chickadee » srfi-197 » chain-when

A variant of chain in which each step has a guard expression and will be skipped if the guard expression evaluates to #f.

<initial-value> and <guard> are expressions. <placeholder> is a literal symbol; this is the placeholder symbol. If <placeholder> is not present, the placeholder symbol is _. The syntax of <step> is (<datum> ... [<_> <datum> ...]), where <_> is the placeholder symbol.

(define (describe-number n)
  (chain-when '()
    ((odd? n) (cons "odd" _))
    ((even? n) (cons "even" _))
    ((zero? n) (cons "zero" _))
    ((positive? n) (cons "positive" _))))

(describe-number 3) ; => '("positive" "odd")
(describe-number 4) ; => '("positive" "even")

Each <step> is evaluated as an application. The return value of the step is passed to the next step as its pipeline value. <initial-value> is the pipeline value of the first step.

The <_> placeholder in each <step> is replaced with that step's pipeline value. If a <step> does not contain <_>, it will ignore its pipeline value

If a step's <guard> is present and evaluates to #f, that step will be skipped, and its pipeline value will be reused as the pipeline value of the next step. The return value of chain-when is the return value of the last non-skipped step, or <initial-value> if all steps are skipped.

Because chain-when may skip steps, it does not support steps with multiple return values. It is an error if a step returns more than one value.