Re: [Haskell-cafe] DSL for data definition (e.g. compiling Haskell type defs into Google's protocol buffers type defs)

On Tue, Oct 4, 2011 at 20:33, Karel Gardas wrote:

data PersonType = Person {        id :: Int        , name :: String        , email :: Maybe String        }        deriving (Show, Data, Typeable)

so I have `PersonType' as type constructor and Person as value constructor (or data constructor) -- speaking using terms from Real World Haskell, Chapter 3[1]. And now I see that none of typeOf/dataTypeOf/toContr is applicable to *type constructor* but all are applicable to *value/data constructor*. Ditto happen when testing Color versus RED, GREEN, BLUE. At least GHCi complains this way:

*Main> typeOf Color

<interactive>:0:8: Not in scope: data constructor `Color' *Main> typeOf PersonType

<interactive>:0:8: Not in scope: data constructor `PersonType'

But, I'd like to start processing of data definition from the *type constructor*. So:

emit_proto PersonType 1 emit_proto Color 1

Is that possible at all? I mean in the scope/context of GHC's Data/Data.Data/Data.Typeable etc. modules. (w/o considering TH now).

A definition of 'typeOf' is not supposed to use its argument, since the normal way to call it is to pass undefined. The documentation says: "The value of the argument should be ignored by any instance of Typeable, so that it is safe to pass undefined as the argument. " So you should call it like: typeOf (undefined :: PersonType). Erik