[Haskell-cafe] "import" functionality in DSLs

[Nikhil A. Patil]

On Sat, Apr 16, 2011 at 13:49 CDT, Luke Palmer wrote:
> You can get away with this using {-# LANGUAGE RecordWildCards #-}, if you
> put your prelude into a record.  Here's a test I did to make sure the
> technique worked:
> 
> {-# LANGUAGE RecordWildCards #-}
> 
> import Prelude hiding ((+))
> 
> data Foo = Foo {
>     (+) :: Int -> Int -> Int,
>     n0  :: Int
> }
> 
> prelude :: IO Foo
> prelude = return $ Foo { (+) = (*), n0 = 1 }
> 
> doit :: IO Int
> doit = do
>     Foo{..} <- prelude
>     return $ n0 + 3 + 4

Oh, that's pretty sweet! Thank you very much! :)

nikhil

> 
> 
> ghci> doit
> 12
> 
> On Sat, Apr 16, 2011 at 7:29 AM, Nikhil A. Patil <patil.nikhil at gmail.com>wrote:
> 
> > 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
> >
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> >