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S9 LIB (unless <test-expression> <body>) ==> object
(when <test-expression> <body>) ==> object
(load-from-library "when.scm")
The WHEN syntax first evaluates <test-expression>. When it evaluates
to a true value, it also evaluates <body>, which is a sequence of
expressions. The expressions will be evaluated in order and the value
of the last expression will be returned. When <test-expression>
evaluates to #f, WHEN does not evaluate its <body> and returns an
unspecific value.
UNLESS is evaluates its body only if <test-expression> evaluates to #F.
It evaluates its <body> exactly if WHEN would not evaluate it and vice
versa.
(when (= 1 1) 'foo 'bar 'baz) ==> baz
(unless (= 1 2) 'foo 'bar 'baz) ==> baz
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