Add function filter and a test

lib/Minilisp.pm

@@ -284,6 +284,13 @@ my %stdctx = (
     $v = $cb->($v,$_) foreach (@copy);
     return $v;
   },
+  'filter' => sub {
+    my ($cb, $list) = @_;
+    die "map: First parameter must be a function" unless ref($cb) eq "CODE";
+    die "map: Second parameter must be a list" unless ref($list) eq "ARRAY";
+
+    return [ grep { $cb->($_) } @$list ];
+  },
   'cons' => sub {
     my ($v, $list) = @_;
     return [ $v, @$list ];

t/lists.t

@@ -34,3 +34,7 @@
 
 (expect "short notation can create empty lists"
         (zerop (length '())))
+
+(expect "filter - evenp"
+        (equal (filter evenp '(1 2 3 4 5 6))
+               '(2 4 6)))