Alex Shinn <alexsh...@gmail.com> writes: > On Tue, Dec 6, 2011 at 4:48 PM, Marijn <hk...@gentoo.org> wrote: >> >> Couldn't help but wonder why they don't return the value of the last >> body form, so I looked around a bit and both CLHS[1] and my racket >> REPL seem to agree that they should: >> >> $ racket >> Welcome to Racket v5.2.0.4. >>> (when #t 'hello) >> 'hello >>> (unless #f 'hi) >> 'hi >> >> Is there some other source that suggests that the return value should >> be unspecified? > > Because the result is meaningless when the condition is false. > > CLHS returns nil in this case, but that fits with CL idioms and > not Scheme idioms. when and unless are for side-effects - it's > better to write (and #t 'hello) if you want the result. > > R7RS also leaves it unspecified.
I've actually wondered if it would not make sense to return *unspecified* in the case of the plain else-less if even if the condition is true, namely when you write (if #t #t). There is probably code relying on this to be #t, but frankly, this appears like a recipe for breakage. -- David Kastrup
