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Sub-turing complete programming languages

Here is an interesting intuition: the key to liberating software development is to use programming languages that are not, by themselves, turing-complete.

That means no loops, no recursion 'in-language'.

Why? Two reasons: any program that is subject to the halting problem is inherently unknowable: in general, the only way to know what a turing-complete program means is to run it. This puts very strong limitations on the combinatorics of turing-complete programs and also on the kinds of support tooling that can be provided: effectively, a debugger is about the best that you can do with any reasonable effort.

On the other hand, a sub-turing language is also 'decidable'. That means it is possible to predict what it means; and paradoxically, a lot easier to provide a rich environment for it etc. etc. An interesting example of two languages on easier side of the turing fence are TeX and CSS. Both are designed for specifying the layout of text, TeX is turing complete and CSS is not.

CSS is still young but, for all its warts, an order of magnitude easier to work with than TeX. Further more, there is actually no much that TeX can do that CSS cannot; with the proviso that sometimes missing functionality must be 'buried' in the CSS language.

For example, TeX is powerful enough to implement an indexing scheme, CSS is not. It would be easy enough to extend a CSS engine to provide key indexing mechanisms.

I think that there are many fundamental merits to this approach to programming languages. The biggest is that a sub-turing complete language would (have to) be inherently more high-level than a turing-complete language. Secondly I believe (no evidence presented here) that such a language could be closer to the way people think about tasks than programming in Java (or even Haskell).

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