Keywords

Rakefile

@@ -2,7 +2,7 @@ require 'rubygems'
 require File.expand_path('../lib/heist', __FILE__)
 
 file "lib/heist/builtin/compiled_library.rb" do |t|
-  library_source = %w[util logic numeric list character string vector].
+  library_source = %w[util logic numeric list character string vector keyword].
                    map { |l| File.read "lib/heist/builtin/lib/#{l}.scm" }.
                    join("\n\n")
 

lib/heist/builtin/lib/keyword.scm

@@ -0,0 +1,30 @@
+; (get-keyword keyword args [default])
+; Returns the subsequent element to +keyword+ in +args+ if
+; present. Otherwise returns the return value of applying the thunk
+; +default+ or, if +default+ is not given, returns +#f+.
+(define (get-keyword keyword args . thunk)
+  (let ((default (if (pair? thunk) (car thunk) (lambda () #f))))
+    (let loop ((args args))
+      (cond ((null? args) (default))
+            ((eq? keyword (car args))
+             (cadr args))
+            (else
+             (loop (cddr args)))))))
+
+; (let-keywords args ((keyword default) keyword ...) body ...)
+; Convenience syntax for binding a list of keyword arguments to variables.
+; Note that this macro is very inefficient at the moment.
+(define-syntax let-keywords
+  (syntax-rules ()
+    ((_ args () body ...) (begin body ...))
+    ((_ args ((keyword default) rest ...) body ...)
+     (let ((keyword (get-keyword
+                     (string->keyword
+                      (symbol->string 'keyword))
+                     args
+                     (lambda () default))))
+       (let-keywords args (rest ...)
+         body ...)))
+    ((_ args (keyword rest ...) body ...)
+     (let-keywords args ((keyword #f) rest ...)
+       body ...))))

lib/heist/builtin/primitives.rb

@@ -204,6 +204,10 @@
   Vector === value
 end
 
+define('keyword?') do |value|
+  Keyword === value
+end
+
 #----------------------------------------------------------------
 
 # Numerical functions
@@ -336,6 +340,20 @@
 
 #----------------------------------------------------------------
 
+# Keyword functions
+
+# Returns the string representing the name of the given keyword
+define('keyword->string') do |keyword|
+    keyword.keyword_name
+end
+
+# Returns the keyword whose name is the given string
+define('string->keyword') do |string|
+  Keyword.new(string)
+end
+
+#----------------------------------------------------------------
+
 # Character functions
 
 # Returns true iff the two characters are equal

lib/heist/runtime/data/keyword.rb

@@ -0,0 +1,22 @@
+module Heist
+  class Runtime
+
+    # A +Keyword+ is a a self-quoting identifier ending in ":". The
+    # separate class is merely a convenience for implementing the
+    # +keyword?+ predicate.
+    class Keyword < Identifier
+
+      attr_reader :keyword_name
+
+      # Since keywords need never be renamed they don't need an
+      # +original_name+ argument.
+      def initialize(name)
+        @keyword_name = name
+        super("#{name}:")
+      end
+
+    end
+
+  end
+end
+

lib/heist/runtime/runtime.rb

@@ -18,7 +18,7 @@ module Heist
   #
   class Runtime
 
-    %w[ data/expression     data/identifier   data/character
+    %w[ data/expression     data/identifier   data/character  data/keyword
         data/cons           data/vector       data/value
         callable/function   callable/syntax   callable/macro  callable/continuation
         frame               stack             stackless

lib/heist/runtime/scope.rb

@@ -38,6 +38,7 @@ def initialize(parent = {})
       #
       def [](name)
         name = to_name(name)
+        return Keyword.new(name[0..-2]) if name.end_with?(':')
         bound = @symbols.has_key?(name)
 
         raise UndefinedVariable.new(