Re: [Haskell-cafe] Is it possible to print out types of instances in scope

16 Jan 2005

      At Sun, 16 Jan 2005 12:09:43 +0100,
Lemmih wrote:
...
On Sun, 16 Jan 2005 00:13:08 -0800, Jeremy Shaw
 wrote:
...
Hello,
Is it possible to write a function that would display all the instances
of a class currently in scope[1]?
For example, for 'Show a' it should output something like:
The instances of 'Show a' currently in scope are:
Show Int
Show Float
Show Char
Show ...
In GHC 6.3:
Prelude> :i Show
class Show a where
  showsPrec :: Int -> a -> String -> String
  show :: a -> String
  showList :: [a] -> String -> String
-- Imported from `GHC.Show'
instance Show IOMode    -- Imported from `GHC.IOBase'
instance Show IOException       -- Imported from `GHC.IOBase'
instance Show IOErrorType       -- Imported from `GHC.IOBase'
instance Show HandleType        -- Imported from `GHC.IOBase'
[...]
Neat! Is there some way I can get access to that information from
within my program ? Maybe something like:

import Language.Haskell.TH.Syntax
instancesOf :: Name -> [Type]

thanks!
Jeremy Shaw.