Ruby: How can I kill "warning: `*' interpreted as argument prefix"?

Question

How can I remove the "warning: `*' interpreted as argument prefix" from the following code?

hash = {"a" => 1,
        "b" => 2,
        "s" => 3,}

if "string".start_with? *hash.keys then
  puts "ok"
else
  puts "ng"
end

When I run the code above, I get:

$ ruby -w /tmp/a.rb
/tmp/a.rb:5: warning: `*' interpreted as argument prefix
ok

What is the best way to fix this warning?

I've tried to put parenthesis around hash like this:

hash = {"a" => 1,
        "b" => 2,
        "s" => 3,}

if "string".start_with? (*hash.keys) then
  puts "ok"
else
  puts "ng"
end

then you get:

$ ruby -w /tmp/a.rb
/tmp/a.rb:5: syntax error, unexpected *
if "string".start_with? (*hash.keys) then
                          ^
/tmp/a.rb:5: syntax error, unexpected ')', expecting '='
if "string".start_with? (*hash.keys) then
                                    ^
/tmp/a.rb:7: syntax error, unexpected keyword_else, expecting end-of-input

And this is the problem described in Why does white-space affect ruby function calls?, and clearly not the way to fix the warning I'm trying to fix.

My ruby version is:

$ ruby --version
ruby 2.3.3p222 (2016-11-21) [x86_64-linux-gnu]

@AlexGolubenko, this question is about argument prefix warning, not the white space between frunction name and a parenthesis. — Yasushi Shoji, Jan 24 '17 at 08:11
I agree with Yashushi. The main way that people find answers on stack overflow is to google the warning. If they have this problem, they will not google ```why does white space affect ruby function calls``` because to do that would imply they already knows what is causing the warning. I think the duplicate tag should be removed. — Obromios, Oct 31 '19 at 06:46

mu is too short · Accepted Answer · 2017-03-21T02:35:18.680

If you're going to use method-calling-parentheses then you must avoid putting a space between the method name and the opening parentheses:

if "string".start_with?(*hash.keys)
  puts "ok"
else
  puts "ng"
end

Also, then is rather archaic so we'll pretend that was never there. If there is a space between the method name and the opening parentheses then your parentheses are interpreted as expression-grouping-parentheses and that's where your syntax error comes from.

Once you add the method-calling-parentheses you remove any possible hint of ambiguity as to what your * is supposed to mean and the warning should go away.

~~BTW, the warning you're getting in this case is rather, um, silly.~~ On second thought, the warning isn't so silly because Ruby can be whitespace sensitive in surprising ways. This:

o.m *x

can be interpreted as:

o.m(*x)

or as:

o.m() * x

but these:

o.m * x
o.m*x
o.m* x

can be interpreted in the same ways. Of course, all three of those are interpreted as o.m() * x and only o.m *x is seen as o.m(*x). Sane whitespace usage would suggest that o.m *x is obviously a splat whereas o.m * x is obviously a multiplication but a couple days on SO should convince you that whitespace usage is hardly sane or consistent.

That said, -w's output in the Real World tends to be so voluminous and noisy that -w is nearly useless.

Ruby: How can I kill "warning: `*' interpreted as argument prefix"?

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