Wednesday, March 18, 2015

Nearly full circle: A functional programming language

My mind is like an attic, stuffed with everything I can't bear to get rid of, and occasionally I bring something out and work on it (only to put it back later).

The earliest posts that I wrote on this blog related to my plan to create a functional programming language. Over the last few weeks, I have actually done that. It's nothing pretty, but it really does work, with lambda expressions and everything.

I worked backwards, starting with how the language would work on the inside, without worrying about what it would look like on the outside. The final language isn't finished yet, but I created a kind of intermediate testing language to use for checking the architecture.

The testing language is not supposed to be pretty. My main consideration here was to create a language that could be parsed by a very simple parser (with no lookahead), so that meant leaving out a lot of syntactic sugar and salt.

Here is my first program in the testing language.

using System
using FBaseUnitTests
let plus
    method Program _test006plus [ Int32 , Int32 ]
let ifzero
    lazyMethod Program _test006ifzero [ Int32 , Int32 , Int32 ]
let func
    lambda [ Int32 ] [ x ]
        apply variable ifzero [
            variable x ,
            value Int32 1 ,
            apply variable plus [
                variable x ,
                apply variable func [
                    apply variable plus [
                        variable x ,
                        value Int32 -1
let r
    apply variable func [ value Int32 16 ]

The most important thing here is that functions are first-class citizens. The variables plus and ifzero hold native methods (one of which has some lazy parameters), while the variable func holds a lambda expression.

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