Allow coderefs to be passed via "defaults"

Sometimes it's useful to execute something when a value is not passed at the
command line.  This patch provides an easy way to do that, by allowing
coderefs to be passed via "default", and executed if no value is provided.

lib/opts.pm

@@ -37,6 +37,9 @@ sub opts {
         # XXX: should we provide ways to check the type of invocant?
     }
 
+    # track our coderef defaults
+    my %default_subs;
+
     my @options;
     my %requireds;
     my %generaters;
@@ -50,8 +53,16 @@ sub opts {
         my $rule = _compile_rule($_[$i+1]);
 
         if (exists $rule->{default}) {
-            $_[$i] = $rule->{default};
+
+            if (ref $rule->{default} && ref $rule->{default} eq 'CODE') {
+                $default_subs{$i} = $rule->{default};
+                $_[$i] = undef;
+            }
+            else {
+                $_[$i] = $rule->{default};
+            }
         }
+
         if (exists $rule->{required}) {
             $requireds{$name} = $i;
         }
@@ -69,6 +80,9 @@ sub opts {
         local $SIG{__WARN__} = sub { $err = shift };
         GetOptions(@options) or Carp::croak($err);
 
+        do { $_[$_] = $default_subs{$_}->() unless defined $_[$_] }
+            for keys %default_subs;
+
         while ( my ($name, $idx) = each %requireds ) {
             unless (defined($_[$idx])) {
                 Carp::croak("missing mandatory parameter named '\$$name'");
@@ -180,7 +194,8 @@ opts is DSL for command line option.
     define option value is required.
 
   default
-    define options default value.
+    define options default value. If passed a coderef, it
+    will be executed if no value is provided on the command line.
 
   alias
     define option param's alias.

t/010_simple/04_default.t

@@ -4,11 +4,19 @@ use opts;
 use Test::More;
 use Test::Exception;
 
-is foo(), 99;
+is default(),            99;
+is default_as_coderef(), 99;
+
 done_testing;
+
 exit;
 
-sub foo {
+sub default {
     opts my $p => { isa => 'Int', default => 99 };
     return $p;
 }
+
+sub default_as_coderef {
+    opts my $x => { isa => 'Int', default => sub { 99 } };
+    return $x;
+}