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I first played around with closed typefamilies early 2013 and wrote up the following simple example

{-# LANGUAGE ConstraintKinds, DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}
 
import GHC.Prim (Constraint)
 
type family Restrict (a :: k) (as :: [k]) :: Constraint
type instance where
    Restrict a (a ': as) = (a ~ "Oops! Tried to apply a restricted type!")
    Restrict x (a ': as) = Restrict x as
    Restrict x '[] = ()
 
foo :: Restrict a '[(), Int] => a -> a
foo = id

This worked fine, functioning like id for types other than () and Int and returning a type error for () and Int.

After hearing comments that my example broke when the closed type families syntax changed I decided to update my version of 7.7 and change the code for that. The new code is:

{-# LANGUAGE ConstraintKinds, DataKinds, PolyKinds, TypeFamilies, TypeOperators #-}
 
import GHC.Prim (Constraint)
 
type family Restrict (a :: k) (as :: [k]) :: Constraint where
    Restrict a (a ': as) = (a ~ "Oops! Tried to apply a restricted type!")
    Restrict x (a ': as) = Restrict x as
    Restrict x '[] = ()
 
foo :: Restrict a '[(), Int] => a -> a
foo = id

This code is accepted by the compiler just fine, but the Constraint gets thrown out. When querying ghci for the type of foo the following is returned:

λ :t foo
foo :: a -> a
λ :i foo
foo :: (Restrict a '[(), Int]) => a -> a

Additionally, in the recent 7.7 foo () returns () rather than a type error. After some playing around this seems to be caused by the "(a ~ "Oops! Tried to apply a restricted type!")" equality constraint. It seems GHC decides that it doesn't like the fact that types with a polymorphic kind and Symbol kind are compared. Leading it to silently discard the Constraint.

This raises two issues:

1. This should probably produce an error, rather than silently discarding the Constraint
2. A better way to produce errors in type families is needed, personally I would love an "error" Constraint that takes a String/Symbol and never holds, producing it's argument String as type error (This would remove the need for the hacky unification with Symbol to get a somewhat relevant type error).

Trac metadata

Trac field	Value
Version	7.7
Type	Bug
TypeOfFailure	OtherFailure
Priority	high
Resolution	Unresolved
Component	Compiler
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