[ ghc-Bugs-1106171 ] GHCi does not find instances

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Bugs item #1106171, was opened at 2005-01-20 10:20
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Category: Compiler
Group: 6.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: GHCi does not find instances

Initial Comment:
This report concerns GHC HEAD (6.3) as downloaded from
CVS around 2PM on 20 Jan 2005.

It seems GHCi does not find some instance declarations.

Consider the following trivial example:

module Foo where
 
data Foo = Foo
 
instance Show Foo where
    show Foo = "Foo"

When loaded into GHCi, regardless of whether it is
first compiled or not, GHCi claims that Foo is not an
instance
of Show. E.g.

Prelude> :load Foo
Compiling Foo              ( Foo.hs, interpreted )
Ok, modules loaded: Foo.
*Foo> show Foo
 
<interactive>:1:0:
    No instance for (Show Foo)
      arising from use of `show' at <interactive>:1:0-3
    Probable fix: add an instance declaration for (Show
Foo)
    In the definition of `it': it = show Foo
*Foo> :i Foo
data Foo = Foo
-- Defined at Foo.hs:3:5

But Foo is plainly an instance of Show.

GHC itself has no problems with this
(of course).

Best,

/Henrik

e-mail: nhn at cs.nott.ac.uk


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