Comment out the last line (main = etc...) and do this in ghci,
  :t (show thing)

Type of (show thing) is String. This is input to gmapQ function which has
following type.

  :t gmapQ   gmapQ :: Data a => (forall d. Data d => d -> u) -> a -> [u]

First argument to this function has the type (d -> u). Surely, this can't be String.

--
Dilawar EE, IITB

On Sat, Jul 13, 2013 at 04:48:49PM +0800, Adrian May wrote:

Hi All,

I got this far:

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE DeriveDataTypeable #-}
import Data.Text
import Data.Typeable
import Data.Data

data Thing = Thing { foo :: Int, bar :: String}
  deriving (Read, Show, Typeable, Data)
thing :: Thing
thing = Thing 1 "wop"
con = toConstr thing
fields = constrFields con
main = putStrLn $ show con ++ show fields ++ ( Prelude.concat $ gmapQ show
thing )

But it's barfing like this:

   Could not deduce (Show d) arising from a use of `show'
   from the context (Data d)
     bound by a type expected by the context: Data d => d -> [Char]
     at w.hs:76:65-80

I can see why, but not how to fix it.

Any help much appreciated,
Adrian.

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