7.5.4 SRFI-2 - and-let*

The following syntax can be obtained with

(use-modules (srfi srfi-2))

or alternatively

(use-modules (ice-9 and-let-star))

library syntax: and-let* (clause …) body …

A combination of and and let*.

Each clause is evaluated in turn, and if #f is obtained then evaluation stops and #f is returned. If all are non-#f then body is evaluated and the last form gives the return value, or if body is empty then the result is #t. Each clause should be one of the following,

(symbol expr)

Evaluate expr, check for #f, and bind it to symbol. Like let*, that binding is available to subsequent clauses.

(expr)

Evaluate expr and check for #f.

symbol

Get the value bound to symbol and check for #f.

Notice that (expr) has an “extra” pair of parentheses, for instance ((eq? x y)). One way to remember this is to imagine the symbol in (symbol expr) is omitted.

and-let* is good for calculations where a #f value means termination, but where a non-#f value is going to be needed in subsequent expressions.

The following illustrates this, it returns text between brackets ‘[...]’ in a string, or #f if there are no such brackets (ie. either string-index gives #f).

(define (extract-brackets str)
  (and-let* ((start (string-index str #\[))
             (end   (string-index str #\] start)))
    (substring str (1+ start) end)))

The following shows plain variables and expressions tested too. diagnostic-levels is taken to be an alist associating a diagnostic type with a level. str is printed only if the type is known and its level is high enough.

(define (show-diagnostic type str)
  (and-let* (want-diagnostics
             (level (assq-ref diagnostic-levels type))
             ((>= level current-diagnostic-level)))
    (display str)))

The advantage of and-let* is that an extended sequence of expressions and tests doesn’t require lots of nesting as would arise from separate and and let*, or from cond with =>.