Kotlin has more constructs that are expressions than Java has (e.g.

if

,

when

,

try

expressions in Kotlin, but not in Java). What was the reason that the assignment operator

=

is not an expression?

Mainly because

=

is used in named arguments syntax:

foo(arg = value)

This is a call of

foo

with

arg

argument, rather than a call of

foo

with the result of assignment of

value

to some

arg

variable/property.

Ah! Thank you! I thought it was done to discourage coding expressions with side-effects. But then I realized that

(value++)

would exactly be such an expression 🙂

It also prevents accidental mistakes when == is intendet, right?

@ghedeon How often does that happen? It would only compile for a boolean and you never do

== true

anyway. You can compare two booleans but that's even more rare.

Note that you can achieve the same effect as assignment expression with

also

function:

foo(value.also { someVar = it })