This will be a cool demo once the kinks get worked out. It lets you see first-hand that you can choose which redex to beta-expand first. Haskell is an excellent choice for this because -- unlike almost every other language -- it does not matter the order you evaluate subexpressions in. I feel like this could be useful to show someone when teaching them to program.

The language in the function editor on the right looks like Haskell. I'm confused as to what the language you run is supposed to be, though. It doesn't seem to have Haskell syntax or semantics. The lists aren't comma-separated, and the main expression seems to require parenthesis? And what is this program doing?

    (map (plus 1) [12])
    (( 1) : (map (plus 1) [2]))
    (( 1) : (( 2) : (map (plus 1) [])))
    (( 1) : (( 2) : []))
    (( 1) : [( 2)])
    [( 1)( 2)]

vs. in Haskell

    Prelude> map (+ 1) [1, 2]
    [2,3]

It is supposed to be Haskell-ish, but admittedly I haven't done much Haskell in a while. I just went roughly with what Steve suggested. But yeah, the missing commas in lists is a pretty big mistake!

In any case, the parser can be found here, https://github.com/stevekrouse/hs.js/blob/812945939e14e3370f..., the transformations on the AST here, https://github.com/stevekrouse/hs.js/blob/812945939e14e3370f..., and the specs here, https://github.com/stevekrouse/hs.js/blob/812945939e14e3370f....

Fixed the commas in https://github.com/stevekrouse/hs.js/commit/d087de3c6dee2827... :-)

Lists don't need to be comma-separated in haskell, you can use the colon infix operator to construct lists. http://stackoverflow.com/questions/1696751/what-does-the-inf...

Very nice. It took me a while to realize that array literals aren't supported, but once I did that I was able to write concatMap and watch it being evaluated.

A couple of points:

- If there is a syntax error, the box just turns red. It would be nice to see what the issue is.

- You could probably make it clearer that it's possible to execute arbitrary expressions, by editing the map term, for example.

I've added error messages in https://github.com/stevekrouse/hs.js/commit/2ed274d4607a4c9d... and https://github.com/stevekrouse/hs.js/commit/812945939e14e337....

The (edit) and (clear) links are now always visible to encourage editing, per https://github.com/stevekrouse/hs.js/commit/e301961c03d92c58...

Thanks!!

Pretty cool, although it keeps crashing my Safari on OSX Yosemite beta somehow :)

Neat, however it feel clunky. Few things that bugged me:

1. Clear does not work. Firefox 22.0, private mode.

2. Clicking to expand gets tedious fast. Adding shortcut like space would be nice.

3. No syntax highlighting on function editor, also lists could use some colors.

Thanks. Fixed 1. with https://github.com/stevekrouse/hs.js/commit/265c0a6632f793f4..., but not sure why that didn't work in FF.. :\

2. But what should we expand then? Just the first one? Part what makes it interesting is that you can decide for yourself what to expand, and it doesn't matter in which order you do it (Church-Rosser).

3. Feel free to hack CodeMirror or Ace in there! :)

4. Hitting 'enter' after editing only sometimes gets you out of editing mode; seemingly only if your expression is well-formed?

5. 'Clear' clears some lines but not all. It usually seems to clear all but the top two lines for me.

Chrome v.36 in Ubuntu

there is a typo

  This document implements a small, dyanmically-typed, 
  subset of Haskell that includes integers, lists, functions,      
  pattern matching and recursion.

Also, Haskell is strictly typed... but most of us aren't ready for that anyway :-P

Thanks, fixed in https://github.com/stevekrouse/hs.js/commit/9882af6312f93a29...

Really awesome stuff JP and Steve! I've been having similar ideas about the subject myself :)

Thanks Michael ;)

I submitted this to /r/Haskell and chrisdoner left you some praise and feedback (he thought I wrote it).

http://www.reddit.com/r/haskell/comments/2d1dpg/λ/cjlakwi

Doesn't appear to work on ipad.

Thanks. Hopefully this helps, tested it in the XCode simulator: https://github.com/stevekrouse/hs.js/commit/3558d2575a194d73...

Would love to play with this, but clicking the expansions crashes Safari on my machine

Hm, cannot reproduce, sorry!