Lock type via := declaration

[jonas-schulze]

I would like to suggest

a := 42

as a shortcut for

a::typeof(42) = 42

The same idea but with different syntax had already been proposed in #43671 (comment).

[jonas-schulze]

Just if people search for it: implicit type annotation.

[quinnj]

That seems pretty clever/useful. One thing to think about is that currently you're not allowed to have multiple type declarations, regardless of the scope of the variable in a function, i.e.:

function foo()
       x::Int = 0
       for _ = 1:10
           x::Float64 = _ / 10
       end
       return x
  end
ERROR: syntax: multiple type declarations for "x"
Stacktrace:
 [1] top-level scope
   @ REPL[6]:1

I mention that because I could see that being a confusing error if I had the same variable name in different scopes within a function, even if the desired type declarations are the same.

[jonas-schulze]

Good point. I am in favor of keeping that restriction, though, even if the annotation is for the same type. I personally like it a lot if the compiler gives me hints, and this is a very clear error message. To me, more than one type annotation hints at a potential code smell, even if it's the same type. The syntax x := 0 is used in mathematics as "define x to be equal to 0", and every variable should only be declared once. The syntax x = 0 feels more like "give zero the name x".