alistcall

Use x!foo for association list lookups

diff -u a/ac.scm b/ac.scm
--- a/ac.scm	2009-11-21 19:15:40.000000000 -0500
+++ b/ac.scm	2009-11-21 19:23:39.000000000 -0500
@@ -646,11 +646,14 @@
 ; default vals with them.  To make compatible with existing written tables, 
 ; just use an atom or 3-elt list to keep the default.
 
+(defarc alref arc-alref)
+
 (define (ar-apply fn args)
   (cond ((procedure? fn) 
          (apply fn args))
         ((pair? fn) 
-         (list-ref fn (car args)))
+         ((if (number? (car args)) list-ref arc-alref)
+          fn (car args)))
         ((string? fn) 
          (string-ref fn (car args)))
         ((hash-table? fn) 

In arc3, lists can be called with an integer argument and will return the item in the list at that position:

arc> ('(a b c d e) 2)
c

Since calling a list with a non-integer argument results in an error, this means that we could make calling lists with other types do something useful, if we think of something.

I was inspired by thaddeus’ post about adding a syntax to Arc for lookups in association lists to think that perhaps we could have calling a list with a non-integer do an association list lookup.

I often use tables instead of association lists merely because with tables I can use the x!foo notation.

With this patch the x!foo notation will also work for association lists, as long as “foo” is a symbol or something else that isn’t a number.

It doesn’t bother me that x!2 still returns the item at that position in the list, since when I'm using a table (or now an association list) as an “object”, a container of named values, my keys are symbols anyway.

Taking thaddeus’ example:

arc> (= x '((first "First Name")(last "Last Name")))
((first "First Name") (last "Last Name"))

arc> x!first
"First Name"

arc> x!last
"Last Name"

Note that this patch doesn’t implement setting values in an association list. I.e.,

(= alist!foo 3)

doesn’t work.

Get this hack

Download alistcall0.patch and its prerequisite: defarc.

Or, using the hackinator:

hack ycombinator.com/arc/arc3.1.tar \
  awwx.ws/defarc0.patch \
  awwx.ws/alistcall0.patch

unLicense

This patch is in the public domain. The combination of this patch and Arc may be redistributed or modified under the same terms as Arc itself.

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