User output should not show eta-contracted data instances
Consider
data family Sing (a :: k)
data instance Sing (a :: [k]) = SNil
data instance Sing Bool = SBool
If you load this into ghci and say :info Sing
you get
:i Sing
type role Sing nominal
data family Sing (a :: k)
-- Defined at T8557.hs:4:1
data instance Sing Bool -- Defined at T8557.hs:6:15
data instance Sing -- Defined at T8557.hs:5:15
The data instance
is eta-contracted (see Note [Eta reduction for data family axioms]
in TcInstDcls
).
This is jolly confusing for our users. We should eta-expand before displaying.
Trac metadata
Trac field | Value |
---|---|
Version | 7.6.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |