Bad error message around lack of impredicativity
(Broken out from #12557 (closed).)
Consider
foo :: ((forall a. f a) -> f r) -> f r
foo g = undefined
x = \g -> foo g
Compiling this code produces
Scratch.hs:24:15: error:
• Couldn't match expected type ‘(forall (a :: k). f a) -> f r’
with actual type ‘t’
‘t’ is a rigid type variable bound by
the inferred type of x :: t -> f r at Scratch.hs:24:1-15
• In the first argument of ‘foo’, namely ‘g’
In the expression: foo g
In the expression: \ g -> foo g
• Relevant bindings include
g :: t (bound at Scratch.hs:24:6)
x :: t -> f r (bound at Scratch.hs:24:1)
The code should be rejected. But, really, the problem is that the type of a lambda-bound variable can't be a polytype (unless the type is obvious at the binding site). We should improve this error message.
Trac metadata
Trac field | Value |
---|---|
Version | 8.0.1 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |