[GHC] #9881: GHCi gives misleading error message when looking up info of ambiguous type

12 Dec 2014

      #9881: GHCi gives misleading error message when looking up info of ambiguous type
-------------------------------------+-------------------------------------
       Reporter:  RyanGlScott        |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  GHCi               |                 Version:  7.8.3
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:  Other
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 When GHCi imports multiple functions with the same name, using {{{:i}}}
 results in an ambiguous occurrence error:

 {{{
...
import Data.Text
import Data.Text.Lazy
:i length
Top level:
     Ambiguous occurrence ‘length’
     It could refer to either ‘Data.Text.Lazy.length’,
                              imported from ‘Data.Text.Lazy’
                           or ‘Prelude.length’,
                              imported from ‘Prelude’ (and originally
 defined in ‘GHC.List’)
                           or ‘Data.Text.length’, imported from ‘Data.Text’
 }}}

 However, if you use {{{:i}}} on a type name that has imported multiple
 times in GHCi, a misleading error is given:

 {{{
...
import Data.Text
import Data.Text.Lazy
:i Text
Top level: Not in scope: data constructor ‘Text’
 }}}

 The real problem is that the type name {{{Text}}} was imported twice with
 two different definitions, so the error should probably look something
 like this:

 {{{
 Top level:
     Ambiguous occurrence ‘Text’
     It could refer to either ‘Data.Text.Lazy.Text’,
                              imported from ‘Data.Text.Lazy’ (and
 originally defined in ‘Data.Text.Internal.Lazy’)
                           or ‘Data.Text.Text’,
                              imported from ‘Data.Text’ (and originally
 defined in ‘Data.Text.Internal‘)
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9881
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler