[GHC] #4017: Unhelpful error message in GHCi

[GHC]

#4017: Unhelpful error message in GHCi
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  supersven
            Type:  bug               |               Status:  new
        Priority:  lowest            |            Milestone:
       Component:  Compiler          |              Version:  6.12.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by supersven):

 Looks like things (or symptoms) changed a bit over time - The problem
 seems to be the same.

 Starting a current version of GHC in interactive mode:
 {{{
 [nix-shell:~/src/ghc]$ inplace/bin/ghc-stage2 --interactive
 GHCi, version 8.3.20170930: http://www.haskell.org/ghc/  :? for help
 }}}

 `:i GHC.Integer.Type.Integer` now responds with a "Not in scope" error.
 `:i Integer` still shows that `GHC.Integer.Type.Integer` is reachable:
 {{{
 Prelude> :i GHC.Integer.Type.Integer

 <interactive>:1:1: error: Not in scope: ‘GHC.Integer.Type.Integer’
 Prelude> :i Integer
 data Integer
   = integer-gmp-1.0.1.0:GHC.Integer.Type.S# GHC.Prim.Int#
   | integer-gmp-1.0.1.0:GHC.Integer.Type.Jp# {-# UNPACK #-}integer-
 gmp-1.0.1.0:GHC.Integer.Type.BigNat
   | integer-gmp-1.0.1.0:GHC.Integer.Type.Jn# {-# UNPACK #-}integer-
 gmp-1.0.1.0:GHC.Integer.Type.BigNat
         -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
 instance Eq Integer
   -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
 instance Ord Integer
   -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
 instance Show Integer -- Defined in ‘GHC.Show’
 instance Read Integer -- Defined in ‘GHC.Read’
 instance Enum Integer -- Defined in ‘GHC.Enum’
 instance Num Integer -- Defined in ‘GHC.Num’
 instance Real Integer -- Defined in ‘GHC.Real’
 instance Integral Integer -- Defined in ‘GHC.Real’
 }}}

 Setting the "integer-gmp" package doesn't change the situation:
 {{{
 Prelude> :set -package integer-gmp
 package flags have changed, resetting and loading new packages...
 Prelude> :i Integer
 data Integer
   = integer-gmp-1.0.1.0:GHC.Integer.Type.S# GHC.Prim.Int#
   | integer-gmp-1.0.1.0:GHC.Integer.Type.Jp# {-# UNPACK #-}integer-
 gmp-1.0.1.0:GHC.Integer.Type.BigNat
   | integer-gmp-1.0.1.0:GHC.Integer.Type.Jn# {-# UNPACK #-}integer-
 gmp-1.0.1.0:GHC.Integer.Type.BigNat
         -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
 instance Eq Integer
   -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
 instance Ord Integer
   -- Defined in ‘integer-gmp-1.0.1.0:GHC.Integer.Type’
 instance Show Integer -- Defined in ‘GHC.Show’
 instance Read Integer -- Defined in ‘GHC.Read’
 instance Enum Integer -- Defined in ‘GHC.Enum’
 instance Num Integer -- Defined in ‘GHC.Num’
 instance Real Integer -- Defined in ‘GHC.Real’
 instance Integral Integer -- Defined in ‘GHC.Real’
 Prelude> :i GHC.Integer.Type.Integer

 <interactive>:1:1: error: Not in scope: ‘GHC.Integer.Type.Integer’
 }}}

 `GHC.Integer.Type` is still a hidden module:
 {{{
 Prelude> import GHC.Integer.Type

 <no location info>: error:
     Could not find module ‘GHC.Integer.Type’
     it is a hidden module in the package ‘integer-gmp-1.0.1.0’
 }}}

 === Summary ===
 The error message of `:i GHC.Integer.Type.Integer` changed to "Not in
 scope". But `:i Integer` still shows that the requested information would
 be available.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/4017#comment:15>
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