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

Keith Wansbrough
Fri, 04 May 2001 16:52:14 +0100

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.

--KW 8-)