Sounds like a pointless misfeature that will only generate mandatory "do not @#$$ do this" entries in future Nim coding style documents, and Nim linting programs that find and flag abuses.
The motivation is good, if the intent is to get rid of ___unwanted___crap___ like this.
But if I, as a language designer, wanted to ban such identifiers, I would just go ahead and ban them, rather than making them equivalent to ones that do not have repeated underscores.
The principle is: don't make unwanted/undesirable forms equivalent to acceptable forms in hopes that people will then just stick to the acceptable forms when they discover that the the unwanted forms don't bring about the difference they were hoping for. People won't. People will go to town with the equivalence to make problems for other people.
People who don't know about the equivalences, or have forgotten, will be tripped up. Someone might think that a variable called __foo in an inner scope is different from an outer (or global) _foo, yet their definition will shadow _foo, with some behavior-altering consequences.
It's possible to argue about this endlessly from a theoretical perspective, but Nim is not a new language, there are substantial code bases (not as substantial as C, Java or Python, of course, but still substantial), and these rules have been worked out through real world cases, and are very effective in practice.
From a theoretical perspective, your argument can equally be applied to C's case sensitivity or Pascal's case insensitivity when someone "doesn't know" or "forgets" about the equivalece - isn't it absurd that FOO and foo are different (C) / equivalent (Pascal) when you are coming from the other one? Similarly, Lisp-1 vs. Lisp-2 . In practice, it's just one more convention -- among several others.
The motivation is not to get rid of __unwanted___crap___ (nim bans multiple sequential underscores and leading underscores, so they are rid of), but rather: Nim arguably has the best built-in FFI of any modern language with nontrivial use, and this FFI has been a factor in the language design. Nim's rules allow you to keep a mostly uniform coding style in _your_ parts, yet integrate it naturally into projects using snake_case, CamelCase, javaCase and ALLCAPSCASE and SHOUTING_MATCH_CASE, or all of the above in the same project.
Nim started out case-insensitive (which is less popular, but definitely common choice made e.g. by Pascal, Excel and others). IIRC, the "first letter's case does matter" is a relatively recent addition (as in, 2 years out of the project's 9) to simplify FFI to conventions like OpenGL, which have the same identifier in both lower or upper case.
In theory, everything could go wrong. In practice, Nim gets it exceptionally right.
Are you saying that the FFI transparently renames identifiers, so you think you're calling foo_bar, but the actual foreign function is FooBar, with no traces of FooBar in the program (like in some definition which indicates that the two are mapped together)?
If so, that's an incredibly bad idea.
If a program calls some foreign function called FooBar, the identifier FooBar better appear somewhere in it, if you know what's good for the maintainer seven years from now.
Ah, but does the real name appear together with the Nim name in some line of code that binds them together, which your editor can jump to if you want to know where that Nim name is defined?
If it's just some construct that defines a call to FooBar in the foreign library, with no mention of foo_bar, but elsewhere in the Nim code we call it as foo_bar, I'm afraid I cannot agree with this being a good technical decision in language design.
I wouldn't sign off on such a concealment ruse even if it were someone's macro, not being upstreamed into a language implementation at all.
That _is_ the nim name. It's just case-and-underscore insensitive so you definitely can if your editor is Nim aware (at the very least emacs, vim and vscode have been aware for a long time; nimgrep is a command line tool similar to grep that is aware of nim's sensitivity rules.
You don't have to agree, you don't have to like it, you don't have to use Nim, and you don't have to think it's a good idea. But any reasonable sampling of real world use shows that it's not a bad technical decision. I don't think it shows it's a great technical decision either; It just shows that it's acceptable and comparable in mental and operational burden to any other decision.
It seems whether it is a little better or a little worse, it will take years of experiment to get any kind of statistical significance.
Nim does not allow variables starting with underscore.
Also, the compiler errors cannot if any of "useHTTP", "usehttp" or "use_http" is acceptable or unwanted, but it can error out with a clear warning if you are trying to define different variables with those names in the same scope.
You shouldn't modify code in a programming language you don't have sufficient familiarity with - at least not code that you depend on.
C has shortcut boolean ops, so "if (have_peace_treaty || launch_missiles() == LM_SUCCESS) cross_border()" would not launch missiles if you have a peace treaty.
C++ has shortcut boolean ops as well... unless you have overloaded "operator&&". So the same line in C++, depending on the types involved, might not short-circuit and start a war even if you have a peace treaty.
In Pascal and Python 3, evaluating 1/(1/2) gives 2. In python 2 and C, you get an integer overflow (with differing semantics). But 1.0/(1.0/2) gives 2 in all cases, even though 1==1.0 is true in all languages mentioned.
And "if (0.3*3 == 0.9) all_is_well(); else apocalypse_go_to_bunker()" might also surprise you if you are not aware of how FP math is implemented (in APL/K/J, all_is_well(), but not in any other language I'm familiar with).
The only example I can think of that could bite you is when shadowing an outer scope -- which is just as much a problem in C, Python etc. Some compilers warn about it, some don't, I don't know if Nim does.
In practice, Nim choices put it in an amazingly sweet spot.
The motivation is good, if the intent is to get rid of ___unwanted___crap___ like this.
But if I, as a language designer, wanted to ban such identifiers, I would just go ahead and ban them, rather than making them equivalent to ones that do not have repeated underscores.
The principle is: don't make unwanted/undesirable forms equivalent to acceptable forms in hopes that people will then just stick to the acceptable forms when they discover that the the unwanted forms don't bring about the difference they were hoping for. People won't. People will go to town with the equivalence to make problems for other people.
People who don't know about the equivalences, or have forgotten, will be tripped up. Someone might think that a variable called __foo in an inner scope is different from an outer (or global) _foo, yet their definition will shadow _foo, with some behavior-altering consequences.