On Thu, Nov 25, 2004 at 10:07:20AM +0100, George Russell wrote:
John Meacham wrote:
Now, my mdo proposal as written would have "hello" outputed exactly once at module start up time no matter what, whether x is demanded or not. it is equivalant to a program transformation that collects all the top level initializers and declarations, puts them all in a mdo block and runs it with the semantics explained in the fixIO paper. (with a deterministic, but partially undefined order)
What happens if you write the following?
y <- f x <- .... let f = something depending on x
Will it know to evaluate x before y, or is this not allowed?
What about if the assignments to y and x occur in different modules? Will the RTS have to analyse the dependencies to work out which order to run the assignments in?`
No. The semantics are completly and precicely defined by the fixIO paper. there is a simple desugaring to haskell which I gave in my original proposal. The basic gist is that things occur almost* identically to if everything were simply placed in a single mdo block. no dependency analysis or strange translation beyond the normal haskell desugaring mechanisms is required. recursive top level declarations are no more tricky than are normal recursive lets. Yes it is possible to write divergent statements, in the same way it is possible to say bottom = bottom you can also say bottom' <- return bottom' (these are actually equivalant.) Remember, that unlike normal monadic binding, this is using recursive monadic binding, meaning the results of future computations can be used as long as they arn't strictly evaluated, in the same way you can write mutually recursive variables but if both are strict in each other they will diverge :). The fact that everyone seems to get along fine despite the ability to write divergent computations with recursive lets makes me think this won't be too much for anyone to handle. John * The differences, which I elaborated on earlier are due to a monomorphic restriction on user-written 'mdo' let-bound variables which is irrelevant to the desugaring so the translation from mdo -> haskell expression is slightly different than presented in the paper. -- John Meacham - ⑆repetae.net⑆john⑈