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

Hi Karel, You can use SYB's toConstr/dataTypeOf [1] to obtain information about the name of the constructor and datatype. Alternatively, you can also use the new generic programming framework of ghc-7.2 [2]. Cheers, Pedro [1] http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Data.h... [2] http://haskell.org/ghc/docs/latest/html/libraries/ghc-prim-0.2.0.0/GHC-Gener... On Tue, Oct 4, 2011 at 17:02, Karel Gardas wrote:

Hello,

I'm trying to find out if it's possible to use Haskell data type definition capability to define types and compile defined types into other languages, for example into Google's protocol buffers data definition language. So basically speaking I'm thinking about using Haskell sub-set as a data-definition DSL together with some functions which will generate some code based on supplied defined data types. My idea is:

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

emit_proto Person 1

where emit_proto is function which will translate Person data type definition into Google's proto language (the 1 is index from which start to index type's fields) by traversing data type definition and translating all its children plus do some header/footer generation etc:

message Person { required int32 id = 1; required string name = 2; optional string email = 3; }

I've looked for something like that and found SYB papers which works on top of data instance (i.e. actual data, not data type). I also found JSON lib which again works on top of data and not data type. I've tried to look into Data.Typetable etc, but have not found function which will print data type's field name and field type name (although JSON lib seems to use field name for JSON generation so I'll need to investigate this more). I've tested `typeOf' function and it's quite useful, but its limitation is that it's not working on ADT name:

data Color = RED|GREEN|BLUE

*Main> typeOf Color

<interactive>:1:8: Not in scope: data constructor `Color'

*Main> typeOf RED Main.Color

and I would need that in order to translate Color defined above into enum like:

enum Color { RED = 0; GREEN = 1; BLUE = 2; }

My question is: do you think I'm looking into good direction (i.e. Data/Typeable) or do you think I'll need to use something different for data definition DSL (Template Haskell?, or impossible in Haskell so write my own language with full parser? etc?)

Thanks for any idea or opinion on this! Karel

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