[Haskell-beginners] Example using gmapQ

[Dilawar Singh]

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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