life is funny, isn't it? so many people so eagerly discussing conversion between non-monadic and monadic code, yet when we asked for your opinions and suggestions on this very topic only a short while ago, we got a total of 4 (four) replies - all quite useful, mind you, so we were grateful, but still one wonders.. we might have assumed that not many people cared after all:
http://www.haskell.org//pipermail/haskell/2005-March/015557.html shall I assume that all participants in this discussion have joined the Haskell parade since then, and have proceeded rapidly to the problems of monadic lifting?-) in which case I'd invite you to have a look at that survey and the papers mentioned. > > I thought the easy answer would be to inject non-monadic values into the > > monad (assuming one already rejiggered things to do automatic lifting). I'd phrase it slightly differently: what (I think) one wants are implicit coercions between monadic and non-monadic types of expressions, where the coercions lift non-monadic values into the monad in question, while embedding monadic computations in the current monad to get a non-monadic result if only that is needed (although one might think of the latter as partially lifting the operation that needs the non-monadic result). only I wouldn't want those implicit coercions to be introduced unless programmers explicitly ask for that (one usually only converts code from non-monadic to monadic once, and while the details of that step might be tiresome and in need of tool-support, the step itself should be explicit - see my comment on (2) below). > Note that in (a), "pure" values are never used where monads are asked > for, only the other way around. that is probably where some would beg to differ - if you lift operations, why not lift values as well? > I think that supporting syntax (a) for semantics (b) should be a > feature because: (1) it is (usually) obvious what (a) means; (2) it > eliminates the single-use variable 'v' - single-use variables like > this occur a lot in monadic Haskell code, and I think they make it > harder to read and write; (3) it would support the math-like syntax > that I presented in my original message. (1) "(usually) obvious" is tech-speak for "(perhaps) possible to figure out, though probably not uniquely determined"?-) when mathematicians abuse notation in the "obvious" way, there is usually an assumed context in which the intended abuses are clearly defined (if not, there is another context in which the "obvious" things will go unexpectedly awry). (2) the nice thing about Haskell is that it *distinguishes* between monadic and non-monadic computations, and between evaluation and execution of monadic computations. if you want everything mixed into one soup, ML might be your language of choice (http://portal.acm.org/citation.cfm?id=178047 , if I recall correctly? see the paper discussed in http://lambda-the-ultimate.org/node/view/552 for one application that demonstrates the power/danger of such implicit monads). (3) using math-like syntax for lifted expressions is common practice in some families of Haskell-DSELs, eg. Conal Elliot's Fran. As John pointed out, the predefined class-hierarchy is not really helpful for such endeavours, but if one isn't picky, one may ignore classes not used.. the "trick" is to lift even constants, so when you get to applications, all components are already lifted, and lifting most arithmetic works out fine (Boolean operations are another matter). note, however, that the resulting language, while looking mathematically pure and permitting concise expression of complex circumstances, may not have the reasoning properties you expect.. > It might be hard to modify the type checker to get it to work, but I > think it is possible, and I see no reason not to be as general as > possible. here I'd agree, although in contrast to you, I'd be talking about a complex refactoring under programmer control, not about an implicitly invoked collection of coercions. I played with that idea after Martin Erwig visited our refactoring project in March, and got to a prototype type-coercion inference system for a very simple functional language, because I found the situation with various existing and, as Erwig/Ren pointed out, apparently unrelated monadification algorithms confusing. apart from the various styles of monadification, which we'd like to permit, and have the programmer select, e.g., by type annotations, there is the slight problem that there are an unbounded number of different monadifications (more if one wants to keep annotations to a minimum), so one needs a sensible bound (one that does not exclude any of the alternatives one might want). one also might want to be able to choose between the alternatives (or tune the system so that taking the first choice works out ok most of the time). oh, and it shouldn't be too inefficient, and it is really a pain to re-implement a type-system just to add a few coercion rules to it (which is why I haven't extended my mini fpl to Haskell yet..). in light of this, perhaps some more participants in this discussion might want to look into contributing their suggestions to our old survey? cheers, claus _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
