On Sun, May 24, 2020 at 11:17 PM Tim Peters <tim.pet...@gmail.com> wrote:
> [Guido] > >> I’ve never been able to remember whether (f@g)(x) means f(g(x)) or > g(f(x)). That pretty much kills the idea for me. > > [David Mertz] > > Well, it means whichever one the designers decide it should mean. But > obviously it's a thing to remember, > > and one that could sensibly go the other way. > > > > On the other hand, when I showed an example using filter() a couple days > ago, I had to try it to remember whether > > the predicate or the iterable came first. Lots of such decisions are > pretty arbitrary. > > Best I know, f@g applies g first in every language that implements a > composition operator, and in mathematics. While that may be arbitrary, > it's easy to remember: (f@g)(x) "looks a heck of a lot more like" > f(g(x)) than g(f(x)) because the former leaves the identifiers in the > same order. > I personally find it easy to remember, but I can imagine (perhaps incorrectly) how others may find it difficult. There's a 'wrong' rule that's equally easy to remember: f@g means call f then call g. In fact, when you think of it like that, you might not even notice that this means g(f(x)). So even if you know that f@g means f(g(x)), you can still mess this up by not thinking it through. This is similar to when you have a list of decorators. Lots of people expect them to be called top to bottom ( https://stackoverflow.com/questions/27342149/decorator-execution-order) and it's not easy to make the real order feel more intuitive.
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