[swift-evolution] Enable omitting `let` for constant declarations

[Radosław Pietruszewski]

I can’t easily find it, but there’s been at least one thread proposing this exact thing, and there was very little interest in the proposal.

TL;DR is that Swift *by design* wants to make the difference between these three concepts:

- assignment
- declaration of a constant
- declaration of a mutable variable

as explicit and obvious as possible.

— Radek

> On 01 Apr 2016, at 13:58, Yuta Koshizawa via swift-evolution <swift-evolution at swift.org> wrote:
> 
> I think it would be good if the following three declarations were equivalent
> 
> let a: Int = 42
> a: Int = 42
> a := 42
> 
> and also the following two were.
> 
> let a: Int
> a: Int
> 
> Then constant declarations become shorter than variable declarations.
> It encourages people to use constants in preference to variables.
> 
> It also prevents repeating `let` for property declarations and makes
> type declarations simpler.
> 
> struct Person {
>    firstName: String
>    lastName: String
>    age: Int
> }
> 
> Omitting `let` is consistent with that we don't write `let` for
> arguments of functions and iterated values in for-in loops.
> 
> Not `=` but `:=` for type inferences because `=` cannot distinguish
> whether it means a constant declaration or an assignment to a variable
> declared in an outer scope. I think `:=` is a natural notation for
> type inferences because omitting the type from `a: Int = 42` makes
> `a:= 42`. Because I have not strictly checked if it can be parsed in
> Swift properly, it may have some other parsing issues.
> 
> What do you think about it?
> 
> -- Yuta
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