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

[Erik Hesselink]

On Tue, Oct 4, 2011 at 20:33, Karel Gardas <karel.gardas at centrum.cz> 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