Mutating function in Julia (function that modifies its arguments)

Question

How do you define a mutating function in Julia, where you want the result to be written to one of it\'s inputs.

I know functions exist like push!(list, a),

Answer 1

It is possible to assign new values to a constant within a function if you're willing to feed that function a symbol corresponding to the constant as an argument, although I think that some might argue that this doesn't satisfy Julia programming best practices:

function modify_constant!(constant_symbol::Symbol, other_arg)
    new_val = eval(constant_symbol) + other_arg
    eval(Main, Expr(:(=), constant_symbol, new_val))
end

y = 2
modify_constant!(:y, 3)

julia> y
5

Or, if one wanted, a bit more concisely:

function modify_constant!(constant_symbol::Symbol, other_arg)
    eval(Expr(:(+=), constant_symbol, new_val))
end

For more discussion of a related issue, see this Github discussion