[mikeash]

It seems to me that it's a cultural problem more than anything. People know that they shouldn't take an existing, well-known method name and repurpose it for something completely different. But somehow a lot of people think it's OK to repurpose an operator to mean something completely different just because it's convenient. C++ being the classic example. << means bitshift. Just because it looks like arrows doesn't mean it's sane to repurpose it into a completely different "output this" operator. You wouldn't override "leftShift" to output stuff, so why do people do it with <<?

I think it depends a lot on the languages. Operator overloading is easy and common in Python, but in my (limited) experience, it's used to create new classes which respond to existing operators with existing semantics, e.g. to make new number classes. For this, it's great. There's no particular reason the types built-in to the language should get special treatment for operators, as long as programmers can resist the temptation to be idiots.

[pcote]

>Operator overloading is easy and common in Python, but in my (limited) experience, it's used to create new classes which respond to existing operators with existing semantics, e.g. to make new number classes.

In Python, it's called "special method attributes" and they can do a lot more than just create classes. The __eq__ attribute returns a straight boolean for example in response to the "==" operator. And yes, by the way, you actually can accidentally alter attributes in the calling and callee objects if you aren't careful.

[mikeash]

Sorry if I was unclear, but I meant that this facility lets you create classes whose instances work like built-in numbers, not that the facility itself is used to create clases somehow.

[stelonix]

"But somehow a lot of people think it's OK to repurpose an operator to mean something completely different just because it's convenient. C++ being the classic example."

Do you have any data to prove such assertions? A C++ STL operator chosen more than a decade ago is not enough statistical data for one to have such a narrow-minded vision and say these bold statements. Maybe you need to check all the nice C++ libraries that make use of operator overloading.

[tragomaskhalos]

There's basic language bigotry going on here: an aged internet meme declares that C++ operator overloading is bad because you can indeed, in theory, do idiotic counter-intuitive things with it. Yet the identical capability in Ruby, Scala and Haskell (inter alia) is deemed A Good Thing by the consensus.

[ScottBurson]

I think it's an interesting phenomenon in the sociology of programming languages.

A powerful new feature is introduced in a niche language, and novices abuse it. The feature gets a bad reputation. Yet eventually, as new languages adopt it, the community learns how to use it and how not to use it, and people become accustomed to it, and come to agree that it's not such a bad thing.

I'd say that's happened, to some degree, with garbage collection and lambda expressions. We seem to be in the middle of the process with operator overloading, and still in the early stages with Lisp-style macros (I can hope, anyway :-).

[ethzero]

It's not an identical capability. Scala doesn't technically have operator overloading, it has symbolic method names and operator syntax for methods. This means you can use any valid name as an operator.

With C++, you can only overload the built-in symbolic operators. This means instead of choosing the best symbolic name to use as an operator you're forced to re-purpose some built-in operator like "<<". In Scala you can pick any name or symbol you want.

[tragomaskhalos]

Yes I know that, but the point is that the "operator overloading is terrible" complaint typically cites either plainly daft straw men like "what if someone overrides + to mean subtraction", or else hones in on << for streams, which as others have pointed out here is actually perfectly readable and sensible. I have yet to see a single sensible criticism that holds water (OK, MS overloading address-of & on a smart pointer, that one was stupid ..)

Conversely, there was an XML library for Scala posted on Reddit about a year ago that used things like <:<, <<? and <> as method names; clearly the freedom to concoct your own symbolic names is far more open to abuse than the oft-maligned C++ mechanism.

[mikeash]

Two big problems with overloading << for streams are operator precedence (the precedence of << makes little sense for a stream operator) and the fact that the chained syntax can make the whole thing ambiguous (which is kind of a subset of the precedence problem), e.g.: cout << x << 3, does that shift x by 3, or output x and 3 separately?

[mercurial]

Same with Haskell, which makes it compact but not always very readable (it effectively turns many libraries into their own non-intuitive DSL).

[kzrdude]

You can't overload operators in Haskell. You can define new operators though. You're not going to change the meaning of + unless you remove the old.

[mikeash]

For me, and perhaps others, this is because C++ enshrines idiotic counter-intuitive operator overloading in its standard library.

[tragomaskhalos]

Examples ?

[mikeash]

The use of << and >> for streams is the one that always comes to mind. So minor, and yet so awful.

[tragomaskhalos]

Well this is clearly a matter of taste; the visual implication of movement (http://images.google.com/images?q=diversion+sign+chevrons) is quite elegant IMHO, and the precedence seems natural also. But others' mileage obviously varies ...

[pjmlp]

The common example people pull off the hat with C++ is the use of << operator for streams and some UI toolkits.

Which I personally never had a problem with.

[mikeash]

You seem to have confused the phrase "classic example" with "the worst thing out there, with data to prove it".

It's an example. Do you dispute that? It's classic. That's subjective, and I claim it by raw assertion. That's it.