Simon Peyton Jones's book "The Implementation of Functional Programming Languages", chapter 13 (Supercombinators and Lamba-lifting), page 222, describes a "multi-argument" beta-reduction strategy whereby several free variables of a multi-argument abstraction may be instantiated simultaneously. Then, on page 225 the following sentence appears: "A crucial point in the definition of a supercombinator given above is that a supercombinator reduction only takes place when all the arguments are present." I feel that there's some big idea in the pages between these two references, but I can't quite grasp it. I can appreciate that there are good reasons (e.g. efficiency) to instantiate several variables at once, but I don't quite understand why this restriction is "crucial". Is it crucial just for reasons of efficiency - and in actual fact it's entirely possible, albeit inefficient, to instantiate one variable at a time? Or is it, given that the whole point of supercombinators is to allow compilation to fixed code sequences for the G-machine, that there is a good reason to avoid having these fixed code sequences deal with instantiating one free variable at a time (simplicity of design, or some kind of restriction on the way that G-machine operations work)? Or is there a deeper theoretical reason related to the combinators themselves that has nothing to do with the downstream compilation process? _________________________________________________________________ Be the first to hear what's new at MSN - sign up to our free newsletters! http://www.msn.co.uk/newsletters