"Illegal constraint in a type" error  is it fixable?
I recently sketched out a solution to #13327. Here is the type signature that I wanted to write:
dataCast1 :: forall (c :: Type > Type) (t :: forall (k :: Type). k > Type).
Typeable t
=> (forall d. Data d => c (t d))
> Maybe (c a)
But this doesn't typecheck:
• Could not deduce (Typeable (t k0))
from the context: (Data a, Typeable (t k))
bound by the type signature for:
dataCast1 :: forall a.
Data a =>
forall k (c :: * > *) (t :: forall k1. k1 > *).
Typeable (t k) =>
(forall d. Data d => c (t * d)) > Maybe (c a)
at NewData.hs:(170,3)(173,26)
The type variable ‘k0’ is ambiguous
• In the ambiguity check for ‘dataCast1’
To defer the ambiguity check to use sites, enable AllowAmbiguousTypes
When checking the class method:
dataCast1 :: forall a.
Data a =>
forall k (c :: * > *) (t :: forall k1. k1 > *).
Typeable (t k) =>
(forall d. Data d => c (t * d)) > Maybe (c a)
In the class declaration for ‘Data’

170  dataCast1 :: forall (c :: Type > Type) (t :: forall (k :: Type). k > Type).
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^...
This makes sense, since GHC has no way to conclude that the k
in t
's kind is also Typeable
. I tried to convince GHC of that fact:
dataCast1 :: forall (c :: Type > Type) (t :: forall (k :: Type). Typeable k => k > Type).
Typeable t
=> (forall d. Data d => c (t d))
> Maybe (c a)
But this also doesn't work:
NewData.hs:171:25: error:
• Illegal constraint in a type: Typeable k0
• In the first argument of ‘Typeable’, namely ‘t’
In the type signature:
dataCast1 :: forall (c :: Type > Type)
(t :: forall (k :: Type). Typeable k => k > Type).
Typeable t => (forall d. Data d => c (t d)) > Maybe (c a)
In the class declaration for ‘Data’

171  Typeable t
 ^
NewData.hs:172:40: error:
• Illegal constraint in a type: Typeable k0
• In the first argument of ‘c’, namely ‘(t d)’
In the type signature:
dataCast1 :: forall (c :: Type > Type)
(t :: forall (k :: Type). Typeable k => k > Type).
Typeable t => (forall d. Data d => c (t d)) > Maybe (c a)
In the class declaration for ‘Data’

172  => (forall d. Data d => c (t d))
 ^^^
At this point, I'm stuck, since I have no idea how to work around this Illegal constraint in a type
error. This error message appears to have originated as a part of the TypeInType
patch, since there's even a test case checking for this behavior.
But is this a fundamental limitation of kind equalities? Or would it be possible to lift this restriction?
Trac metadata
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 
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