Monday, September 21, 2015

Syntax is a minor matter

I just learnt of the existence of Marpa: Basically this generates a BNF parser that is simple and efficient. I'm not sure how easily Wombat's user-defined syntax system can be implemented with Marpa, but the key point is this:

Having syntax built in to a language definition is going to alienate people. It isn't necessary. A core aim of Wombat is that different folk can use different syntax systems, including rolling their own, and still use each others libraries without difficulty. Of course Wombat will have default syntax for the standard libraries, based on the prejudices of the core developers, but even that can be easily changed. You prefer to separate your list elements with commas instead of spaces, or you like your "if" expressions to end with "fi": easy to do. Wombat may provide standard WCTL (Wombat Compile Time Language) functions to allow programmers to specify common styles.

What is important is the structure of the Type system. This is where I think Wombat is different (though the relevant literature is vast, so I could well be wrong). If a Rational has a denominator of 1 (or a Gaussian Integer has an imaginary part of 0), then it is an Integer, and not distinguishable from an Integer that arose in a more obviously Integer way. If two types have a non-Empty intersection (as Gaussian Integers and Rationals do) then you are not allowed to define a lossy conversion from one to the other: Conversion can only happen losslessly by going down to the intersection and then up (and it may then, naturally, fail).

I trained as a Mathematician, and I think this is the way most Mathematicians operate most of the time. Sometimes they will show a bit of guilt by writing "by abuse of notation", but I think they really feel it is ok. By contrast Mathematicians involved in the foundations of Mathematics don't allow this sort of thing at all. I suspect this is a big part of the disconnect between the two camps. Programming is in a funny state, being a mixture of very hacky stuff on one side, plus stuff that has come over from work on the Foundations of Mathematics. I think that Wombat, by contrast, has the right balance, corresponding to the way most people think.

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