[swift-evolution] Mutability inference

[Darko Damjanovic]

In the current Swift version the compiler is warning me if a variable is never written to and therefore should be a "let" constant. So now the compiler knows best if "let" or "var" should be applied. During writing code I experience repetitive hints about using "let" or "var" so why do I have to declare it all the time by myself if the compiler anyway knows best?

My proposal would be to make the declaration of "let" and "var" optional.

Example:
x = 0 // <- implicitly declared as "var x" because later was written to, no need to declare it manually
x = 5  

Example:
y = 0 // <- implicitly declared as "let y" because later was not written to
print(y) 

Example Optional Binding:
if index = myArray.indexOf("A") {
	print(index) // here it's clear that index can be "let index"
}

etc...

This would _not_ mean to disallow or remove the "var and "let" mutability declarations - just to make them optional. If I still want to write it to make it clear just by reading thru the code then this is ok. But I can omit "var and "let" if I want -  why bother about it at all if I can go sure that the compiler is already doing the best?

Another option (if this is "too much" change) would be to just make "let" optional and "var" still should be explicitly written. So "let" would be the default and if I want mutability I have explicitly declare it as "var". This is already the rule for function parameters.

Kind regards,
Darko Damjanovic-Lichtfuss