Re: Macros (Was: Interesting: "Lisp as a competitive advantage")

Keith Wansbrough quotes :

Jerzy Karczmarczuk writes:

...

Macros in Scheme are used to unfold n-ary control structures such as COND into a hierarchy of IFs, etc. Nothing (in principle) to do with laziness or HO functions.

Isn't this exactly the reason that macros are less necessary in lazy languages? In Haskell you can write myIf True x y = x myIf False x y = y

and then a function like recip x = myIf (abs x < eps) 0 (1 / x)

works as expected. In Scheme,

(define myIf (lambda (b x y) (if b x y))) does *not* have the desired behaviour! One can only write myIf using macros, or by explicitly delaying the arguments.

========================== Well, my point was very different *here*. Lazy functions *may* play the role of control structures (as your myIf in Haskell). In Scheme, with macros or without macros you CANNOT implement "if". But you can write (cond (cond1 seq1) (cond2 seq2 etc) ... (condN und so weiter) ) translated into (if cond1 seq1 (if cond2 (begin seq2 etc) (if ... ))) In the same way the LET* constructs are developed. And a multifunction DEFINE. So, here it is not the question of laziness, but of syntactic extensions. (Of course, one can "cheat" implementing a user-defined IF as you proposed above, delaying the last two arguments of this ternary operator, but the decision which one will be evaluated, will pass through the "real" IF anyway.) However, if you have already your primitive control structures (i.e. your underlying machine), you can play with macros to implement lazy continuations, backtracking, etc.Usually this is awkward, and less efficient than if done at a more primitive level, where you can use easily unboxed data stored on true stacks, execute real, fast branching, etc. I am not sure about this vise-grip analogy. For me macros are façades, a way to present a surface of the programm. The real work, where you need wrenches, pipes, dynamite and Bible, all that is hidden behind. Jerzy Karczmarczuk Caen, France