On Mon, 2009-10-05 at 19:22 -0400, David Menendez wrote:
The two obvious options are call-by-name and call-by-value.
I wonder how easily one can provide both, like in Algol.
Fairly easy, you can either do a language that has an explicit monad (a bit like Haskell with only the IO monad), or you can go the whole hog and embed Paul Levy's Call-by-push-value. This requires a bit more cleverness, and an explicit representation of the embedded types in order to define the algebra maps on the computation types.
Obviously, doing a deterministic version is simpler. You can probably get away with representing values as simple self-evaluating thunks.
data Thunk s a = STRef s (Either a (ST s a))
evalThunk :: Thunk s a -> ST s a evalThunk r = readSTRef r >>= either return update where update m = m >>= \x -> writeSTRef r (Left x) >> return x
I had a go at this and it seems to work. Happily, it is a tiny tweak to the CBV implementation I gave: you just replace "return" with your "evalThunk" (or something like it) and wrap the argument in the 'app' case with a function that creates a thunk. Bob -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.