let syntax to distinguish bindings vs body

[jariji]

The documentation says

Additionally, the syntax has a special meaning for comma-separated assignments and variable names that may optionally appear on the same line as the let....The variables introduced on this line are local to the let block

These look similar but do very different things:

function foo()
    x = 1
    let x = 2
        x
    end
    x
end
foo() == 1


function bar()
    x = 1
    let
        x = 2
        x
    end
    x
end
bar() == 2

I request a change to this syntax to make it easier to

• notice the difference between binding and assignment
• put bindings on separate lines for readability without risk of mutating the enclosing scope (currently done with commas, but not very readable if the expressions are long)

One way to do this is adding a token to separate the place where = makes bindings from where it makes assignments, such as within:

let x = 1 # binding
    y = 2 # binding
within 
    x = 2 # assignment
    x + y
end

This could be a 2.0 change but ideally there is change to address the issues in 1.x, allowing within.

Other languages

In Scheme, bindings are separated with parentheses and assignments are made with set!:

(let 
  ((x 1) ;; binding
   (y 5)) ;; binding
  (+ x y))

In OCaml, in separates the bindings component from the expression that produces the value:

let n = 2
in n * n

and likewise in Nix:

let 
  x = 1;
  y = 1;
in
  x + y

[jariji]

Separating the bindings from the body also means we can use ; for multiple bindings instead of ,, which currently is not nice:

julia> let a,b = (1,2); (a,b) end
ERROR: UndefVarError: `a` not defined