Comment by zephen

> You have the hidden inner iteration loop inside the `.extend` standard library method driving the lazy generator expression with _unspecified_ one-step-at-a-time semantics

That's why it wasn't the first thing I wrote.

> To explain what this does to a beginner, or even intermediate programmer.... oooooh boy.

As if the raku were better in that respect, lol.

> Some people program in APL/J/K/Q just fine, and they prefer their symbols.

APL originally had a lot of its own symbols with very little reuse, and clear rules. Learning the symbols was one thing, but the usage rules were minimal and simple. I'm not a major fan of too many different symbols, but I really hate reuse in any context where how things will be parsed is unclear. In the raku example, what if the elements were to be multiplied?

> Calling it "stupid" is showing your prejudice. (I don't and can't write APL but still respect it) > Reminds me a bit of the fish anecdote told by DFW...

Yeah, for some reason, it's not OK for me to insult a language, but it's OK for you to insult a person.

But you apparently missed that the "twisty" part was about the multiple meanings. Because both those symbols are used in Python (the * in multiple contexts even) but the rules on parsing them are very simple.

perl and its successor raku are not about simple parsing. You are right to worry about the semantics of execution, but that starts with the semantics of how the language is parsed.

In any case, sure, if you want to be anal about paradigm purity, take my first example, and (1) ignore the print statement because the raku version wasn't doing that anyway, although the OP's python version was, and (2) change the accumulation.

  seq = [0,1]
  while len(seq) < 20:
    seq = seq + [seq[-2] + seq[-1]]

But that won't get you very far in a shop that cares about pythonicity and coding standards.

And...

You can claim all you want that the original was "pure" but that's literally because it did nothing. Not only did it have no side effects, but, unless it was assigned or had something else done with it, the result was null and void.

Purity only gets you so far.