[GHC] #14106: Out of scope errors appear after type errors caused by them

[GHC]

#14106: Out of scope errors appear after type errors caused by them
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        Reporter:  EyalLotem         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by simonpj:

Old description:

> Example:
>
>   import Control.Lens (_Just, (&))
>   main = Just 5 & _Just .~ 100 & print
>
> .~ is out of scope, the error output:
>
> ghc-error.hs:2:17-21: error: …
>     • Couldn't match type ‘Maybe’ with ‘p0 a0’
>       Expected type: Maybe (f0 b0) -> p0 (Maybe a0) (f0 (Maybe b0))
>         Actual type: p0 a0 (f0 b0) -> p0 (Maybe a0) (f0 (Maybe b0))
>     • In the second argument of ‘(&)’, namely ‘_Just’
>       In the first argument of ‘(.~)’, namely ‘Just 5 & _Just’
>       In the expression: (.~) Just 5 & _Just 100 & print
> ghc-error.hs:2:23-24: error: …
>     • Variable not in scope:
>         (.~) :: p0 (Maybe a0) (f0 (Maybe b0)) -> IO () -> t
>     • Perhaps you meant ‘.’ (imported from Prelude)
>       Perhaps you want to add ‘.~’ to the import list in the import of
>       ‘Control.Lens’ (/home/eyal/devel/test/ghc-error.hs:1:1-32).
> Compilation failed.
>
> In larger examples, the out of scope error can be buried deep down.
>
> In the case of operators - the fixity is unknown so it can even cause the
> parse to go wrong - and very weird type errors to result from that.
>
> Out of scope errors should be put BEFORE any type errors that might be
> caused by them.

New description:

 Example:
 {{{
   import Control.Lens (_Just, (&))
   main = Just 5 & _Just .~ 100 & print
 }}}
 .~ is out of scope, the error output:
 {{{
 ghc-error.hs:2:17-21: error: …
     • Couldn't match type ‘Maybe’ with ‘p0 a0’
       Expected type: Maybe (f0 b0) -> p0 (Maybe a0) (f0 (Maybe b0))
         Actual type: p0 a0 (f0 b0) -> p0 (Maybe a0) (f0 (Maybe b0))
     • In the second argument of ‘(&)’, namely ‘_Just’
       In the first argument of ‘(.~)’, namely ‘Just 5 & _Just’
       In the expression: (.~) Just 5 & _Just 100 & print
 ghc-error.hs:2:23-24: error: …
     • Variable not in scope:
         (.~) :: p0 (Maybe a0) (f0 (Maybe b0)) -> IO () -> t
     • Perhaps you meant ‘.’ (imported from Prelude)
       Perhaps you want to add ‘.~’ to the import list in the import of
       ‘Control.Lens’ (/home/eyal/devel/test/ghc-error.hs:1:1-32).
 Compilation failed.
 }}}
 In larger examples, the out of scope error can be buried deep down.

 In the case of operators - the fixity is unknown so it can even cause the
 parse to go wrong - and very weird type errors to result from that.

 Out of scope errors should be put BEFORE any type errors that might be
 caused by them.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14106#comment:1>
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