A Haskell JSON service and updated Try Haskell!

UPDATE: The service went down 5 April at about 19:50 GMT+1 to about 20:21. I had to update the watchdog to be more aggressive for Linode (as opposed to my development machine). Just letting you guys know.

Make your own Try Haskell!

You can now run Try Haskell! as a simple HTML file locally, or on your own domain, if you like! All you need is an internet connection.

Grab the source with git (or download a .zip or .tar.gz of the latest version):

$ git clone git://github.com/chrisdone/tryhaskell.git

Get writing interactive tutorials! Please let me know how you get on.

Evaluate Haskell code anywhere

You can now also use the JSONP service that Try Haskell uses from anywhere!

It is located at: http://tryhaskell.org/haskell.json

And you can download the haskell-json source from Github:

$ git clone git://github.com/chrisdone/haskell-json.git

Here is a simple Haskell evaluating text box. Download the source.

Haskell evaluation

Expression:

Result: Output here.

Type: Type here.

Haskell loading

Code:

Result: Output here.

Feel free to make this pretty and use it in your blog or books, Haskell courses, etc. Trying Haskell should be easy!

Methods and options

eval method

There is an eval method for evaluating expressions, used like this:

http://tryhaskell.org/haskell.json?method=eval&expr=23*52

and you get back:

{"result":"1196","type":"(Num t) => t","expr":"23*52"}

Note that computations are limited to 750ms. This limit may be softened if people have problems doing useful things.

load method

And there is a load method for loading your own file into the service for later evaluation, used like this:

http://tryhaskell.org/haskell.json?method=load&contents=x=1

and you get back:

{"success":"wrote file."}

Then you can request

http://tryhaskell.org/haskell.json?method=eval&expr=x

and get back:

{"result":"5","type":"Integer","expr":"x"}

Hurrah!

guid for unique states

If you are writing a tutorial or a blog post, maybe you want your visitors to be able to maintain two different states depending on what post/chapter they have open in what tab. You can do this by adding a guid to your request:

http://tryhaskell.org/haskell.json?method=load&contents=x=1&guid=ch2

For “chapter2”, or maybe you could use the ID of your blog post, or whatever you want, with the condition that it is alphanumeric.

pad or “JSONP”

If you are making your request from JavaScript, you will need to follow the JSON style of making requests. haskell.json supports this by adding a pad parameter:

http://tryhaskell.org/haskell.json?method=eval&expr=Just%201&pad=doStuff

and you get back:

doStuff({"result":"Just 1","type":"(Num t) => Maybe t","expr":"Just 1"})

Implementing simple JSONP in JavaScript

I use jQuery in these examples, because it is really easy.

So for example I wrote a little function for tryhaskell for making JSONP requests:

function jsonp(url,func) {
    handleJSON = function(r){
        script.remove();
        func(r);
    };
    var script = $('<script type="text/javascript" src="'+url+'"></script>');
    script.attr('src',url);
    $('body').append(script);
}

And it references a global handleJSON function that I define at the top level so that any script file could access it:

var handleJSON = function(a){ alert('Unassigned JSONP: ' + a); }

(Just have a little alert message in case we get something wrong.)

It adds a script tag to the body with the URL we want to query. This is how it gets around the domain restriction of normal AJAX. It is kind of an ugly solution if you ask me, but there you go!

Once the script has been loaded, the global function handleJSON is called. So before that happens, we need to overwrite it handleJSON with our closure to remove our script tag from the DOM and then call the function provided to jsonp.

Bringing it all together we can write something like this:

jsonp("http://tryhaskell.org/haskell.json?method=eval&pad=handleJSON&expr="
       + encodeHex("34*23"),
      function(resp){
         alert("Result: " + resp.result);
      });

Encoding what you provide in the URL is important.

See the sample zip file above for an example of this.

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© 2010-04-05 Chris Done <chrisdone@gmail.com>