2.12 Conditionals: if, cond, and, and or

Conditionals in Guide: PLT Scheme introduces conditionals.

(if test-expr then-expr else-expr)

Evaluates test-expr. If it produces any value other than #f, then then-expr is evaluated, and its results are the result for the if form. Otherwise, else-expr is evaluated, and its results are the result for the if form. The then-expr and else-expr are in tail position with respect to the if form.

Examples:

> (if (positive? -5) (error "doesn't get here") 2)

2

> (if (positive? 5) 1 (error "doesn't get here"))

1

(cond cond-clause ...)

cond-clause   =   [test-expr then-expr ...+]     |   [else then-expr ...+]     |   [test-expr => proc-expr]     |   [test-expr]

Chaining Tests: cond in Guide: PLT Scheme introduces cond.

A cond-clause that starts with else must be the last cond-clause.

If no cond-clauses are present, the result is #<void>.

If only a [else then-expr ...+] is present, then the then-exprs are evaluated. The results from all but the last then-expr are ignored. The results of the last then-expr, which is in tail position with respect to the cond form, are the results for the whole cond form.

Otherwise, the first test-expr is evaluated. If it produces #f, then the result is the same as a cond form with the remaining cond-clauses, in tail position with respect to the original cond form. Otherwise, evaluation depends on the form of the cond-clause:

[test-expr then-expr ...+]

The then-exprs are evaluated in order, and the results from all but the last then-expr are ignored. The results of the last then-expr, which is in tail position with respect to the cond form, provides the result for the whole cond form.

[test-expr => proc-expr]

The proc-expr is evaluated, and it must produce a procedure that accepts on argument, otherwise the exn:fail:contract exception is raised. The procedure is applied to the result of test-expr in tail position with respect to the cond expression.

[test-expr]

The result of the test-expr is returned as the result of the cond form. The test-expr is not in tail position.

Examples:

> (cond)

> (cond       [else 5])

5

> (cond      [(positive? -5) (error "doesn't get here")]      [(zero? -5) (error "doesn't get here, either")]      [(positive? 5) 'here])

here

> (cond      [(member 2 '(1 2 3)) => (lambda (l) (map - l))])

(-2 -3)

> (cond      [(member 2 '(1 2 3))])

(2 3)

else

Recognized specially within forms like cond. An else form as an expression is a syntax error.

=>

Recognized specially within forms like cond. A => form as an expression is a syntax error.

(and expr ...)

Combining Tests: and and or in Guide: PLT Scheme introduces and.

If no exprs are provided, then result is #f.

If a single expr is provided, then it is in tail position, so the results of the and expression are the results of the expr.

Otherwise, the first expr is evaluated. If it produces #f, the result of the and expression is #f. Otherwise, the result is the same as an and expression with the remaining exprs in tail position with respect to the original and form.

Examples:

> (and)

#t

> (and 1)

1

> (and (values 1 2))

1

2

> (and #f (error "doesn't get here"))

#f

> (and #t 5)

5

(or expr ...)

Combining Tests: and and or in Guide: PLT Scheme introduces or.

If no exprs are provided, then result is #t.

If a single expr is provided, then it is in tail position, so the results of the and expression are the results of the expr.

Otherwise, the first expr is evaluated. If it produces a value other than #f, that result is the result of the or expression. Otherwise, the result is the same as an or expression with the remaining exprs in tail position with respect to the original or form.

Examples:
> (or)
#f
> (or 1)
1
> (or (values 1 2))
1
2
> (or 5 (error "doesn't get here"))
5
> (or #f 5)
5