[swift-evolution] Mutability inference

[Radosław Pietruszewski]

This is by design:

1. You do have to explicitly declare a variable with let or var to disambiguate, in the eyes of the reader, declaration and assignment. This way, when you see `let x = y`, you know for sure, without double checking, that “x” has no previous value. This is also makes the code more resilient, because if you accidentally reuse a name or whatever, the compiler will warn you.

2. You have to explicitly opt into mutability not to aid the compiler, but to let the compiler aid you!

This teaches you to make immutable things by default, because immutability gives you a guarantee that a thing won’t change. You don’t have to think about it. You’re protected from your future self trying to change a value of a constant. And when you declare a `var`, but don’t need mutability capabilities, the compiler pushes you to change it to `let`, again, to protect yourself from future yourself’s mistakes.

Fun fact: Rust goes even further in pushing you to be immutable by default. Variables are declared by “let x = y”, and to opt into mutability you have to write “mut let x = y”.

— Radek

> On 24 Feb 2016, at 08:06, Darko Damjanovic via swift-evolution <swift-evolution at swift.org> wrote:
> 
> 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
> 
> 
> 
> 
> 
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