Re: [Haskell-cafe] "import" functionality in DSLs

On Sat, Apr 16, 2011 at 08:55 CDT, Felipe Almeida Lessa wrote:

On Sat, Apr 16, 2011 at 10:29 AM, Nikhil A. Patil wrote:

...

...

doit :: DSL Term doit = do (+) <- (+)           n0  <- n0           k   <- k           -- begin beautiful DSL code           let x = k + n0           return $ x + x

I guess the core problem is that on each time you say '(+) <- (+)', you may actually get something different depending on what 'define_function' does. You say yourself that these functions change a hidden state. So, without any internal changes, I doubt you could do something better.

Thanks very much for your response! You are right, I haven't defined the semantics of this operation in my post. For the example in my post, the define_* functions are idempotent on the hidden state, when used with the same arguments (i.e. the multiple calls are redundant). I explicitly handle this in the implementation of the DSL monad: basically, there is a Set that tracks what functions have been previously defined, and when a conflicting re-definition is found, I throw a run-time error. But: how can I get rid of the boilerplate code, and give the user the appearance that she is "import"-ing identifier bindings from another file? nikhil

One possible solution may be to have a special case for your Prelude functions and constants that never change. That is, if currently you have

data Term = Term Key ... type Key = Integer

and you store other informations about each term on your hidden state, then you may use

data Term = Term Key ... data Key = Prelude Integer | User Integer

Your define_* functions always return User keys, however now you can have unsafe versions of them that take a key as argument. Then your Prelude would be

...

module DSLPrelude where

(+) :: Term -> Term -> Term n0 :: Term k :: Term (+) = unsafe_define_function t1 "+" 2 n0 = unsafe_define_constant t2 "0" k = unsafe_define_constant t3 "k"

t1, t2, t3 :: Key (t1:t2:t3:_) = map Prelude [1..]

Of course, this is a lot of handwaving, but you haven't provided any details about your internal implementation.

HTH,

-- Felipe.