doco srfi-2 and-let*

[Kevin Ryde <user42@zip.com.au>]

I'd like to propose

	* srfi-modules.texi (SRFI-2): Rewrite and-let*, adding plain
	expression clauses and improving the examples.

New text below.  I quite like the first example, which I think is
realistic and sensible.  But the second is a bit of a contrivance and
I'd be open to better ideas.


SRFI-2 - and-let*
=================

The following syntax can be obtained with

     (use-modules (srfi srfi-2))


 - 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.  Each
     CLAUSE should be one of the following forms,

    `(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 `=>'.


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