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

[Antoine Latter]

On Tue, Oct 4, 2011 at 1:33 PM, Karel Gardas <karel.gardas at centrum.cz> wrote:
>
> Hello,
>
> thanks a lot to Edward, Jose, Ryan and Stephen for fast reply in this
> thread. I see I've not been that precise in specification of what I need
> exactly so I need to add this: I've changed a little bit definition of
> person to:
>
> 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
>

You could use Data.Proxy and pass a type proxy as the first argument:

> emit_proto (Proxy :: Proxy MyType) 1

It seems like you could then do something like `dataTypeOf
(asProxyTypeOf undefined proxy)` to unpack the 'Data' instance of the
proxied type. There might be a better way, but this is just what I can
think of off the top of my head.

The 'Poxy' type is here:

http://hackage.haskell.org/packages/archive/tagged/0.2.3.1/doc/html/Data-Proxy.html

Antoine