Add rudimentary support for keywords

lib/Minilisp.pm

@@ -10,6 +10,7 @@ use constant {
   IDENT => 3,
   STRING => 4,
   NUMBER => 5,
+  KEYWORD => 6,
 };
 
 sub tokenize {
@@ -33,6 +34,13 @@ sub tokenize {
         {
           push @tokens, { type => RPAREN };
         }
+      elsif($str =~ s/^'([^\s()"]+)//)
+        {
+          push @tokens, {
+            type => KEYWORD,
+            value => $1,
+          };
+        }
       elsif($str =~ s/^"(([^"\\]|\\.)+)"//)
         {
           my $value = $1;
@@ -86,6 +94,10 @@ sub lisp_format {
     {
       return "(" . join(" ", map { lisp_format($_) } @$e) . ")";
     }
+  elsif (ref($e) eq "KEYWORD")
+    {
+      return "'" . $e->{value};
+    }
   else
     {
       return "$e";
@@ -305,6 +317,11 @@ sub parser_expr {
         return $ctx->{$name};
       }
     }
+  elsif($tok->{type} == KEYWORD)
+    {
+      my $k = bless { value => $tok->{value} }, "KEYWORD";
+      return sub { return $k; };
+    }
   elsif($tok->{type} == STRING || $tok->{type} == NUMBER)
     {
       return sub {