Typechecking error: "Could not deduce (m ~ m1)..."
This program:
import Control.Monad
f x = let
g y = let
h z = liftM not x
in h 0
in g 0
is expected to compile. Instead, it produces an error:
test.hs:5:21:
Could not deduce (m ~ m1)
from the context (Monad m)
bound by the inferred type of f :: Monad m => m Bool -> m Bool
at test.hs:(3,1)-(7,8)
or from (m Bool ~ m1 Bool, Monad m1)
bound by the inferred type of
h :: (m Bool ~ m1 Bool, Monad m1) => t1 -> m1 Bool
at test.hs:5:5-21
`m' is a rigid type variable bound by
the inferred type of f :: Monad m => m Bool -> m Bool
at test.hs:3:1
`m1' is a rigid type variable bound by
the inferred type of
h :: (m Bool ~ m1 Bool, Monad m1) => t1 -> m1 Bool
at test.hs:5:5
Expected type: m1 Bool
Actual type: m Bool
In the second argument of `liftM', namely `x'
In the expression: liftM not x
In an equation for `h': h z = liftM not x
Trac metadata
Trac field | Value |
---|---|
Version | 7.6.3 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler (Type checker) |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |