Bad error message using poly pat bind with MonoPatBinds
This program:
foo :: a -> a
bar :: a -> a
(foo, bar) = (\x -> x, \y -> y)
main = print $ foo $ bar 1
Is trying to pattern-bind a polymorphic function. The error message is dreadful:
/Users/mbolingbroke/Junk/PolyPatBinds.hs:5:15:
Couldn't match expected type `t -> t1'
with actual type `forall a. a -> a'
The lambda expression `\ x -> x' has one argument one argument,
but its type `forall a. a -> a' has none
In the expression: \ x -> x
In the expression: (\ x -> x, \ y -> y)
The issues are:
- It repeats itself: "one argument one argument"
- It is patently incorrect: the type forall a. a -> a DOES have one argument in the standard nomenclature
- It does not point the user towards the fix (-XNoMonoPatBinds)
Trac metadata
Trac field | Value |
---|---|
Version | 7.0.1 |
Type | FeatureRequest |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |