Even in your example, multi-character names are used for Branch, Empty, Tree, and List. And those are much more helpful than single-character names would be.
Plus, the math tradition of one-character variable names means that they've had to adopt several different alphabets just to get enough identifier uniqueness (greek, hebrew, etc., plus specialized symbols like the real, natural, and integer number set symbols). Which makes all that stuff a pain to type. And even then, there are still identifier collisions where different sub-disciplines have different conventions for the meaning of a particular character.
It's also annoying because single-character names are impossible to google for.
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.
Plus, the math tradition of one-character variable names means that they've had to adopt several different alphabets just to get enough identifier uniqueness (greek, hebrew, etc., plus specialized symbols like the real, natural, and integer number set symbols). Which makes all that stuff a pain to type. And even then, there are still identifier collisions where different sub-disciplines have different conventions for the meaning of a particular character.
It's also annoying because single-character names are impossible to google for.