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

[Peter Gammie]

Nick,

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.)

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.

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).

cheers
peter