[GHC] #10021: Add "Error:" prefix for compile-time error messages (was: Add "Error:" prefix for error-messages)

[GHC]

#10021: Add "Error:" prefix for compile-time error messages
-------------------------------------+-------------------------------------
        Reporter:  k-bx              |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:  7.12.1
       Component:  Compiler          |                 Version:  7.8.4
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
      Blocked By:                    |               Test Case:
 Related Tickets:                    |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
Description changed by k-bx:

Old description:

> Similar to what we have for "Warning:".
>
> E.g., we have:
>
> {{{
> ➜ cat test.hs
> main = putStrLn "Hello!"
> ➜ ghc --make -fforce-recomp -Wall ./test.hs
> [1 of 1] Compiling Main             ( test.hs, test.o )
>
> test.hs:1:1: Warning:
>     Top-level binding with no type signature: main :: IO ()
> Linking test ...
> ➜ cat test2.hs
> main = foo
> ➜ ghc --make -fforce-recomp -Wall ./test2.hs
> [1 of 1] Compiling Main             ( test2.hs, test2.o )
>
> test2.hs:1:8: Not in scope: ‘foo’
> }}}
>
> And I propose having:
>
> {{{
> ➜ cat test2.hs
> main = foo
> ➜ ghc --make -fforce-recomp -Wall ./test2.hs
> [1 of 1] Compiling Main             ( test2.hs, test2.o )
>
> test2.hs:1:8: Error: Not in scope: ‘foo’
> }}}
>
> This proposal only affects error messages produced by GHC itself.  For
> example, the behaviour of the `error` function is unaffected.
>
> == Motivation ==
>
> Main motivation point right now is that there's a ticket #9219 (parallel
> build's output of errors doesn't occur at the end of output), due to
> which, error-reporting of your program might get lost somewhere in the
> middle of a build-log. As another example, if your program has a lot of
> warnings, and you build it with multiple cores, you almost always get an
> error lost somewhere in the middle of warnings and it's almost impossible
> to find.
>
> So, this feature's main purpose is to ease navigation in textual output
> of ghc in order to find errors quickly, not being tied to output
> ordering.
>
> == Disadvantages ==
>
> Many regression tests might need a fix.

New description:

 The proposal is to modify GHC compile-time output for error-messages, by
 adding a prefix, similar to what we currently have for warnings.

 An example of a warning message (see `Warning:` prefix):

 {{{
 ➜ cat test.hs
 main = putStrLn "Hello!"
 ➜ ghc --make -fforce-recomp -Wall ./test.hs
 [1 of 1] Compiling Main             ( test.hs, test.o )

 test.hs:1:1: Warning:
     Top-level binding with no type signature: main :: IO ()
 Linking test ...
 }}}

 An example of current output for an error:

 {{{
 ➜ cat test2.hs
 main = foo
 ➜ ghc --make -fforce-recomp -Wall ./test2.hs
 [1 of 1] Compiling Main             ( test2.hs, test2.o )

 test2.hs:1:8: Not in scope: ‘foo’
 }}}

 The proposal is to change it to become:

 {{{
 ➜ cat test2.hs
 main = foo
 ➜ ghc --make -fforce-recomp -Wall ./test2.hs
 [1 of 1] Compiling Main             ( test2.hs, test2.o )

 test2.hs:1:8: Error: Not in scope: ‘foo’
 }}}

 This proposal only affects error messages produced by GHC itself.  For
 example, the behaviour of the `error` function is unaffected.

 == Motivation ==

 Main motivation point right now is that there's a ticket #9219 (parallel
 build's output of errors doesn't occur at the end of output), due to
 which, error-reporting of your program might get lost somewhere in the
 middle of a build-log. As another example, if your program has a lot of
 warnings, and you build it with multiple cores, you almost always get an
 error lost somewhere in the middle of warnings and it's almost impossible
 to find.

 So, this feature's main purpose is to ease navigation in textual output of
 ghc in order to find errors quickly, not being tied to output ordering.

 == Disadvantages ==

 Many regression tests might need a fix.

 == Additional consideration ==

 @hvr proposed that during work on this bug we could make GHC output more
 consistent with other compiler's (GCC and clang) by also lower-casing
 "Error" and "Warning" prefixes.

 This might be potentially useful for existing tools that work on top of
 compiler output for finding errors/warning, and GHC's output will become
 "more valid" for them.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10021#comment:6>
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