dorchard

Consider a data type representing syntax trees.
 
> data Expr = BinOp Expr Expr | Unop Expr 
> data Stmt = Seq Stmt Stmt   | Assign String Expr 
 
Let's say I want this in a module, but I also want to be able to arbitrarily extend this AST type 
in modules that import it. So, I add an extra constructor and a type parameter
to all the data type definitions:
 
> data Expr ext = BinOp (Expr ext) (Expr ext) | Unop (Expr ext) | Ext ext

dorchard

> {-# LANGUAGE QuasiQuotes #-}
> {-# LANGUAGE EmptyDataDecls #-}

> import Control.Comonad
> import Language.Haskell.Codo

> data Input a 
> data Output a