On Feb 21, 2012, at 7:18 AM, Rodolfo Carvalho wrote:
> BTW I just found the nested-cond pattern appearing on HtDP, throughout
> section 9:
> http://htdp.org/2003-09-26/Book/curriculum-Z-H-13.html
That happens when you follow the HtDP design recipe directly and there are
conditionals for two unrelated reasons, e.g.
(cond [(empty? L) ...]
[(cons? L)
(cond [(snark? (first L)) ...]
[(boojum? (first L)) ...]
[(beeblebrox? (first L)) ...]
)])
Yes, this COULD be collapsed into
(cond [(empty? L) ...]
[(snark? (first L)) ...]
[(boojum? (first L)) ...]
[(beeblebrox? (first L)) ...]
)
but that doesn't match the data structure as transparently.
Incidentally, if you want to bulletproof the above code against non-lists, the
first version is easy: just add
[else (error ...)]
as a new third branch of the outer "cond". The second version doesn't
bulletproof as neatly: you can either add
[(not (cons? L)) (error ...)]
as a new second branch of the "cond", or you can revert to the nested-cond
structure.
It's a judgment call, which can go different ways in different circumstances.
Stephen Bloch
sbl...@adelphi.edu
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