# Writing Christmas Trees, or Building A Cute Turing Tarpit

## December 27, 2012

My wife and I recently decided to enter this month’s contest over at PLT Games. We produced a variation of the lambda calculus called Tannenbaum in which every program looks like (or hopefully looks like) a Christmas tree. For example, here is omega, which is normally written ((lambda (x) (x x )) (lambda (x) (x x))). It’s just a cute little bush!

To be honest, the language interpretation itself isn’t difficult, once in the proper form—once everything is lambdas and application tags and variable and line references. But properly stripping it apart is a fancy trick. Luckily, most of the string processing tools I needed got written while I was writing that blogging software last week. The harder part, though, was properly picking apart application, and that’s the algorithm I’m going to spend this talking about.

The idea is simple: given application tags and the guarantee that every application is of one argument, properly group up the arguments, separated only by %. For example, (@ @  % $%$ ) is, in Scheme, (((var 1) (var 0)) (var 2)). But given a chain of application tags and delimiters, how does one group up and properly partition these?

I wouldn’t call my solution elegant, but it gets the job done. I have never done this before, so it was certainly a first for me. (I imagine that automatic currying follows a similar pattern, so maybe this is a more useful exercise than I initially suspected.) But I’ve pontificated enough; onto this algorithm:

The algorithm I have uses three helpers: app-splitter, app-builder, and app-parser (which really just calls app-builder on app-splitter’s output). The app-splitter function walks over the input, as shaped above, using two stacks. Any time it sees a symbol such as a +, \$, or 0, it simply pushes that symbol onto stack A. On a %, the current contents of stack A are moved to stack B as their own element (as a list). This means that the delimiters chunk up the input.

When an @—application—tag is found, if stack A is empty an app tag is pushed onto stack B. If stack A is empty, it is moved onto stack B before the app tag is pushed on. At the end of all of this, the string is split into tokens based on its application and argument delimiters, but still shaped incorrectly (and backwards).

This moves us to app-builder, which takes this reversed list and adds proper nesting by using its own stack as follows: it ‘pops’ something off the original stack handed to it (stack B from before), and if it’s not an app tag, the element gets pushed onto app-builder’s own stack C. An app tag pops the previous two elements off of stack C and pushes the whole (app [element 1] [element 2]) back onto the stack (in case it’s nested in another application tag). This repeats until stack B is empty, at which point (if the input was correct), stack C contains the properly-shaped result.

This completes the parsing, handing out the result. I thought this approach was neat (though possibly a little expensive); it requries two passes over the code for O(2n) complexity based on input size. It is certainly possible to do all of this in one pass, but it would require performing building up tokens while reshaping applications, and must be done back-to-front.

As an interesting aside, I find this entire exercise has led me to one interesting historical insight: I suspect the reason that APL is right-to-left precedence is for precisely this reason of application. Since this requires far less backtracking when done in reverse, if a computer with little power is reading from left to right, providing precedence from right to left allows you to perform this analysis while reading input.

(It’s worth noting that doing this in one fell swoop only requires one stack, and would probably be far more efficient after the initial reversal. That said, I feel like this implementation is more clear at the cost of efficiency.)

I hope this article helps someone somewhere; writing a parser like this is a nice academic endeavor, if nothing else, and I encourage you to write one for your favorite language some time.

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