[Haskell-cafe] "import" functionality in DSLs

[Nikhil A. Patil]

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
> <patil.nikhil at gmail.com> 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.