Re: [GHC] #4099: Slighlty confusing error message with data families
#4099: Slighlty confusing error message with data families
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Reporter: rl | Owner:
Type: bug | Status: closed
Priority: normal | Milestone:
Component: Compiler |Version: 6.13
Resolution: fixed| Keywords:
Difficulty: | Os:
Unknown/Multiple
Testcase: indexed-types/should_fail/T4099 | Architecture:
Unknown/Multiple
Failure: None/Unknown |
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Changes (by simonpj):
* status: new => closed
* testcase: => indexed-types/should_fail/T4099
* resolution: => fixed
Comment:
Yes good point. Fixed by
{{{
Mon May 31 15:04:13 BST 2010 simo...@microsoft.com
* Fix Trac #4099: better error message for type functions
Now we only want about "T is a type function and might not be
injective" when matchin (T x) against (T y), which is the case
that is really confusing.
M ./compiler/typecheck/TcTyFuns.lhs -14 +9
}}}
--
Ticket URL:
GHC
[GHC] #4099: Slighlty confusing error message with data families
#4099: Slighlty confusing error message with data families
-+--
Reporter: rl| Owner:
Type: bug | Status: new
Priority: normal| Component: Compiler
Version: 6.13 |Keywords:
Os: Unknown/Multiple |Testcase:
Architecture: Unknown/Multiple | Failure: None/Unknown
-+--
Example code:
{{{
data family T a
foo :: T a -> a
foo (Just x) = x
}}}
Error message:
{{{
Couldn't match expected type `T a' against inferred type `Maybe a1'
NB: `T' is a type function
In the pattern: Just x
In the definition of `foo': foo (Just x) = x
}}}
Calling `T` a type function is confusing, I had to double check that I
hadn't accidentally declared it as a type family. I would
* call these things type families and data families in messages and
* omit the message for data families altogether because it doesn't
matter that `T` is one in this case.
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Ticket URL:
GHC
