Opa Tutorial - Intermission

Cedric Soulas of MLState - the creators of Opa - has very kindly put up some suggestions on writing code based on my tutorials. They're available via GitHub at https://github.com/cedricss/ian-oliver-tutorials . My code resides over on SourceForge.

While these are based on part 4 of the series they are applicable to later parts and indeed I will integrate these in due course.

One thing I do want to point out now is the use of a parser instead of an explicit pattern match dispatching URLs to specific functions. Very briefly the code which reads:

function start(url)

{

  match (url)

   {

       case {path: [] ... }: hello();

       case {path: ["expressions" ] ... } : expressionsRESTendpoint();

       case {path: ["expressions" | [ key | _ ] ] ...} : expressionWithKeyRESTendpoint(key);

       case {~path ...}: error();

   }

}

Server.start(

   Server.http,

     [ {resources: @static_include_directory("resources")} , {dispatch: start} ]

);

can be rewritten using a parser:

start = parser {
   case "/": hello();
   case "/expressions" : expressionsRESTendpoint();
    case "/expressions/" key=((!"/".)*) (.*):      expressionWithKeyRESTendpoint(Text.to_string(key));
   default: error();
}

Server.start(
   Server.http,
   [ {resources: @static_include_directory("resources")} , {custom: start} ]
);

Which performs more or less the same function with the bonus that we obtain much more flexibility in how we process the URL - rather than treating it statically in the pattern matching. Note how we're no longer calling an explicit function but now have a parser as first-class object :-)


At this time there's relatively little documentation on the parser in Opa, but on the Opa Blog there's a tantalizing glimpse of what is possible.


So with that to whet your appetites go and download the latest nightly build and start coding...while I finalise composing part 5...