Opened 18 months ago
Closed 18 months ago
#9033 closed bug (fixed)
Type error suppresses defaulting, causes spurious errors
Reported by: | jun0 | Owned by: | |
---|---|---|---|
Priority: | normal | Milestone: | |
Component: | Compiler | Version: | 7.8.2 |
Keywords: | defaulting, ambiguous | Cc: | |
Operating System: | Unknown/Multiple | Architecture: | Unknown/Multiple |
Type of failure: | None/Unknown | Test Case: | typecheck/should_fail/T9033 |
Blocked By: | Blocking: | ||
Related Tickets: | Differential Rev(s): |
Description
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.
Attachments (1)
Change History (3)
Changed 18 months ago by jun0
comment:1 Changed 18 months ago by Simon Peyton Jones <simonpj@…>
comment:2 Changed 18 months ago by simonpj
- Resolution set to fixed
- Status changed from new to closed
- Test Case set to typecheck/should_fail/T9033
Good point, thank you!
Simon
In ba2e20149e2addaccf5ce3122d3a6e93da696a0a/ghc: