The COND => construct


The COND form has this syntax:

 (COND clause1
       clause2
       clause3)

Each clause can take a couple possible forms.  Previously we discussed
two of these forms:

 clause =   (test result)

in which "test" is evaluated, and if true the result is evaluated and
if false we go on to the next clause.

We've also seen

 clause =   (ELSE result)

which is equivalent to just writing (#T result) but intended to be
more readable.

We today encounter a new form of cond clause,

 clause =    (test => function)

which evaluates the test, and if false moves on to the next clause,
however if the test is true it evaluates function, and then applys the
returned value to the result of evaluating test.

Eg

  (cond (12 => (lambda (x) (+ x 1)))
	(else 'nope))

returns 13.

similarly

  (cond ('(a b c) => cadr)
	(else 'nope))

returns B.


There is actually yet another form of cond clause, very rarely used

 clause =    (test)

is the same as writing

 clause =    (test => (lambda (x) x))

ie if the test is true the value of test is directly returned.