Infix notation for function call / invocation.

[Centril]

There doesn't seem to be much support for supporting custom operators (see #818) in Rust. But what about calling normal binary functions as if they were operators, like haskell allows you to?

The syntax would be:

a `dot` b

to convey scalar product of two vectors a and b.

Another option suggested by @bluss:

a \dot b

Or if it possible without parsing conflicts (@thepowersgang: "but probably not dersirable"):

a dot b

Which is converted to

a.dot( b )

or, depending on the function:

dot( a, b )

[ticki]

This is a TIMTOWTDI feature with very little (any?) gain. It doesn't read terribly well either.

[Centril]

Also discussed in: rust-lang/rust#8824

[Centril]

TIMTOWTDI = There's more than one way to do it

There's nothing inherently wrong with having more ways to do one thing.
Some prefer reading a dot b to a.dot( b ) since it gives a more "mathematical" feel to it.

[bluss]

a \dot b is the nicest syntax I have seen proposed for this (taken from the previous discussion).

The idea is to introduce one more operator to end all custom operators. Instead of defining custom operators, we can just use functions for this purpose.

I know the argument that methods are infix, x.dot(y) is infix. The infix binary function call proposal has advantages:

• Functions already support: namespacing, importing and renaming. Functions can also use their own traits that they defined for the operands, so we get quite a lot of flexibility by reusing an abstraction we already have (functions are great!).
• It staves off parenthesis build up. Methods may result in nested calls like x.dot(y.dot(z.dot(w)))).
  • x \dot y \dot z \dot w is easier to read and even a parenthesized infix call version is easier to read: x \dot (y \dot (z \dot w)).
• Free functions can use a conversion trait bound on both arguments, where methods can not do that for the self argument.

[ketsuban]

It doesn't seem beyond the wit of man that someone might reasonably expect a library-defined operator whose return type is the same as that of the first operand to be usable as an augmented assignment. How easy is that to do?

[Centril]

Discussion on fixity from IRC, for future reference...:

151205            vfs | Centril: are those to have some hard-coded precedence?
151242          bluss | Centril: if you want to. You can just say that either one or the other is a possible
                      | implementation
151301          bluss | proposals like this are judged from superficial issues like syntax too
151321        Centril | vfs: it would have to I guess, how high it should bind I don't know yet
151331          bluss | I mean, the syntax is important in the end, but it doesn't decide if the feature is worth
                      | having or not by itself
151357        Centril | bluss: would it be possible to skip the \ alltogether and just have a dot b ?

                                                            [...]

151924            vfs | Centril: what about associativity? bluss comment seems to suggest right associative, but
                      | afaik all rust operators are all left associative.
151942        Centril | vfs: haskell says:
                      | http://stackoverflow.com/questions/8139066/haskell-infix-function-application-precedence
151956        Centril | infixl 7 `dot`
152204          bluss | vfs: wouldn't the comment suggest left associative, if the (x (y  z)) version is the one
                      | needing explict ()'s
152220        Centril | so in haskell, a `plus` b `plus` c == (a `plus` b) `plus` c
152256            vfs | bluss: `x.dot(y.dot(z.dot(w))))` <-- this does if i read it correctly
152353        Centril | vfs is correct that it is right associative because it evaluates z.dot(w) fist
152357          bluss | anyway, it's not intended to suggest anything
152453        Centril | vfs bluss: one could (possibly) add an attribute to a function specifying its fixity if a
                      | default one isnt good
152533            vfs | what if function is in another crate?
152549       oli_obk_ | Centril: and how would that work together with other infix functions?
152636        Centril | vfs: then the fixity information would have to be carried over into the metadata in some
                      | way i guess
152644          bluss | vfs: the attribute should be on the function
152655          bluss | You'd use this with a function intended to be callable infix
152754        Centril | or, maybe you could know if it is left associative or right depending on where \ is, so x
                      | \dot y or x dot\ y
152807        Centril | maybe thats a bad idea, just brainstorming atm
152828        Centril | oli_obk_: ideas :) ?
152842            vfs | bluss: so you would need to external crate metadata to know how to parse an expression?
                      | (maybe it is the case already now, idk)
152917          bluss | vfs: that's a good point, that doesn't sound workable to me
152919        Centril | guess we need someone who knows the compiler better to flesh that out
152931          bluss | even if crates can inject macros etc
152947        Centril | so maybe it a better idea to specify fixity at call site?
153039       oli_obk_ | Centril: you can't decide it globally. So you can say that a function has a higher fixity
                      | than another function. This way you get a tree of rules depending on the crate dependency
                      | tree. Similar to impl orphans
153117       oli_obk_ | Centril: then you can simply force infix to require brackets if you're specifying it at
                      | the call site
153129       oli_obk_ | and if you are doing that, you can go the macro way
153212       oli_obk_ | a `$lhs:expr $fun:path $rhs:expr` => { $fun($lhs, $rhs) } rule should do it
153255        Centril | so, umh, rephrase .P ?
153351       oli_obk_ | which part? the one about fixity at the call site or the fixity tree?
153423        Centril | all of the above :P
153555       oli_obk_ | if you want to specify the fixity at the call site, you can simply use brackets and force
                      | them. so it would be something like `\(a dot b)` transforms to `dot(a, b)`, but then you
                      | don't gain much over macros: `i!(a dot b)` might be possible, maybe one needs `i!(a \dot
                      | b)`
153722       oli_obk_ | if you want to specify the fixity at the definition site, you need to specify it for all
                      | infix functions that your infix function can be in the same expression with. That might
                      | not be very feasible, but at least complete.
153724        Centril | well, if you have a default fixity, say infixl, then you can write a \dot b \dot c , or
                      | force it: a \dot (b \dot c)
153822        Centril | oli_obk_: haskell does it at definition site, but it has a default fixity of infixl 9
153902       oli_obk_ | that works if you only have one function
153914       oli_obk_ | but what about `a \foo b \bar c` ?
153921        Centril | problems =)
153932        Centril | or: first come first served
154108       oli_obk_ | Centril: what about `vec \add_each_value vec2 \mul_each_value vec3` ? (in matlab terms:   
                      | `vec + vec2 .* vec3`
154206        Centril | well yes, that isnt so nice since that would eval to (vec1 .+ vec2) .* vec3

                                                            [...]

154759        Centril | oli_obk_: but i think that left associative with first-come-first-serve still might be a  
                      | good simplification that makes it crystal clear
154831        Centril | otherwise you have to read about the fixity in the documentation, etc.

[bungcip]

I like custom infix operator as long as limited in ascii letter. But I don't like if you can turn any arbitrary function to infix call. When I create function foo, I expect the function to be used like this: foo(a,b). When I want to create custom infix notation, I expected to implement the necessary trait.

see previous discussion in https://users.rust-lang.org/t/infix-functions/4724

[durka]

Does it interact with generics? Would we end up with a \dot::<Foo<'a>, Bar> b?

[Centril]

@durka Probably, you'd have to specify it somehow with generics, and what you've come up with seems best. But I guess you'd save \dot for cases when the types can be inferred from the Exprs.

[strega-nil]

I don't want this in the language. I think it adds a lot of complication, and it's unimportant to me personally. However, I wouldn't be too opposed to this. It seems nice, especially, for mathematical code.

[brendanzab]

Infix functions are great in languages like Haskell where operators are more common, but it doesn't seem to give Rust much.

[alilleybrinker]

Are there examples of projects where this would result in a decent ergonomic improvement?

[Centril]

@AndrewBrinker I venture a guess that writing mathematics libraries, e.g: linear algebra, and using them, as well as the low level parts of physics engines might benefit from the increased ergonomics.
Essentially, inside and outside anywhere where a lot of math is needed.

There might be other projects of course, but off of my head, that's the typical use case.

[ticki]

@Centril The existing overloadable operators covers 95% of those use cases.

[burdges]

Would this mean we could write obfuscated code that compiled both in Rust and TeX? ;)

I love the infix operator notation in Haskell, partially because not all mathematical operators to *, +, etc. even when mathematicians write them that way. I love Haskell's lack of parentheses on function calls too though. Yet, Rust is a rather different language, so tools like currying do not always fit.

At first blush, I'm thinking one should start with just knowing when to avoid, or argue against, using object . notation around mathematical objects. If for example, we're doing an action, scalar multiplication, etc. then object notation looks like a left action, which gets annoying if you are not a group theorist from Britain or something.

I fear this proposal makes the left vs right action issue worse. In other words, a \dot b might be more useful for mathematics as b.dot(a) than as a.dot(b), but that seems confusing in non-mathematical situations.

In the long run, one could probably build a strict functional language with Haskell-like syntax, maybe an Idris fork, that compiles to Rust code, but does not introduce a garbage collector. I'd suspect that's a better way to build DSLs that compile to Rust.