Syntax of multiple let assignments vs tuples

General Usage
1

One question has been continually bothering me—why do we have this syntax let x = 1, y = 2?
Since we used to write x, y = 1, 2 directly in julia REPL.
Otherwise it’s invalid as follows

julia> x = 1, y = 2
ERROR: syntax: invalid assignment location "1" around REPL[7]:1
Stacktrace:
 [1] top-level scope
   @ REPL[7]:1

Then why do we need to invent the additional style x = 1, y = 2 and exclusively associate it to let?

My intuition was acute to find that we have this Cartesian For-loop usage.

for i = 1:2, j = 1:3

And this fact

julia> for i = 1 #


ERROR: ParseError:
# Error @ REPL[2]:2:2
for i = 1 #

#└ ── premature end of input
Stacktrace:
 [1] top-level scope
   @ none:1

Note that this ParseError is not as severe as the one let x = 1 # has, because the latter cannot tolerate an Enter stroke, whereas the former can.

2

You’re forgetting about the = in () syntax of named tuples and keyword arguments in function calls, though the latter parses to :kw subexpressions instead of :(=).

1 Like
3

Yes, I do forget them. Because I hardly use them.
I only write normal functions function f(a, b ,c) in my daily code.
And I almost never use named tuples either. :sweat_smile:

4

Seems I understand now. let/for can create a dependence tree, whereas x, y = 1, 2 style is not.

for i = 1:3, j = 1:i, k = 1:i+j
# some body
end

julia> let x = 3, y = 4, x = y, y = x
       println("x = $x, y = $y")
       end
x = 4, y = 4

julia> let x = 3, y = 4, (x, y) = (y, x)
       println("x = $x, y = $y")
       end
x = 4, y = 3
5

Is there a good reason that the second style is not supported?

julia> for _ = 1:3
           local x, y = 7, 8
           println("x = $x, y = $y")
       end
x = 7, y = 8
x = 7, y = 8
x = 7, y = 8

julia> for _ = 1:3
           local x = 7, y = 8
           println("x = $x, y = $y")
       end
ERROR: syntax: invalid assignment location "7" around REPL[2]:2
Stacktrace:
 [1] top-level scope
   @ REPL[2]:1