Perl 6 Grammars – not only for parsing

Yesterday on #perl6 edenc, (a Catalyst dev), together with ruoso started discussing an idea of dispatching http requests with Perl 6 grammars. My nose smelled some awesomeness being baked around, so I decided to work something out too.

And that’s it. A grammar with paths (being regexes), methods-controllers building responses, in this case only plaintext ones, but that could have been objects as well.

grammar Mysite {
    token TOP {
        <main>    |
        <about>   |
        <contact> |
        <cookie>  |
        <api>     |
        <e404>
    }
    token main    { ^ '/' $ }
    token about   { ^ '/about' $ }
    token contact { ^ '/about/contact' $ }
    token cookie  { ^ '/cookie' $ }
    token api     { ^ '/api/' $<type>=[ \S ]+ $ }
    token e404    { }
}

class Mysite::Actions {
    method TOP($/) {
        make $/.values[0].ast;
    }
    method main($/)    { make "Hello from the main page!" }
    method about($/)   { make "About me" }
    method contact($/) { make "Reach me at 555 01 23" }
    method cookie($/)  { make $*REQUEST.cookie }
    method api($/)     {
        given $<type>.Str {
            when 'json' { make '{"foo":"bar"}' }
            when 'xml'  { make '<foo>bar</foo>' }
            default     { make 'Wrong serialization method passed' }
        }
    }
    method e404($/)    { make "404: Not found" }
}

class Request is Cool {
    has $.path;
    has $.cookie;
    method Str {
        $.path
    }
}

my ($*REQUEST, $req, $res);
$*REQUEST = $req = Request.new(path => '/cookie', cookie => 'monster');
$res = Mysite.parse($req, :actions(Mysite::Actions.new));
say $res.ast; # monster

$*REQUEST = $req = Request.new(path => '/nope');
$res = Mysite.parse($req, :actions(Mysite::Actions.new));
say $res.ast; # 404: Not found

Few interesting things:

• method TOP is just picking out the right result from the matched token
• token e404 is empty so it matches everything that wasn’t alredy matched
• Request is a class acting as a String, so it can store things like cookies, POST etc. aside from the query string
• $*REQUEST is a contextual variable (no worries, it’s not global although it may look like one) so we can extract the Request fields from inside the action methods

And there it is, shiny and pretty. Look out for some future posts, once I get more $time I’ll probably code some reusable components and present them here as well.

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