Re: [Haskell-cafe] Re: Does laziness make big difference?

[Nick]

Peter, Roughly, I'd say you can fudge laziness in data structures in a strict language without too much bother. (I don't have much experience with this, but the existence of a streams library for OCaml is the sort of thing I mean. There are plenty of papers on co-iterative streams and suchlike that show the general pattern.) Yes, agree. And this was my initial point. If you wish to add control structures you would need to use the lazy keyword a lot, e.g.: if cond then *lazy* S1 else *lazy* S2 and for more complicated structures it's not going to be always clear what needs to be suspended. Laziness is a conservative default here. (If you want to write an EDSL in a non-lazy language, you'll need to use some kind of preprocessor / macros / ... - in other words, a two-level language - or do thunking by hand, as above, or live with doing too much evaluation.) One way to gauge how useful laziness really is might be to look through big ML projects and see how often they introduce thunks manually. A thunk there is usually something like "fn () => ..." IIRC. Also IIRC, Concurrent ML is full of them. Probably, dealing with macros is not so scary and Paul Graham and Piter Siebel show that it is quite easy. :-) Ok, let's go from the another side: I have searched through Darcs source and found 17 datastructures with strict members (for example data Patch = NamedP !PatchInfo ![PatchInfo] !Patch) and 36 occurrence of this dreaded seq. And Darcs is an application being not speed critical. And if one try to write cipher decoder on Haskell, I guess he has to make his program full of '!' and 'seq' (or FFI). Dare I say the tradeoff is between a relatively simple operational model (so you can judge space and time usage easily) and semantic simplicity (e.g. the beta rule is unrestricted, easing program transformation). Cool! Thank you. Best regards, Nick.

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