Re: #16358: Confusing ghci error message

#16358: Confusing ghci error message
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        Reporter:  lerkok            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Description changed by lerkok:

Old description:

> {{{
> $ ghci
> GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
> Prelude> import qualified Data.Set as S
> Prelude S> S.foo
>
> <interactive>:2:1: error:
>     Not in scope: ‘S.foo’
>     No module named ‘S’ is imported.
> }}}
>
> I'm curious why `ghci` says `No module named 'S' is imported`. I found it
> very confusing.
>
> I'd have expected it to say one of three things:
>
> 1. `Module Data.Set referenced via S does not export foo`
> 2. `Constructor S not in scope`
> 3. `Variable foo not in scope`
>
> The first one would be the most informative, but I can see that the
> second and the third messages might be quite reasonable as well due to
> the interpretation of dot as composition. But the message `No module S is
> imported` is quite confusing given I just issued `import qualified
> Data.Set as S`.

New description:

 {{{
 $ ghci
 GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
 Prelude> import qualified Data.Set as S
 Prelude S> S.foo

 <interactive>:2:1: error:
     Not in scope: ‘S.foo’
     No module named ‘S’ is imported.
 }}}

 I'm curious why `ghci` says `No module named 'S' is imported`. I found it
 very confusing.

 I'd have expected it to say one of three things:

 1. `Module Data.Set referenced via S does not export foo`
 2. `Constructor S not in scope`
 3. `Variable foo not in scope`

 The first one would be the most informative, but I can see that the second
 and the third messages might be quite reasonable as well due to the
 interpretation of dot as composition. But the message `No module S is
 imported` is quite confusing given I just issued `import qualified
 Data.Set as S`.

 Indeed, if I put this in a file, the error message is crystal clear:

 {{{#!hs
 $ cat a.hs
 import qualified Data.Set as S

 bar = S.foo

 $ ghci a.hs
 GHCi, version 8.6.3: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Main             ( a.hs, interpreted )

 a.hs:3:7: error:
     Not in scope: ‘S.foo’
     Module ‘Data.Set’ does not export ‘foo’.
   |
 3 | bar = S.foo
   |       ^^^^^
 Failed, no modules loaded.
 }}}

 I'm curious why there's a difference between these two scenarios.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16358#comment:1>