if [test expr] ... [else-expr]
Arc is the basic conditional operation.
It takes a sequence of tests and expressions. The expression corresponding
to the first true test is returned. Other expressions are not evaluated.
>(if nil "Nil is true"
0 "0 is true"
"What is true?")
"0 is true"
iflet var expr then-expr [test1 expr1] ... [else-expr]
Evaluates
expr. If true, expr is assigned to var and then-expr is evaluated and returned. Otherwise, the remaining arguments are processed as normal if clauses.
>(iflet x 42 (+ x 1))
43
>(iflet x nil (+ x 1))
nil
>(iflet x nil (+ x 1)
(< 1 2) 55)
55
when test [body ...]
Executes body if test is true. This is similar to
if, except it allows multiple body statements but only has a single test clause. >(when 1 (pr "a") (pr "b")) ab "b"
whenlet var expr [body ...]
Evaluates
expr. If true, the value is assigned to var and body is executed.
>(whenlet x nil (prn "hi") (+ x 1))
nil
>(whenlet x 1 (prn "hi") (+ x 1)) hi 2
unless test [body ...]
Executes body if test is false. This is the opposite of 'when'.
>(unless 1 (pr "a") (pr "b"))
nil
case arg [test1 expr1] ... [else-expr]
arg is evaluated. It is then compared to the test values in sequence. If it matches one, the corresponding expr is evaluated and returned. If there is no match and no else-expr, nil is returned.
>(case 'b
a 1
b 2
3)
2
>(case 42
10 "foo"
42 "bar")
"bar"
caselet var arg [test1 expr1] ... [else-expr]
arg is evaluated and assigned to var. It is then compared to the test values in sequence. If it matches one, the corresponding expr is evaluated and returned. If there is no match and no else-expr, nil is returned.
>(caselet x 'b
a 1
b 2
3)
2
>(caselet x 42
10 (+ x 3)
42 (+ x 5))
47
check expr test [alt]
Evaluates
expr and applies predicate test. If true, returns the evaluated value. Otherwise returns alt or nil. alt can be a function to try again, even calling check recursively.
>(check (+ 10 10) odd "foo")
"foo"
>(check (+ 10 10) even "bar")
20