V: 'a, b += foo()' doesn't result in an error

[SleepyRoy]

V version:
latest master with mingw64

OS:
win10

What did you do?

fn foo() (int,int) {
    return 10, 10
}

fn main() {
    mut a, mut b := foo()
    a, b += foo() 
    println(a)
    println(b)
}

What did you expect to see?
compile error

What did you see instead?
10, 10

_And the same applies to -=, *=, /=, ..._

[medvednikov]

it shouldn't compile:

syntax error: unexpected +=, expecting := or = or comma

[SleepyRoy]

@medvednikov Then how to write code to achieve a, b += foo()?

[medvednikov]

a += foo()
b += foo()

[SleepyRoy]

@medvednikov
But that would result in assignment mismatch error. foo() returns two values while lhs only has one variable.

[medvednikov]

x, y := foo()
a += x
b += y

[SleepyRoy]

@medvednikov Yes it works, but isn't that kind of verbose? Why not allowing a, b += foo()?

[yuyi98]

The syntax itself is not very reasonable, we must first give up some special needs.

[Delta456]

@medvednikov is right. You can't do this in languages which have multiple assignments.

Go:

package main

import (
    "fmt"
)

func r() (int, int) {
    return 12, 12
}

func main() {
    var a, b = 2, 3
    a, b += r()
    fmt.Println("Hello, playground")
}

Output: syntax error: unexpected +=, expecting := or = or comma

Python:

def foo()
   return 12, 12

a, b = 2, 3
a, b += foo()

Output: SyntaxError: illegal expression for augmented assignment

[SleepyRoy]

@Delta456 Thanks for the info! I'm just thinking about why. Is it due to ambiguity? But I didn't come up with such an example.

[Delta456]

@SleepyRoy I think it is because you have multiple assignment for declaration and changing values only but not for declare assignments. This works.

>>> mut a, mut b := 2, 3
>>> a, b = a+2, b+2
>>> a
4
>>> b
5
>>> 

[SleepyRoy]

@Delta456 Thanks. But I still don't understand why a, b += foo() shouldn't be allowed.

[medvednikov]

Because it's complicated and confusing, and I've never seen a use case for it.

In very rare situations when you need to do this, just use temp vars.

[medvednikov]

Actually should keep it open until it results in an error.