDoc, called at compiler/utils/Outputable.hs:1160:37 in ghc:Outputable\r\n pprPanic, called at compiler/utils/Outputable.hs:1219:5 in ghc:Outputable\r\n assertPprPanic, called at compiler/typecheck/TcMType.hs:316:25 in ghc:TcMType\r\n\r\nPlease report this as a GHC bug: http://www.haskell.org/ghc/reportabug\r\n}}}\r\n\r\nIt's as yet unclear whether the program should be accepted. My best guess is that it should, but that (even with this panic fixed) GHC isn't up to the task.","type_of_failure":"OtherFailure","blocking":[]} -->8.6.1https://gitlab.haskell.org/ghc/ghc/-/issues/15561TypeInType: Type error conditioned on ordering of GADT and type family defini...2022-03-05T13:41:59ZBj0rnTypeInType: Type error conditioned on ordering of GADT and type family definitionsConsider this code which successfully compiles:
```hs
{-# LANGUAGE TypeInType, TypeFamilies, GADTs #-}
module Bug where
class HasIndex a where
type Index a
emptyIndex :: IndexWrapper a
instance HasIndex [a] where
type Inde...Consider this code which successfully compiles:
```hs
{-# LANGUAGE TypeInType, TypeFamilies, GADTs #-}
module Bug where
class HasIndex a where
type Index a
emptyIndex :: IndexWrapper a
instance HasIndex [a] where
type Index [a] = Int
emptyIndex = Wrap 0
data IndexWrapper a where
Wrap :: Index a -> IndexWrapper a
type family UnwrapAnyWrapperLikeThing (a :: t) :: k
type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a
```
The mere act of moving the definition of `IndexWrapper` anywhere below the definition of `UnwrapAnyWrapperLikeThing` makes the type family instance at the bottom of the example fail compilation, with this error:
```
Bug.hs:17:15: error:
• Illegal type synonym family application in instance: Index [b]
• In the type instance declaration for ‘UnwrapAnyWrapperLikeThing’
|
17 | type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
This is the smallest example that I could come up with; my real scenario of course has more things going on, but I can share if it would help.
The problem for me (other than that I'm pretty sure reordering definitions in Haskell should never affect anything) is that I would like just the definition of the type family (`UnwrapAnyWrapperLikeThing` in this example) in module `A` and all of the other definitions in module `B` that imports `A`.
Ideally, I would have liked to add a `HasIndex a` constraint to the `Wrap` constructor, but that disqualifies use of `'Wrap` on the type level. This does make me feel like I'm on shaky ground to begin with.
I have reproduced this bug on 8.2.2, 8.4.3 and 8.6.0.20180810 (NixOS). I should note that 8.0.2 rejects even the code that I pasted here.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 8.4.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"TypeInType: Type error conditioned on ordering of GADT and type family definitions","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"8.6.1","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.4.3","keywords":["GADTs","TypeFamilies,","TypeInType,"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"Consider this code which successfully compiles:\r\n{{{#!hs\r\n{-# LANGUAGE TypeInType, TypeFamilies, GADTs #-}\r\n\r\nmodule Bug where\r\n\r\nclass HasIndex a where\r\n type Index a\r\n emptyIndex :: IndexWrapper a\r\ninstance HasIndex [a] where\r\n type Index [a] = Int\r\n emptyIndex = Wrap 0\r\n\r\ndata IndexWrapper a where\r\n Wrap :: Index a -> IndexWrapper a\r\n\r\ntype family UnwrapAnyWrapperLikeThing (a :: t) :: k\r\n\r\ntype instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a\r\n}}}\r\n\r\nThe mere act of moving the definition of `IndexWrapper` anywhere below the definition of `UnwrapAnyWrapperLikeThing` makes the type family instance at the bottom of the example fail compilation, with this error:\r\n{{{\r\nBug.hs:17:15: error:\r\n • Illegal type synonym family application in instance: Index [b]\r\n • In the type instance declaration for ‘UnwrapAnyWrapperLikeThing’\r\n |\r\n17 | type instance UnwrapAnyWrapperLikeThing ('Wrap a :: IndexWrapper [b]) = a\r\n | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\r\n}}}\r\n\r\nThis is the smallest example that I could come up with; my real scenario of course has more things going on, but I can share if it would help.\r\n\r\nThe problem for me (other than that I'm pretty sure reordering definitions in Haskell should never affect anything) is that I would like just the definition of the type family (`UnwrapAnyWrapperLikeThing` in this example) in module `A` and all of the other definitions in module `B` that imports `A`.\r\n\r\nIdeally, I would have liked to add a `HasIndex a` constraint to the `Wrap` constructor, but that disqualifies use of `'Wrap` on the type level. This does make me feel like I'm on shaky ground to begin with.\r\n\r\nI have reproduced this bug on 8.2.2, 8.4.3 and 8.6.0.20180810 (NixOS). I should note that 8.0.2 rejects even the code that I pasted here.","type_of_failure":"OtherFailure","blocking":[]} -->8.6.1https://gitlab.haskell.org/ghc/ghc/-/issues/13650Implement KPush in types2021-10-04T14:41:09ZRichard Eisenbergrae@richarde.devImplement KPush in typesA recent commit contributed a Note (https://gitlab.haskell.org/ghc/ghc/-/commit/ef0ff34d462e3780210567a13d58b868ec3399e0#07ce9a046fb8ea6659690020b0a8551d94cfdf1c_1175_1163) that explains why we need the dreaded KPush rule to be implement...A recent commit contributed a Note (https://gitlab.haskell.org/ghc/ghc/-/commit/ef0ff34d462e3780210567a13d58b868ec3399e0#07ce9a046fb8ea6659690020b0a8551d94cfdf1c_1175_1163) that explains why we need the dreaded KPush rule to be implemented in `splitTyConApp`. Without KPush there, it's possible that we can have two types t1 and t2 such that `t1 `eqType` t2` and yet they respond differently to `splitTyConApp`: t1 = `(T |> co1) (a |> co2)` and t2 = `T a`. Both t1 and t2 are well-kinded and can have the same kind. But one is a `TyConApp` and one is an `AppTy`. (Actually, looking at this, perhaps the magic will be in `mkAppTy`, not `splitTyConApp`.) But I have to look closer.
This ticket serves as a reminder to do so.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | |
| Type | Task |
| TypeOfFailure | OtherFailure |
| Priority | high |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Implement KPush in types","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"8.4.1","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Task","description":"A recent commit contributed a [https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/types/Type.hs;a483e711da7834bc952367f554ac4e877b4e157a$1191 Note] that explains why we need the dreaded KPush rule to be implemented in `splitTyConApp`. Without KPush there, it's possible that we can have two types t1 and t2 such that {{{t1 `eqType` t2}}} and yet they respond differently to `splitTyConApp`: t1 = `(T |> co1) (a |> co2)` and t2 = `T a`. Both t1 and t2 are well-kinded and can have the same kind. But one is a `TyConApp` and one is an `AppTy`. (Actually, looking at this, perhaps the magic will be in `mkAppTy`, not `splitTyConApp`.) But I have to look closer.\r\n\r\nThis ticket serves as a reminder to do so.","type_of_failure":"OtherFailure","blocking":[]} -->8.6.1Richard Eisenbergrae@richarde.devRichard Eisenbergrae@richarde.devhttps://gitlab.haskell.org/ghc/ghc/-/issues/12564Type family in type pattern kind2023-08-19T13:41:26ZVladislav ZavialovType family in type pattern kindI want to write a type family that is analogous to `!!` for lists but requires the index to be no bigger than the length of the list. Usually, in dependently typed languages finite sets are used for this purpose, here's an attempt to do ...I want to write a type family that is analogous to `!!` for lists but requires the index to be no bigger than the length of the list. Usually, in dependently typed languages finite sets are used for this purpose, here's an attempt to do so in Haskell:
```haskell
{-# LANGUAGE TypeInType, TypeFamilies, GADTs, TypeOperators #-}
import Data.Kind
data N = Z | S N
type family Len (xs :: [a]) :: N where
Len '[] = Z
Len (_ ': xs) = S (Len xs)
data Fin :: N -> Type where
FZ :: Fin (S n)
FS :: Fin n -> Fin (S n)
type family At (xs :: [a]) (i :: Fin (Len xs)) :: a where
At (x ': _) FZ = x
At (_ ': xs) (FS i) = At xs i
```
It fails to compile with this error:
```
FinAt.hs:16:3: error:
• Illegal type synonym family application in instance: 'FZ
• In the equations for closed type family ‘At’
In the type family declaration for ‘At’
```
That's because the kind of the `FZ` pattern (first clause of `At`) has the kind `Fin (Len xs)` and the application of `Len` cannot reduce completely. `checkValidTypePat` then disallows the pattern, as it contains a type family application.
I tried to suppress `checkValidTypePat` and the definition of `At` has compiled; however, it's of little use, since `At` doesn't reduce:
```haskell
x :: At '[Bool] FZ
x = True
```
results in
```
FinAt.hs:20:5: error:
• Couldn't match expected type ‘At
* ((':) * Bool ('[] *)) ('FZ 'Z)’
with actual type ‘Bool’
• In the expression: True
In an equation for ‘x’: x = True
```
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 8.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | goldfire, int-index |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Type family in type pattern kind","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"8.2.1","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.0.1","keywords":["TypeFamilies","TypeInType,"],"differentials":[],"test_case":"","architecture":"","cc":["goldfire","int-index"],"type":"Bug","description":"I want to write a type family that is analogous to `!!` for lists but requires the index to be no bigger than the length of the list. Usually, in dependently typed languages finite sets are used for this purpose, here's an attempt to do so in Haskell:\r\n\r\n{{{#!haskell\r\n{-# LANGUAGE TypeInType, TypeFamilies, GADTs, TypeOperators #-}\r\n\r\nimport Data.Kind\r\n\r\ndata N = Z | S N\r\n\r\ntype family Len (xs :: [a]) :: N where\r\n Len '[] = Z\r\n Len (_ ': xs) = S (Len xs)\r\n\r\ndata Fin :: N -> Type where\r\n FZ :: Fin (S n)\r\n FS :: Fin n -> Fin (S n)\r\n\r\ntype family At (xs :: [a]) (i :: Fin (Len xs)) :: a where\r\n At (x ': _) FZ = x\r\n At (_ ': xs) (FS i) = At xs i\r\n}}}\r\n\r\nIt fails to compile with this error:\r\n\r\n{{{\r\nFinAt.hs:16:3: error:\r\n • Illegal type synonym family application in instance: 'FZ\r\n • In the equations for closed type family ‘At’\r\n In the type family declaration for ‘At’\r\n}}}\r\n\r\nThat's because the kind of the `FZ` pattern (first clause of `At`) has the kind `Fin (Len xs)` and the application of `Len` cannot reduce completely. `checkValidTypePat` then disallows the pattern, as it contains a type family application.\r\n\r\nI tried to suppress `checkValidTypePat` and the definition of `At` has compiled; however, it's of little use, since `At` doesn't reduce:\r\n\r\n{{{#!haskell\r\nx :: At '[Bool] FZ\r\nx = True\r\n}}}\r\n\r\nresults in\r\n\r\n{{{\r\nFinAt.hs:20:5: error:\r\n • Couldn't match expected type ‘At\r\n * ((':) * Bool ('[] *)) ('FZ 'Z)’\r\n with actual type ‘Bool’\r\n • In the expression: True\r\n In an equation for ‘x’: x = True\r\n}}}","type_of_failure":"OtherFailure","blocking":[]} -->Richard Eisenbergrae@richarde.devRichard Eisenbergrae@richarde.devhttps://gitlab.haskell.org/ghc/ghc/-/issues/12239Dependent type family does not reduce2024-02-27T13:56:52ZVladislav ZavialovDependent type family does not reduceWhen a type family is used in a kind of an argument for another type family, it does not reduce. Here's example code:
```
{-# LANGUAGE TypeInType, GADTs, TypeFamilies #-}
import Data.Kind (Type)
data N = Z | S N
data Fin :: N -> Type...When a type family is used in a kind of an argument for another type family, it does not reduce. Here's example code:
```
{-# LANGUAGE TypeInType, GADTs, TypeFamilies #-}
import Data.Kind (Type)
data N = Z | S N
data Fin :: N -> Type where
FZ :: Fin (S n)
FS :: Fin n -> Fin (S n)
type family FieldCount (t :: Type) :: N
type family FieldType (t :: Type) (i :: Fin (FieldCount t)) :: Type
data T
type instance FieldCount T = S (S (S Z))
type instance FieldType T FZ = Int
type instance FieldType T (FS FZ) = Bool
type instance FieldType T (FS (FS FZ)) = String
```
Unfortunately, I get these errors during typechecking:
```
[1 of 1] Compiling Main ( fieldtype.hs, fieldtype.o )
fieldtype.hs:19:27: error:
• Expected kind ‘Fin (FieldCount T)’,
but ‘'FZ’ has kind ‘Fin ('S n0)’
• In the second argument of ‘FieldType’, namely ‘FZ’
In the type instance declaration for ‘FieldType’
fieldtype.hs:20:28: error:
• Expected kind ‘Fin (FieldCount T)’,
but ‘'FS 'FZ’ has kind ‘Fin ('S ('S n0))’
• In the second argument of ‘FieldType’, namely ‘FS FZ’
In the type instance declaration for ‘FieldType’
fieldtype.hs:21:28: error:
• Expected kind ‘Fin (FieldCount T)’,
but ‘'FS ('FS 'FZ)’ has kind ‘Fin ('S ('S ('S n0)))’
• In the second argument of ‘FieldType’, namely ‘FS (FS FZ)’
In the type instance declaration for ‘FieldType’
```
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 8.0.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Dependent type family does not reduce","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.0.1","keywords":["TypeFamilies","TypeInType,"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"When a type family is used in a kind of an argument for another type family, it does not reduce. Here's example code:\r\n\r\n\r\n{{{\r\n{-# LANGUAGE TypeInType, GADTs, TypeFamilies #-}\r\n\r\nimport Data.Kind (Type)\r\n\r\ndata N = Z | S N\r\n\r\ndata Fin :: N -> Type where\r\n FZ :: Fin (S n)\r\n FS :: Fin n -> Fin (S n)\r\n\r\ntype family FieldCount (t :: Type) :: N\r\n\r\ntype family FieldType (t :: Type) (i :: Fin (FieldCount t)) :: Type\r\n\r\ndata T\r\n\r\ntype instance FieldCount T = S (S (S Z))\r\n\r\ntype instance FieldType T FZ = Int\r\ntype instance FieldType T (FS FZ) = Bool\r\ntype instance FieldType T (FS (FS FZ)) = String\r\n}}}\r\n\r\nUnfortunately, I get these errors during typechecking:\r\n\r\n{{{\r\n[1 of 1] Compiling Main ( fieldtype.hs, fieldtype.o )\r\n\r\nfieldtype.hs:19:27: error:\r\n • Expected kind ‘Fin (FieldCount T)’,\r\n but ‘'FZ’ has kind ‘Fin ('S n0)’\r\n • In the second argument of ‘FieldType’, namely ‘FZ’\r\n In the type instance declaration for ‘FieldType’\r\n\r\nfieldtype.hs:20:28: error:\r\n • Expected kind ‘Fin (FieldCount T)’,\r\n but ‘'FS 'FZ’ has kind ‘Fin ('S ('S n0))’\r\n • In the second argument of ‘FieldType’, namely ‘FS FZ’\r\n In the type instance declaration for ‘FieldType’\r\n\r\nfieldtype.hs:21:28: error:\r\n • Expected kind ‘Fin (FieldCount T)’,\r\n but ‘'FS ('FS 'FZ)’ has kind ‘Fin ('S ('S ('S n0)))’\r\n • In the second argument of ‘FieldType’, namely ‘FS (FS FZ)’\r\n In the type instance declaration for ‘FieldType’\r\n}}}","type_of_failure":"OtherFailure","blocking":[]} -->Vladislav ZavialovVladislav Zavialovhttps://gitlab.haskell.org/ghc/ghc/-/issues/18308Order of StandaloneKindSignatures and CUSKs extensions significant2021-03-31T15:14:45ZBjörn HegerforsOrder of StandaloneKindSignatures and CUSKs extensions significant## Summary
I have some code that compiles without the `StandaloneKindSignatures` extension, does not compile with it, but does compile again if I also add the `CUSKs` extension _after_ `StandaloneKindSignatures`. However, if I place the...## Summary
I have some code that compiles without the `StandaloneKindSignatures` extension, does not compile with it, but does compile again if I also add the `CUSKs` extension _after_ `StandaloneKindSignatures`. However, if I place the `CUSKs` extension _before_ `StandaloneKindSignatures`, then the code doesn't compile.
I'm not sure if I can avoid a CUSK in this case either, because it revolves around an associated type family, which to my understanding stand-alone kind signatures cannot (yet) be written for.
I generally try to keep my extensions arranged alphabetically, which is unfortunate in this case. But that's not at all a big worry. For now I can put `CUSKs` after `StandaloneKindSignatures` as a workaround.
## Steps to reproduce
Boiling it down to a minimal demonstrative case, this compiles:
```haskell
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE StandaloneKindSignatures #-}
{-# LANGUAGE CUSKs #-}
import Data.Kind (Type)
import Data.Proxy (Proxy)
class Cls where
type Fam (k :: Type) (a :: k) :: Type
mtd :: Proxy k -> Proxy (a :: k) -> Fam k a -> Int
```
but this doesn't:
```haskell
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE CUSKs #-}
{-# LANGUAGE StandaloneKindSignatures #-}
import Data.Kind (Type)
import Data.Proxy (Proxy)
class Cls where
type Fam (k :: Type) (a :: k) :: Type
mtd :: Proxy k -> Proxy (a :: k) -> Fam k a -> Int
```
## Expected behavior
I would expect both of the above to compile.
I should also note that it's not very clear to me what it is about the type signature for `mtd` that makes it problematic. The part that is sensitive here is the `(a :: k)` kind annotation. Whether I instead put that annotation in a `forall` makes no difference. If I simply remove the kind annotation for `a`, and let it be inferred, the code compiles regardless of which of `StandaloneKindSignatures` and `CUSKs` are enabled (and in which order). I don't know if this part behaves as expected or not.
## Environment
* GHC version used: 8.10.1https://gitlab.haskell.org/ghc/ghc/-/issues/18062A cast might get in the way of instantiation2020-04-27T10:20:56ZSimon Peyton JonesA cast might get in the way of instantiationSuppose we have a type signature `f :: forall a. Eq a => blah`, and it somehow kind-checks to
```
forall a. ((Eq a => Blah) |> co)
```
In principle this can happen:
```
tc_hs_type mode (HsQualTy { hst_ctxt = ctxt, hst_body = rn_ty }) ...Suppose we have a type signature `f :: forall a. Eq a => blah`, and it somehow kind-checks to
```
forall a. ((Eq a => Blah) |> co)
```
In principle this can happen:
```
tc_hs_type mode (HsQualTy { hst_ctxt = ctxt, hst_body = rn_ty }) exp_kind
= do { ctxt' <- tc_hs_context mode ctxt
; ek <- newOpenTypeKind -- The body kind (result of the function) can
-- be TYPE r, for any r, hence newOpenTypeKind
; ty' <- tc_lhs_type mode rn_ty ek
; checkExpectedKind (unLoc rn_ty) (mkPhiTy ctxt' ty')
liftedTypeKind exp_kind }
```
That `checkExpectedKind` can add a cast.
If this happens, our instantiation mechanisms would fall over in a heap. We'd instantiate the `forall a`, but then fail to instantiate the `Eq a =>`; instead we'd unify `(Eq a => blah) |> co` with the function body. Bad, bad.
This popped up when fixing #18008, when a missing zonk in `tcHsPartialSigType` was producing just such a forall (with a Refl coercion). But it seems plausible that it could happen for real.
EDIT: And it does:
```hs
{-# LANGUAGE KindSignatures, TypeFamilies, DataKinds #-}
module Bug where
import Data.Kind ( Type )
type family Star where
Star = Type
f :: ((Eq a => a -> Bool) :: Star)
f x = x == x
```
produces
```
Bug.hs:11:1: error:
• Couldn't match kind ‘Constraint’ with ‘*’
When matching types
a0 :: *
Eq a :: Constraint
Expected type: Eq a => a -> Bool
Actual type: a0 -> Bool
• The equation(s) for ‘f’ have one argument,
but its type ‘Eq a => a -> Bool’ has none
• Relevant bindings include
f :: Eq a => a -> Bool (bound at Bug.hs:11:1)
|
11 | f x = x == x
| ^^^^^^^^^^^^
```
Solution: teach instantiation to look through casts.https://gitlab.haskell.org/ghc/ghc/-/issues/17621Type-level multiplication parsed as application at kind *, no guidance provided2020-01-05T03:00:19ZGeshType-level multiplication parsed as application at kind *, no guidance provided## Summary
Location of documentation issue: GHC error message
To reproduce:
```
Prelude> :set -XDataKinds -XTypeOperators
Prelude> import GHC.TypeNats
Prelude GHC.TypeNats> :k 2 * 4
<interactive>:1:1: error:
• Expected kind ‘* ->...## Summary
Location of documentation issue: GHC error message
To reproduce:
```
Prelude> :set -XDataKinds -XTypeOperators
Prelude> import GHC.TypeNats
Prelude GHC.TypeNats> :k 2 * 4
<interactive>:1:1: error:
• Expected kind ‘* -> GHC.Types.Nat -> k0’,
but ‘2’ has kind ‘GHC.Types.Nat’
• In the type ‘2 * 4’
```
The error message is correct, but useless for someone getting started working
at the type level. At the very least, a suggestion of adding ```NoStarIsType```
is in order.
## Proposed improvements or changes
In a kinding error of form "expected ```* -> r```, found ```s```" with
```StarInType```, suggest that this error might be the due to ```StarInType```.
## Environment
* GHC version used (if appropriate): 8.6.5. Asked on IRC, and @solonarv checked
this still exists in 8.8https://gitlab.haskell.org/ghc/ghc/-/issues/15710Should GHC accept a type signature that needs coercion quantification?2023-03-23T12:00:56ZSimon Peyton JonesShould GHC accept a type signature that needs coercion quantification?Consider
```
f :: forall k (f :: k) (x :: k1). (k ~ (k1 -> *)) => f x
f = error "uk"
```
Should we accept it? Now that we have coercion quantification (Trac #15497), I think the answer should be yes, with the elaborated signature being...Consider
```
f :: forall k (f :: k) (x :: k1). (k ~ (k1 -> *)) => f x
f = error "uk"
```
Should we accept it? Now that we have coercion quantification (Trac #15497), I think the answer should be yes, with the elaborated signature being
```
f :: forall k (f::k) (x::k1). forall (co :: k ~# (k1->*)). (f |> co) x
```
But there is a problem: the user wrote `k ~ (k1 -> *)`, and that's a boxed value that we can't take apart in types. I'm not sure what to do here.
These thoughts arose when contemplating `Note [Emitting the residual implication in simplifyInfer]` in `TcSimplify`; see [ticket:15710\#comment:161240](https://gitlab.haskell.org//ghc/ghc/issues/15710#note_161240) in #15497https://gitlab.haskell.org/ghc/ghc/-/issues/14795Data type return kinds don't obey the forall-or-nothing rule2022-07-13T16:54:51ZRyan ScottData type return kinds don't obey the forall-or-nothing ruleOriginally noticed [here](https://github.com/ghc-proposals/ghc-proposals/pull/103#issuecomment-364562974). GHC accepts this:
```hs
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeInType #-}
import Data.Kind
data Fo...Originally noticed [here](https://github.com/ghc-proposals/ghc-proposals/pull/103#issuecomment-364562974). GHC accepts this:
```hs
{-# LANGUAGE GADTs #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeInType #-}
import Data.Kind
data Foo :: forall a. a -> b -> Type where
MkFoo :: a -> Foo a b
```
Despite the fact that `Foo`'s return kind is headed by an explicit `forall` which does not quantify `b`.
The users' guide doesn't explicitly indicate that the `forall`-or-nothing rule should apply to data type return kinds, but goldfirere believes that not doing so is an [inconsistent design](https://github.com/ghc-proposals/ghc-proposals/pull/103#issuecomment-364670215), so I'm opening this ticket to track this.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 8.2.2 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Data type return kinds don't obey the forall-or-nothing rule","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.2.2","keywords":["TypeInType"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"Originally noticed [https://github.com/ghc-proposals/ghc-proposals/pull/103#issuecomment-364562974 here]. GHC accepts this:\r\n\r\n{{{#!hs\r\n{-# LANGUAGE GADTs #-}\r\n{-# LANGUAGE RankNTypes #-}\r\n{-# LANGUAGE TypeInType #-}\r\n\r\nimport Data.Kind\r\n\r\ndata Foo :: forall a. a -> b -> Type where\r\n MkFoo :: a -> Foo a b\r\n}}}\r\n\r\nDespite the fact that `Foo`'s return kind is headed by an explicit `forall` which does not quantify `b`.\r\n\r\nThe users' guide doesn't explicitly indicate that the `forall`-or-nothing rule should apply to data type return kinds, but goldfirere believes that not doing so is an [https://github.com/ghc-proposals/ghc-proposals/pull/103#issuecomment-364670215 inconsistent design], so I'm opening this ticket to track this.","type_of_failure":"OtherFailure","blocking":[]} -->https://gitlab.haskell.org/ghc/ghc/-/issues/14668Ordering of declarations can cause typechecking to fail2023-06-30T17:10:59ZheptahedronOrdering of declarations can cause typechecking to failThe following will successfully typecheck:
```hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeFamilies #-}
data CInst
data G (b :: ()) = G
class C a where
type family F a
...The following will successfully typecheck:
```hs
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}
{-# LANGUAGE TypeInType #-}
{-# LANGUAGE TypeFamilies #-}
data CInst
data G (b :: ()) = G
class C a where
type family F a
class (C a) => C' a where
type family F' a (b :: F a)
-- data CInst
instance C CInst where
type F CInst = ()
instance C' CInst where
type F' CInst (b :: F CInst) = G b
```
But if the `data CInst` declaration is moved to where it is currently commented out, typechecking fails with this error:
```
Test.hs:23:18: error:
• Expected kind ‘F CInst’, but ‘b’ has kind ‘()’
• In the second argument of ‘F'’, namely ‘(b :: F CInst)’
In the type instance declaration for ‘F'’
In the instance declaration for ‘C' CInst’
|
23 | type F' CInst (b :: F CInst) = G b
|
```
However, the data declaration //can// be in the lower position if the kind annotation for its argument is instead written as `data G (b :: F CInst) = G`.
This behavior is also exhibited when G is a type family (I believe the sort of type family does not matter, but I know for sure closed and open type families).
I was using GHC 8.2.2 when I discovered this, but user `erisco` on `#haskell` confirmed for 8.2.1 as well.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 8.2.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Ordering of declarations can cause typechecking to fail","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.2.1","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"The following will successfully typecheck:\r\n\r\n{{{#!hs\r\n{-# LANGUAGE DataKinds #-}\r\n{-# LANGUAGE KindSignatures #-}\r\n{-# LANGUAGE TypeInType #-}\r\n{-# LANGUAGE TypeFamilies #-}\r\n\r\ndata CInst\r\n\r\ndata G (b :: ()) = G \r\n\r\nclass C a where\r\n type family F a\r\n \r\nclass (C a) => C' a where\r\n type family F' a (b :: F a)\r\n\r\n-- data CInst\r\n\r\ninstance C CInst where\r\n type F CInst = ()\r\n\r\ninstance C' CInst where\r\ntype F' CInst (b :: F CInst) = G b\r\n}}}\r\n\r\nBut if the `data CInst` declaration is moved to where it is currently commented out, typechecking fails with this error: \r\n\r\n{{{\r\nTest.hs:23:18: error:\r\n • Expected kind ‘F CInst’, but ‘b’ has kind ‘()’\r\n • In the second argument of ‘F'’, namely ‘(b :: F CInst)’\r\n In the type instance declaration for ‘F'’\r\n In the instance declaration for ‘C' CInst’\r\n |\r\n23 | type F' CInst (b :: F CInst) = G b\r\n | \r\n}}}\r\n\r\nHowever, the data declaration //can// be in the lower position if the kind annotation for its argument is instead written as `data G (b :: F CInst) = G`.\r\n\r\nThis behavior is also exhibited when G is a type family (I believe the sort of type family does not matter, but I know for sure closed and open type families).\r\n\r\nI was using GHC 8.2.2 when I discovered this, but user `erisco` on `#haskell` confirmed for 8.2.1 as well.","type_of_failure":"OtherFailure","blocking":[]} -->https://gitlab.haskell.org/ghc/ghc/-/issues/14645Allow type family in data family return kind2021-11-12T13:25:10ZRichard Eisenbergrae@richarde.devAllow type family in data family return kindGHC currently allows
```hs
data family DF1 :: k1 -> k2
```
where it's expected (and checked) that all data *instances* have a return kind of `Type`. (Perhaps `k2` expands to `Type -> Type`, for example.)
However, it rejects
```hs
typ...GHC currently allows
```hs
data family DF1 :: k1 -> k2
```
where it's expected (and checked) that all data *instances* have a return kind of `Type`. (Perhaps `k2` expands to `Type -> Type`, for example.)
However, it rejects
```hs
type family TF (x :: Type) :: Type
data family DF2 :: x -> TF x
```
when that's clearly just as sensible as the first definition.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 8.4.1-alpha1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Allow type family in data family return kind","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.4.1-alpha1","keywords":["TypeFamilies","TypeInType,"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"GHC currently allows\r\n\r\n{{{#!hs\r\ndata family DF1 :: k1 -> k2\r\n}}}\r\n\r\nwhere it's expected (and checked) that all data ''instances'' have a return kind of `Type`. (Perhaps `k2` expands to `Type -> Type`, for example.)\r\n\r\nHowever, it rejects\r\n\r\n{{{#!hs\r\ntype family TF (x :: Type) :: Type\r\ndata family DF2 :: x -> TF x\r\n}}}\r\n\r\nwhen that's clearly just as sensible as the first definition.","type_of_failure":"OtherFailure","blocking":[]} -->https://gitlab.haskell.org/ghc/ghc/-/issues/14420Data families should not instantiate to non-Type kinds2022-01-25T21:49:09ZRichard Eisenbergrae@richarde.devData families should not instantiate to non-Type kinds```hs
data family Any :: k -- allowable now due to fix for #12369
type family F (a :: Bool) :: Nat where
F True = 0
F False = 1
F Any = 2
```
```
ghci> :kind! F True
F True :: Nat
= 0
ghci> :kind! F False
F False :: Nat
= 1
g...```hs
data family Any :: k -- allowable now due to fix for #12369
type family F (a :: Bool) :: Nat where
F True = 0
F False = 1
F Any = 2
```
```
ghci> :kind! F True
F True :: Nat
= 0
ghci> :kind! F False
F False :: Nat
= 1
ghci> :kind! F Any
F Any :: Nat
= 2
```
Oh dear.
We should require that any instantiation of a data family be to a kind that ends in `Type`.
Inspired by [ticket:9429\#comment:144757](https://gitlab.haskell.org//ghc/ghc/issues/9429#note_144757)
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 8.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Data families should not instantiate to non-Type kinds","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.3","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"{{{#!hs\r\ndata family Any :: k -- allowable now due to fix for #12369\r\n\r\ntype family F (a :: Bool) :: Nat where\r\n F True = 0\r\n F False = 1\r\n F Any = 2\r\n}}}\r\n\r\n{{{\r\nghci> :kind! F True\r\nF True :: Nat\r\n= 0\r\nghci> :kind! F False\r\nF False :: Nat\r\n= 1\r\nghci> :kind! F Any\r\nF Any :: Nat\r\n= 2\r\n}}}\r\n\r\nOh dear.\r\n\r\nWe should require that any instantiation of a data family be to a kind that ends in `Type`.\r\n\r\nInspired by comment:31:ticket:9429","type_of_failure":"OtherFailure","blocking":[]} -->https://gitlab.haskell.org/ghc/ghc/-/issues/14319Stuck type families can lead to lousy error messages2020-01-23T19:27:40ZDavid FeuerStuck type families can lead to lousy error messagesI first noticed this problem at the type level:
```hs
{-# language TypeFamilies, TypeInType, ScopedTypeVariables #-}
module ArityError where
import Data.Kind
import GHC.TypeLits
import Data.Proxy
type family F (s :: Symbol) :: Type
ty...I first noticed this problem at the type level:
```hs
{-# language TypeFamilies, TypeInType, ScopedTypeVariables #-}
module ArityError where
import Data.Kind
import GHC.TypeLits
import Data.Proxy
type family F (s :: Symbol) :: Type
type family G (s :: Symbol) :: F s
type instance G "Hi" = Maybe
```
This produces the error message
```hs
ArityError.hs:10:24: error:
• Expecting one more argument to ‘Maybe’
Expected kind ‘F "Hi"’, but ‘Maybe’ has kind ‘* -> *’
• In the type ‘Maybe’
In the type instance declaration for ‘G’
|
10 | type instance G "Hi" = Maybe
| ^^^^^
```
This looks utterly bogus: `F "Hi"` is stuck, so we have no idea what arity it indicates.
----
I just realized we have a similar problem at the term level:
```hs
f :: forall (s :: Symbol). Proxy s -> F s
f _ _ = undefined
```
produces
```hs
ArityError.hs:14:1: error:
• Couldn't match expected type ‘F s’ with actual type ‘p0 -> a0’
The type variables ‘p0’, ‘a0’ are ambiguous
• The equation(s) for ‘f’ have two arguments,
but its type ‘Proxy s -> F s’ has only one
• Relevant bindings include
f :: Proxy s -> F s (bound at ArityError.hs:14:1)
|
14 | f _ _ = undefined
| ^^^^^^^^^^^^^^^^^
```
The claim that `Proxy s -> F s` has only one argument is bogus; we only know that it has *at least* one argument. The fix (I imagine) is to refrain from reporting arity errors when we don't know enough about the relevant arities.https://gitlab.haskell.org/ghc/ghc/-/issues/14198Inconsistent treatment of implicitly bound kind variables as free-floating2020-12-28T09:20:27ZRyan ScottInconsistent treatment of implicitly bound kind variables as free-floating(Spun off from the discussion starting at https://phabricator.haskell.org/D3872#109927.)
This program is accepted:
```hs
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
import Data.Pro...(Spun off from the discussion starting at https://phabricator.haskell.org/D3872#109927.)
This program is accepted:
```hs
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
import Data.Proxy
data Foo = MkFoo (forall a. Proxy a)
```
There's something interesting going on here, however. Because `PolyKinds` is enabled, the kind of `a` is generalized to `k`. But where does `k` get quantified? It turns out that it's implicitly quantified as an existential type variable to `MkFoo`:
```
λ> :i Foo
data Foo = forall k. MkFoo (forall (a :: k). Proxy a)
```
This was brought up some time ago in #7873, where the conclusion was to keep this behavior. But it's strange becuase the `k` is existential, so the definition is probably unusable.
But to make things stranger, it you write out the kind of `a` explicitly:
```hs
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
import Data.Proxy
data Foo2 = MkFoo2 (forall (a :: k). Proxy a)
```
Then GHC will reject it:
```
Bug.hs:7:1: error:
Kind variable ‘k’ is implicitly bound in data type
‘Foo2’, but does not appear as the kind of any
of its type variables. Perhaps you meant
to bind it (with TypeInType) explicitly somewhere?
|
7 | data Foo2 = MkFoo2 (forall (a :: k). Proxy a)
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```
So GHC's treatment is inconsistent. What should GHC do? We could:
1. Have both be an error.
2. Have both be accepted, and implicitly quantify `k` as an existential type variable
3. Have both be accepted, and implicitly quantify `k` in the `forall` itself. That is:
```hs
MkFoo :: (forall {k} (a :: k). Proxy k a) -> Foo
```
4. Something else. When you try a similar trick with type synonyms:
```hs
{-# LANGUAGE ExistentialQuantification #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE RankNTypes #-}
import Data.Proxy
type Foo3 = forall a. Proxy a
```
Instead of generalizing the kind of `a`, its kind will default to `Any`:
```
λ> :i Foo3
type Foo3 = forall (a :: GHC.Types.Any). Proxy a
```
Would that be an appropriate trick for data types as well?https://gitlab.haskell.org/ghc/ghc/-/issues/14180Permit levity polymorphism in kinds2022-11-30T21:59:43ZDavid FeuerPermit levity polymorphism in kinds```hs
{-# language TypeInType, TypeFamilies, MagicHash #-}
import GHC.Exts
type family MatchInt (f :: Int) :: () where
MatchInt ('I# x) = '()
```
produces
```
<interactive>:2:59: error:
• Couldn't match a lifted type with an un...```hs
{-# language TypeInType, TypeFamilies, MagicHash #-}
import GHC.Exts
type family MatchInt (f :: Int) :: () where
MatchInt ('I# x) = '()
```
produces
```
<interactive>:2:59: error:
• Couldn't match a lifted type with an unlifted type
When matching kinds
k0 :: *
Int# :: TYPE 'IntRep
Expected kind ‘Int#’, but ‘x’ has kind ‘k0’
• In the first argument of ‘ 'I#’, namely ‘x’
In the first argument of ‘MatchInt’, namely ‘( 'I# x)’
In the type family declaration for ‘MatchInt’
```
If, however, I replace `x` in the pattern with `_`, the type checker is satisfied. If I give it a pattern signature,
```hs
MatchInt ('I# (x :: Int#)) = '()
```
I get a different (and equally unhelpful) error message,
```
• Expecting a lifted type, but ‘Int#’ is unlifted
• In the kind ‘Int#’
In the first argument of ‘ 'I#’, namely ‘(x :: Int#)’
In the first argument of ‘MatchInt’, namely ‘( 'I# (x :: Int#))’
```
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ----------------------- |
| Version | 8.3 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler (Type checker) |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Strange/bad error message binding unboxed type variable","status":"New","operating_system":"","component":"Compiler (Type checker)","related":[],"milestone":"8.2.2","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.3","keywords":["TypeInType"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"{{{#!hs\r\n{-# language TypeInType, TypeFamilies, MagicHash #-}\r\n\r\nimport GHC.Exts\r\n\r\ntype family MatchInt (f :: Int) :: () where\r\n MatchInt ('I# x) = '()\r\n}}}\r\n\r\nproduces\r\n\r\n{{{\r\n<interactive>:2:59: error:\r\n • Couldn't match a lifted type with an unlifted type\r\n When matching kinds\r\n k0 :: *\r\n Int# :: TYPE 'IntRep\r\n Expected kind ‘Int#’, but ‘x’ has kind ‘k0’\r\n • In the first argument of ‘ 'I#’, namely ‘x’\r\n In the first argument of ‘MatchInt’, namely ‘( 'I# x)’\r\n In the type family declaration for ‘MatchInt’\r\n}}}\r\n\r\nIf, however, I replace `x` in the pattern with `_`, the type checker is satisfied. If I give it a pattern signature,\r\n\r\n{{{#!hs\r\nMatchInt ('I# (x :: Int#)) = '()\r\n}}}\r\n\r\nI get a different (and equally unhelpful) error message,\r\n\r\n{{{\r\n • Expecting a lifted type, but ‘Int#’ is unlifted\r\n • In the kind ‘Int#’\r\n In the first argument of ‘ 'I#’, namely ‘(x :: Int#)’\r\n In the first argument of ‘MatchInt’, namely ‘( 'I# (x :: Int#))’\r\n}}}","type_of_failure":"OtherFailure","blocking":[]} -->https://gitlab.haskell.org/ghc/ghc/-/issues/14155GHC mentions unlifted types out of the blue (to me anyway)2020-01-23T19:31:01ZIcelandjackGHC mentions unlifted types out of the blue (to me anyway)This one does me 'ead in, I accidentally type `Ran` instead of `Swap`
```hs
{-# Language RankNTypes, DerivingStrategies, TypeApplications, ScopedTypeVariables, GADTs, GeneralizedNewtypeDeriving, InstanceSigs, PolyKinds #-}
import Data....This one does me 'ead in, I accidentally type `Ran` instead of `Swap`
```hs
{-# Language RankNTypes, DerivingStrategies, TypeApplications, ScopedTypeVariables, GADTs, GeneralizedNewtypeDeriving, InstanceSigs, PolyKinds #-}
import Data.Coerce
newtype Ran g h a = Ran (forall b. (a -> g b) -> h b)
newtype Swap p f g a where
Swap :: p g f a -> Swap p f g a
deriving newtype
Show
class IxPointed m where
ireturn :: a -> m i i a
instance IxPointed f => IxPointed (Swap f) where
ireturn :: forall a i. a -> Swap f i i a
ireturn a = Ran (ireturn a)
```
and get this error
```
$ ghci -ignore-dot-ghci /tmp/bug.hs
GHCi, version 8.3.20170605: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Main ( /tmp/bug.hs, interpreted )
/tmp/bug.hs:17:15: error:
• Couldn't match expected type ‘Swap f i i a’
with actual type ‘Ran g0 h0 a0’
• In the expression: Ran (ireturn a)
In an equation for ‘ireturn’: ireturn a = Ran (ireturn a)
In the instance declaration for ‘IxPointed (Swap f)’
• Relevant bindings include
a :: a (bound at /tmp/bug.hs:17:11)
ireturn :: a -> Swap f i i a (bound at /tmp/bug.hs:17:3)
|
17 | ireturn a = Ran (ireturn a)
| ^^^^^^^^^^^^^^^
/tmp/bug.hs:17:20: error:
• Couldn't match a lifted type with an unlifted type
Expected type: (a0 -> g0 b) -> h0 b
Actual type: (->) (a0 -> g0 b) (a0 -> g0 b) a
• In the first argument of ‘Ran’, namely ‘(ireturn a)’
In the expression: Ran (ireturn a)
In an equation for ‘ireturn’: ireturn a = Ran (ireturn a)
|
17 | ireturn a = Ran (ireturn a)
| ^^^^^^^^^
Failed, modules loaded: none.
```
Is GHC right to bring up unlifted types? I would guess this is due to the newly added levity polymorphism of `(->) :: TYPE rep -> TYPE rep' -> Type`
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | ------------ |
| Version | 8.2.1 |
| Type | Bug |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"GHC mentions unlifted types out of the blue (to me anyway)","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.2.1","keywords":[],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Bug","description":"This one does me 'ead in, I accidentally type `Ran` instead of `Swap` \r\n\r\n{{{#!hs\r\n{-# Language RankNTypes, DerivingStrategies, TypeApplications, ScopedTypeVariables, GADTs, GeneralizedNewtypeDeriving, InstanceSigs, PolyKinds #-}\r\n\r\nimport Data.Coerce\r\n\r\nnewtype Ran g h a = Ran (forall b. (a -> g b) -> h b)\r\n\r\nnewtype Swap p f g a where\r\n Swap :: p g f a -> Swap p f g a\r\n deriving newtype\r\n Show\r\n\r\nclass IxPointed m where\r\n ireturn :: a -> m i i a\r\n\r\ninstance IxPointed f => IxPointed (Swap f) where\r\n ireturn :: forall a i. a -> Swap f i i a\r\n ireturn a = Ran (ireturn a)\r\n}}}\r\n\r\nand get this error\r\n\r\n{{{\r\n$ ghci -ignore-dot-ghci /tmp/bug.hs\r\nGHCi, version 8.3.20170605: http://www.haskell.org/ghc/ :? for help\r\n[1 of 1] Compiling Main ( /tmp/bug.hs, interpreted )\r\n\r\n/tmp/bug.hs:17:15: error:\r\n • Couldn't match expected type ‘Swap f i i a’\r\n with actual type ‘Ran g0 h0 a0’\r\n • In the expression: Ran (ireturn a)\r\n In an equation for ‘ireturn’: ireturn a = Ran (ireturn a)\r\n In the instance declaration for ‘IxPointed (Swap f)’\r\n • Relevant bindings include\r\n a :: a (bound at /tmp/bug.hs:17:11)\r\n ireturn :: a -> Swap f i i a (bound at /tmp/bug.hs:17:3)\r\n |\r\n17 | ireturn a = Ran (ireturn a)\r\n | ^^^^^^^^^^^^^^^\r\n\r\n/tmp/bug.hs:17:20: error:\r\n • Couldn't match a lifted type with an unlifted type\r\n Expected type: (a0 -> g0 b) -> h0 b\r\n Actual type: (->) (a0 -> g0 b) (a0 -> g0 b) a\r\n • In the first argument of ‘Ran’, namely ‘(ireturn a)’\r\n In the expression: Ran (ireturn a)\r\n In an equation for ‘ireturn’: ireturn a = Ran (ireturn a)\r\n |\r\n17 | ireturn a = Ran (ireturn a)\r\n | ^^^^^^^^^\r\nFailed, modules loaded: none.\r\n}}}\r\n\r\nIs GHC right to bring up unlifted types? I would guess this is due to the newly added levity polymorphism of `(->) :: TYPE rep -> TYPE rep' -> Type`","type_of_failure":"OtherFailure","blocking":[]} -->Richard Eisenbergrae@richarde.devRichard Eisenbergrae@richarde.devhttps://gitlab.haskell.org/ghc/ghc/-/issues/14119Refactor type patterns2019-07-07T18:18:23ZRichard Eisenbergrae@richarde.devRefactor type patternsType patterns are used in instance heads: class instances, type instances, and data/newtype instances. These type patterns differ from ordinary types in several ways:
- They must never mention a `forall`.
- They must never mention a con...Type patterns are used in instance heads: class instances, type instances, and data/newtype instances. These type patterns differ from ordinary types in several ways:
- They must never mention a `forall`.
- They must never mention a context. (The context in a class instance head is a different part of the instance construct.)
- They must never mention a type family.
Types that appear as arguments on the LHS of a RULE should also be type patterns.
Type patterns are used in GHC differently than types as well: we should match only against patterns, never ordinary types.
I thus propose that a new datatype within GHC to store type patterns. I'll call it `TyPat` here, but perhaps a longer name is better. The matcher (in the `Unify` module) would then take a `TyPat` parameter, making clear which type is the template and which is the concrete instantiation.
We could have a new algorithm to typecheck/desugar source syntax into patterns. This new algorithm could accommodate issues like those mentioned in [ticket:14038\#comment:140786](https://gitlab.haskell.org//ghc/ghc/issues/14038#note_140786). It could also avoid ever putting a type family in a kind parameter, which would fix #12564. Separating out checking type patterns from proper types could also improve GADT pattern matching in types, which is currently more like "wobbly types" than OutsideIn.
<details><summary>Trac metadata</summary>
| Trac field | Value |
| ---------------------- | -------------- |
| Version | 8.2.1 |
| Type | Task |
| TypeOfFailure | OtherFailure |
| Priority | normal |
| Resolution | Unresolved |
| Component | Compiler |
| Test case | |
| Differential revisions | |
| BlockedBy | |
| Related | |
| Blocking | #12564, #14038 |
| CC | |
| Operating system | |
| Architecture | |
</details>
<!-- {"blocked_by":[],"summary":"Refactor type patterns","status":"New","operating_system":"","component":"Compiler","related":[],"milestone":"","resolution":"Unresolved","owner":{"tag":"Unowned"},"version":"8.2.1","keywords":["TypeInType"],"differentials":[],"test_case":"","architecture":"","cc":[""],"type":"Task","description":"Type patterns are used in instance heads: class instances, type instances, and data/newtype instances. These type patterns differ from ordinary types in several ways:\r\n\r\n * They must never mention a `forall`.\r\n * They must never mention a context. (The context in a class instance head is a different part of the instance construct.)\r\n * They must never mention a type family.\r\n\r\nTypes that appear as arguments on the LHS of a RULE should also be type patterns.\r\n\r\nType patterns are used in GHC differently than types as well: we should match only against patterns, never ordinary types.\r\n\r\nI thus propose that a new datatype within GHC to store type patterns. I'll call it `TyPat` here, but perhaps a longer name is better. The matcher (in the `Unify` module) would then take a `TyPat` parameter, making clear which type is the template and which is the concrete instantiation.\r\n\r\nWe could have a new algorithm to typecheck/desugar source syntax into patterns. This new algorithm could accommodate issues like those mentioned in comment:6:ticket:14038. It could also avoid ever putting a type family in a kind parameter, which would fix #12564. Separating out checking type patterns from proper types could also improve GADT pattern matching in types, which is currently more like \"wobbly types\" than OutsideIn.","type_of_failure":"OtherFailure","blocking":[12564,14038]} -->Richard Eisenbergrae@richarde.devRichard Eisenbergrae@richarde.dev