Type error suppresses defaulting, causes spurious errors
I have a strong feeling this must have been reported already but I can't find a ticket for it, so here goes.
If a source file contains a type error, like
bad :: Bool
bad = ()
then GHC seems to turn off defaulting for ambiguous types in the remainder of the file. So if the same file contains for instance
square :: Integral i => i -> i
square x = x^2
then GHC reports another type error at the ^2:
test.hs:4:7:
Couldn't match expected type ‘Bool’ with actual type ‘()’
In the expression: ()
In an equation for ‘bad’: bad = ()
test.hs:7:13:
Could not deduce (Integral b0) arising from a use of ‘^’
from the context (Integral i)
bound by the type signature for square :: Integral i => i -> i
at test.hs:6:11-30
The type variable ‘b0’ is ambiguous
Note: there are several potential instances:
instance Integral Int -- Defined in ‘GHC.Real’
instance Integral Integer -- Defined in ‘GHC.Real’
instance Integral GHC.Types.Word -- Defined in ‘GHC.Real’
In the expression: x ^ 2
In an equation for ‘square’: square x = x ^ 2
test.hs:7:14:
Could not deduce (Num b0) arising from the literal ‘2’
from the context (Integral i)
bound by the type signature for square :: Integral i => i -> i
at test.hs:6:11-30
The type variable ‘b0’ is ambiguous
Note: there are several potential instances:
instance Num Double -- Defined in ‘GHC.Float’
instance Num Float -- Defined in ‘GHC.Float’
instance Integral a => Num (GHC.Real.Ratio a)
-- Defined in ‘GHC.Real’
...plus three others
In the second argument of ‘(^)’, namely ‘2’
In the expression: x ^ 2
In an equation for ‘square’: square x = x ^ 2
The first error is a legitimate type mismatch and it's all fine, but the other two should be resolved with default instance selection -- and it is, once the type error in bad
is fixed. For example, rewriting bad
to
bad :: ()
bad = ()
makes *all* errors go away, including the ones on square
.
Default instance selection seems to be turned back on when square is made monomorphic, like
squareMono :: Int -> Int
squareMono x = x^2
Note that the type of x is being fixed, not the 2 that GHC complains about in square. The effect of this is that in a file with a large number of ^2
, ^3
, and such, the error message pointing to the real type error is drowned in all the "could not deduce" messages.
Trac metadata
Trac field | Value |
---|---|
Version | 7.8.2 |
Type | Bug |
TypeOfFailure | OtherFailure |
Priority | normal |
Resolution | Unresolved |
Component | Compiler |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |