Type error suppresses defaulting, causes spurious errors
|Reported by:||jun0||Owned by:|
|Type of failure:||None/Unknown||Test Case:||typecheck/should_fail/T9033|
|Related Tickets:||Differential Rev(s):|
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.