XPost: comp.lang.scheme

Juho Snellman wrote:

One useful detail that I didn't see mentioned in that chapter is the
loop keyword "it". I find that it pops up often in code like

(let ((x ...))
...
(loop for e in list when (foo-bar e x) collect it))

, where foo-bar returns nil on failure and some sort of state object otherwise.

Gauche Scheme

(define (foo-bar x) (and (positive? x) (sqrt x)))

(filter-map foo-bar '(-3 0 4 9))
===>
(2 3)


Here's an anaphoric "if" that provides "it".
(Adapted from an example in the Gauche documentation.)

(use util.match)

(define-syntax if@
(er-macro-transformer
(lambda (form rename id=?)
(match form
[(if@ test expr1 expr2)
(quasirename rename
`(let ((,'it ,test))
(if ,'it ,expr1 ,expr2)))]))))

(if@ (find odd? '(2 3)) (print it) (print "false is " it))
3

(if@ (find odd? '(2 30)) (print it) (print "false is " it))
false is #f

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