We would use multi-character names for Branch, Empty and Tree because it matters what those things represent. It would be thoroughly confusing if we instead wrote
t a = e | b a (t a) (t a)
However, we don't care what the 'a' represents. It's just a placeholder for an arbitrary piece of information. If we had to write
then we have just introduced a lot of unnecessary line noise.
One difference between programming and mathematics is that programming is mostly interpreted in context, when it matters that this double represents elapsed time, and this double represents dollars in my checking account. Mathematics, on the other hand, is mostly interpreted out of context. I don't care what a represents, all I care about is that it enjoys the relationship ab = ba with some other arbitrary object b.
If the mantra of programming is "names are important" then the mantra of mathematics might be "names aren't important".
Sure, your original example with single-letter type variables makes sense to me, since those variables could represent anything. I never meant to object to those. I just wanted to point out the fact that your example also included multi-letter names, while mathematics generally does not.
So if you really don't care what a variable represents, then I'd agree that a single-letter name is fine. Given that math is almost universally done with single-letter variable names, are you suggesting that in math you almost never care what a variable represents? This wikipedia article makes me think otherwise; clearly, variables often have a fairly specific meaning.
One difference between programming and mathematics is that programming is mostly interpreted in context, when it matters that this double represents elapsed time, and this double represents dollars in my checking account. Mathematics, on the other hand, is mostly interpreted out of context. I don't care what a represents, all I care about is that it enjoys the relationship ab = ba with some other arbitrary object b.
If the mantra of programming is "names are important" then the mantra of mathematics might be "names aren't important".