Redbean: string multiply operator considered harmful

[bjornbm]

• redbean supports a string multiply operator, like Python. For example, you can say "hi" * 2 instead of string.rep("hi", 2).

Maybe I am missing the genius but this “lua enhancement” seems to have limited uses (building TUI frames??). More seriously though, it is inconsistent with pre-exisiting Lua string multiplication which will interpret a string as a number if possible and error otherwise.

Standard Lua:

Lua 5.4.6  Copyright (C) 1994-2023 Lua.org, PUC-Rio
> '2' * 2
4
> 2 * '2'
4
> '2' * 2.0
4.0
> 'hi' * 2
stdin:1: attempt to mul a 'string' with a 'number'
stack traceback:
	[C]: in metamethod 'mul'
	stdin:1: in main chunk
	[C]: in ?
> 2 * 'hi'
stdin:1: attempt to mul a 'number' with a 'string'
stack traceback:
	[C]: in metamethod 'mul'
	stdin:1: in main chunk
	[C]: in ?

redbean:

>: '2' * 2  -- silently diverges from Lua
"22"
>: 2 * '2'  -- like Lua
4
>: '2' * 2.0  -- like Lua
4.
>: 'hi' * 2  -- deliberately not like Lua
"hihi"
>: 2 * 'hi'  -- like Lua
redbean: stdin:1: attempt to mul a 'number' with a 'string'
stack traceback:
	[C]: in metamethod 'mul'
	stdin:1: in main chunk

I'm not going to defend the implicit coercion in standard Lua. But the redbean behavior seems the worst of many worlds:

• Breaks compatibility with Lua for strings representing numbers (but only if the string is the first operand and the second operand is an integer).
• Behaves like Python, but only if the string is the first operand.
• The multiply operator is not commutative, which is not strictly necessary in a mathematical sense but seems like it is asking for subtle bugs by violating programmers' expectations.

I propose removing this feature. In particular the '2' * 2 behavior seems potentially disastrous. A defensive programmer should of course validate and “cast” strings to numbers before arithmetic, but a lazy programmer (me in many situations) will be perfectly happy with a runtime error if a coercion fails.

Example:

-- fact.lua
-- defines a factorial function
function fact (n)
  if n == 0 then
    return 1
  else
    return n * fact(n - 1)
  end
end

a = arg[1]
print(fact(a))

$ lua fact.lua 5
120
$ redbean -i fact.lua 5
555555555555555555555555
$ redbean -i fact.lua 5.0
120.0

Of course, an easy fix is a = arg[1] + 0 in the script, but it is only easy if you anticipate the need.

[bjornbm]

Related, from The Evolution of Lua:

One of the few (and rather minor) innovations in Lua was
the syntax for string concatenation. The natural ‘+’ operator
would be ambiguous, because we wanted automatic coer-
cion of strings to numbers in arithmetic operations. So, we
invented the syntax ‘..’ (two dots) for string concatenation.