Thursday, December 31, 2015

Suneido Functional Compose

I started on one project, found it required another, which then led to code like:


which seemed ugly. I knew that functional programming has a higher order "compose" to handle this more cleanly. (Higher order because it takes functions as arguments and returns another function.)

Suneido isn't specifically a functional language, but it has a bunch of functional methods and functions. I hadn't written Compose yet but it turned out to be quite easy.

function (@fns)
    return {|@args|
        result = (fns[0])(@args)
        for fn in fns[1..]
            result = fn(result)

If you're not familiar with Suneido code, the '@' is used to accept multiple arguments and to expand them again. The {|...| ... } notation is Suneido's way of writing a "block", i.e. a lambda or closure. (Suneido's use of the term "block" and the syntax come from Smalltalk, one of its roots way back when, pre 2000.)  Because, in Suneido, blocks are used for control flow, "return" returns from the containing function, so to return a result from a block we take advantage of how Suneido by default returns the value of the last statement.

Although in some languages the arguments to compose are listed "outside-in", I chose to have them "inside-out", so Compose(a,b,c) returns a function that does c(b(a(...))) in the same way you would write a *nix pipeline a | b | c i.e. in the order they will be applied.

I documented it, like a good programmer, except when I went to add the "see also" section I found a lot of the other functional stuff had never been documented. So I spent a bunch more time documenting some of the that. In the process I almost got sucked into improving the code, but I resisted pushing yet another task onto the stack!

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