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2 Your First System ¶

Suppose a small library in ‘/home/pjb/lisp/geometry/’:

geometry/
  geometry.alpm
  points.lsp
  lines.lsp
  circles.lsp

‘points.lsp’ defines point constructors; ‘lines.lsp’ calls functions from ‘points.lsp’; ‘circles.lsp’ uses both. Write this in ‘geometry.alpm’:

;;;; System definition for GEOMETRY.
(alpm-define-system 1
 '(name        "geometry"
   version     "1.0.0"
   description "2D geometry entities and predicates."
   files       ("points"
                (file "lines"   depends-on ("points"))
                (file "circles" depends-on ("points" "lines")))))

Things to note:

• The leading ‘1’ is the definition format version. It says "the quoted list that follows uses the version-1 grammar". When a future ALPM introduces new definition syntax as format 2, your version-1 files keep working unchanged, and an old ALPM reading a format-2 file stops with a clear message instead of misinterpreting it. Always write the format version you are actually using — today, ‘1’.
• The whole definition is one quoted list passed to the function ‘alpm-define-system’. Don’t forget the quote — AutoLISP has no macros, and the property names (‘name’, ‘version’, ‘files’, ‘depends-on’, …) are plain symbols that must not be evaluated. In particular do not write ‘:name’ — in AutoLISP a ‘:symbol’ is not a keyword, just a symbol like any other; ALPM uses bare symbols throughout.
• System and file names are strings, compared case-insensitively.
• File names are given without extension (‘.lsp’ is added when loading).
• The file is named after the system: system ‘"geometry"’ lives in ‘geometry.alpm’. ALPM finds it either as ‘<registry-dir>/geometry.alpm’ or ‘<registry-dir>/geometry/geometry.alpm’ — the layout above uses the second form, so register the parent directory:

(alpm-register-directory "/home/pjb/lisp" nil)