Sunday, November 10, 2013

A chartreuse analogy

This is a quick follow-up on the last post, where I sketched an idea for a functional programming language where the lambda was implicit.

First, let me repudiate the whole idea of the unresolve metafunction. There's really no need for it, and it just makes things messy. I'm going to go back and cross that bit out.

Second, it might have occurred to you that, if the lambda is implicit in the way I described, then there is no distinction between values and expressions that take no parameters. At first, that seems like a problem, but I don't think it is. In a pure functional context, what is really the difference?

Of course, even the purest functional language must eventually sully itself by making contact with this sordid and messy world, and there you might have a problem, since your parameterless functions need to be evaluated in order for any side-effects to occur.

Which brings me to an idea about how to separate the pure (and provable) side of a program from the impure, messy side: Rather than trying to incorporate both worlds in one language, let them be separate languages.

Functional languages are bad at being imperative, and imperative languages are bad at being functional. So strictly separate the two by writing the core code in a functional language, and accessing it from the real world via an imperative language. Naturally, you want to be able to compile the two together, and you want the languages to have common features, but let it be clear when you are coding in one or the other, and let the compiler enforce the purity of the functional side.

You could think of the functional (sub-)language as being like the eremitical side of a Carthusian monastery, and the imperative (sub-)language as being like the cenobitic side. In a Carthusian monastery you have the hermits, whose lives are dedicated to prayer and contemplation, and the lay monks, who serve the hermits and maintain the monastery. The two are strictly separated and work in different domains, but are integrated with each other in a symbiotic way.

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