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  • Re: replace helper fn with flet or labels

    From B. Pym@Nobody447095@here-nor-there.org to comp.lang.lisp on Mon Aug 11 10:02:23 2025
    From Newsgroup: comp.lang.lisp

    I want to read a list and return a list of cons pairs:

    (a b c d e f) -> ((a . b) (c . d) (e . f))
    (a b c d e f g) -> ((a . b) (c . d) (e . f) (g . ""))

    This function kind of works:

    (defun process-form-x (x)
    "create a list of (key . value) pairs from a list"
    (cond ((endp x) nil);; terminate on empty list
    ((not (endp (rest x))) ;; recurse if there are at least two left
    ( cons (cons (first x) (second x))
    (process-form-x (rest (rest x)))))))

    but if the list has an odd number of members, I want to pair the
    last member with "". The function above just drops the last
    member of a list with an odd number of members.

    [snip]

    Any ideas? Is using flet and labels the right approach? Should I
    just be happy with the helper function calling the recursive
    function and stop there? Should I use loop to walk through a list,
    rather than try recursion?


    How about (to answer your last question first):

    (defun make-pairs (list)
    (loop while list collect (cons (pop list) (or (pop list) ""))))

    Gauche Scheme

    (define (mpairs x)
    (if (null? x) x
    (acons (pop! x) (pop~ x "") (mpairs x))))

    (mpairs '(a b c d))
    ===>
    ((a . b) (c . d))

    (mpairs '(a b c d e))
    ===>
    ((a . b) (c . d) (e . ""))

    Given:

    (define-syntax pop~
    (syntax-rules ()
    [ (_ xs) (pop~ xs #f) ]
    [ (_ xs default)
    (if (pair? xs)
    (begin0 (car xs) (set! xs (cdr xs)))
    default) ] ))
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