Hi, I am planning a simple monadic DSL (frankly, calling it a DSL is a bit of a stretch; it's just a somewhat glorified state monad), and I wish to implement some kind of "import" functionality for it. The DSL code looks something like this:
doit :: DSL Term doit = do (+) <- define_function "+" 2 n0 <- define_constant "0" k <- define_constant "k" -- begin beautiful DSL code let x = k + n0 return $ x + x
The code above adds identifiers "+", "0", "k" to the DSL monad and conveniently exposes haskell identifiers (+), n0, k for the user to use in code that follows. (Note that these define_* functions make state updates in the underlying state monad.) Needless to say, most functions like doit have very similar define_* calls in the beginning. Thus, I want to implement some kind of import functionality. Ideally, the code would look like this:
module DSLPrelude where
prelude :: DSL () prelude = do (+) <- define_function "+" 2 n0 <- define_constant "0" k <- define_constant "k" return ()
module Main where import DSLPrelude
doit :: DSL Term doit = do prelude -- begin beautiful DSL code let x = k + n0 return $ x + x
...but of course that won't work since (+), n0, k are not in scope. I can think of two solutions, both of which I dislike: Solution 1:
module DSLPrelude where
prelude :: DSL (Term -> Term -> Term, Term, Term) prelude = do (+) <- define_function "+" 2 n0 <- define_constant "0" k <- define_constant "k" return ((+), n0, k)
module Main where import DSLPrelude
doit :: DSL Term doit = do ((+), k, n0) <- prelude -- begin beautiful DSL code let x = k + n0 return $ x + x
This is quite unsafe: I have mistakenly swapped k and n0 in doit, without failing typecheck. Solution 2:
module DSLPrelude where
(+) :: DSL (Term -> Term -> Term) n0 :: DSL Term k :: DSL Term (+) = define_function "+" 2 n0 = define_constant "0" k = define_constant "k"
module Main where import DSLPrelude
doit :: DSL Term doit = do (+) <- (+) n0 <- n0 k <- k -- begin beautiful DSL code let x = k + n0 return $ x + x
...which works, but still has quite a bit of boilerplate crap. I feel this would be a common problem with a lot of DSLs, so I am curious to know how others solve it. Any pointers and suggestions are most welcome and greatly appreciated. Thanks! nikhil