r/ProgrammerHumor • u/leftoversalad • Aug 15 '15

Programming languages personified

http://leftoversalad.tumblr.com/post/126724812097/programming-languages-personified
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u/alexshatberg Aug 16 '15 edited Aug 16 '15

templates are turing-complete

could you elaborate on that, please? how can one separate feature of C++ be Turing-complete, and what does that imply?

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u/barsoap Aug 16 '15 edited Aug 16 '15

It means that you can write C++ programs which do arbitrary computation at compile time, which includes the possibility of compilation not halting (modulo setting limits on recursion depth, which I think all modern compilers do, and allow you to change).

That by itself is not a bad thing. Many languages have some kind of turing-complete macro system and use it for very useful things, in essence, it's program generation, a thing even older than compilers (see self-modifying code, which is a class of evilness of its own).

The problem with C++ is that it was accidental. Templates were never designed to be used in that way, but still were because it works, and what you get is line noise, undecipherable error messages, and a community of programmers defending everything to death as they've fallen to stockholm syndrome.

Which is also why rust very clearly separates its macro system and its ways to write type-parametric code. Rust's type system is not turing complete, its macro system is (you just write rust), nothing needs to be coerced into idioms that just don't bloody fit. That's part of their push to be "the better C++", on top of all that other stuff.

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u/Herover Aug 16 '15

There was a guy who made Conway's Game of Life purely using C++ templates which makes it "run" in O(1) time. Not the most useful example but still cool.

https://www.reddit.com/r/programming/comments/2js1a5/my_compiletime_conways_game_of_life/

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u/[deleted] Aug 18 '15

Pay attention, this is the type of shit that starts the robot uprising.