Previous: Iteration Up: Contents Next: Evaluation

Conditionals

The fundamental conditional operator in Arc is if. It is similar to the Lisp if, except it eliminates the parentheses around the clauses. For a simple conditional with multiple body statements, when or its opposite unless can be used.

Arc provides several conditionals that assign the test expression to a variable, similar to let. The iflet, caselet, and whenlet macros are useful if the test expression is used inside the body.

Conditionals

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