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SCHEME: I'm not understanding some for-each behavior.

April 25, 2011 - 10:13 — Eldrich Gaiman

> (list (list 9 8) (list 7 6) (list 5 4) (list 3 2))
((9 8) (7 6) (5 4) (3 2))
> (for-each (lambda x (car x)) (list (list 9 8) (list 7 6) (list 5 4) (list 3 2)))
((7 6) (5 4) (3 2))

Did that in a 2.6 console window. I would have expected the second command to yield (9 7 5 3), car'ing each of the four inner lists within the list. Can someone explain its behavior for me?

Thanks.

***UPDATE***

I figured it out. The for-each was executing as expected and hiding the results (as it should as compared to map). For some reason it always returns the tail of the list. This behavior required me to restructure some of my data and some of my implementation, but I resolved my impasse.

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April 26, 2011 - 05:24 — saulgoode

As a general rule, 'for-each' is concerned with the side effects of each of the iterations through the list; 'map' is concerned with the result produced.

Contrast the output of the following with its resulting evaluation:

(for-each (lambda (x)
                  (display (car x))
                  (newline) )
          '((9 8) (7 6) (5 4) (3 2)) )
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