macrosschemehygienesyntax-rules

How to create Alist from List using syntax-rules in Scheme?


Just as exercise I want to transform:

(alist "foo" 1 "bar" 2 "baz" 3)

into

(("foo" . 1) ("bar" . 2) ("baz" . 3))

is this possible with syntax-rules?

My attempt:

(define-syntax alist
  (syntax-rules ()
     ((_ a b) (cons a b))
     ((_ x y ...)
      (list (alist x y) ...))))

the problem that it expand into:

(("foo" . 2) ("foo" . "bar") ("foo" . 4) ("foo" . "baz") ("foo" . 6))

is macro alist possible with syntax-rules? How it should look like?

EDIT:

Another attempt

(define-syntax alist
  (syntax-rules ()
     ((_ a b) (cons a b))
     ((_ x y z ...)
      (list (alist x y) (alist z ...)))))

it returns (("foo" . 2) (("bar" . 4) ("baz" . 6))).


Solution

  • If it's literals only (like "foo" and 2) you can just do this:

    #!r6rs
    (import (rnrs))
    
    (define-syntax alist
      (syntax-rules (alist-builder)
        ((_ alist-builder () (results ...)) 
         '(results ...))
        ((_ alist-builder (a) . rest) 
         (raise 'bad-alist))
        ((_ alist-builder (a b rest ...) (results ...))
         (alist alist-builder (rest ...) (results ... (a . b))))
        ((_ a ...) (alist alist-builder (a ...) ()))))
    
    (alist)         ; ==> ()
    (alist "a" 2)   ; ==> (("a" . 2))
    (alist a 3 b 4) ; ==> ((a . 3) (b . 4))
    (alist a)       ; ==> uncaught exception: bad-alist
    

    Of course you are not allowed to mutate this because (alist a b c d) is the same as literally writing '((a . b) (c . d)) which you are not allowed to mutate.

    Also if you ever make an alist with alist-builder as the first key the internals will leak. You can fix this by splitting up the internal definition and you can hide it from exposure by having both in a library and only export alist