[Haskell-cafe] "import" functionality in DSLs

[wren ng thornton]

On 4/16/11 9:55 AM, 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.

That really depends. For example, first assume we've hoisted things out:

     module DSLPrelude where
         import qualified Prelude
         (+) = define_function (Prelude.+) 2
         ...

Now, rather than having the define_* functions perform side effects 
themselves, instead we could have the generated (+) register itself in 
the state the first time it's called in each DSL, assuming the DSL has 
(or can have) a way to keep a log of which things it has used (i.e., 
could run when executed). This also has the benefit that the DSL can 
prune out definitions that are unused, and collapse any duplicate 
definitions it detects.

Of course, the downside is that it means (+) is monadic now, which may 
get in the way of your beautiful DSL code. Whether it's worth it or not 
depends on what the DSL is for. If it's pseudo-assembly, then it should 
be fine; if it aims to be more of a high-level mathematical notation, 
then not so much.


Though Luke's -XRecordWildCards approach was my first thought.

-- 
Live well,
~wren