Passing False into a function

[tentus]

Hey, I'm trying to do something like this:

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function object:init(x, y, visible)
	self.x = x or 64
	self.y = y or 64
	self.visible = visible or true
end

function object:draw()
	if self.visible then
		-- drawing code goes here
	end
end

This all works in my head, but it looks like saying object:new(32, 32, false) will make Lua use the "or true", meaning that I can't have a default visibility set.

Am I just showing my newness to Lua or is there some other bug I'm not seeing? I'm a bit under the weather right now (home from work, even) so I won't be offended at all if someone points out a tremendously stupid mistake or obvious error.

[slime]

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self.visible = visible ~= nil and visible or true

Unless there's a cleaner way to do that, I think that's what you're looking for.

[kikito]

I prefer the positive version:

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 self.visible = visible == nil and true or visible 

But in general, if you are dealing with falses, trues and nils, it's probably simpler just to use a plain old if. It ain't much longer:

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 if visible == nil then self.visible = true else self.visible = visible end 

[Boolsheet]

How about:

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self.visible = visible ~= false and true

From the Lua manual ( http://www.lua.org/manual/5.1/manual.html#2.5.3 ):

The disjunction operator or returns its first argument if this value is different from nil and false; otherwise, or returns its second argument.

[slime]

You could simplify that to

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self.visible = visible ~= false

Unless I'm missing something. :p

[tentus]

You could simplify that to

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self.visible = visible ~= false

Unless I'm missing something. :p

Nope, that works exactly how I want it to. Now I just have to figure out how to say that in English, so that I can remember it and/or explain it to other people down the line.

[vrld]

Maybe too late, but if you want your code to look hackish, you could do this:

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self.visible = not not visible

It works like this: If visible is ...

• true, not visible = false and not not visible = not false = true
• false, not visible = true and not not visible = not true = false
• nil, not visible = true and not not visible = not true = false

It's probably more readable to use one of the other solutions though

[Robin]

Maybe too late, but if you want your code to look hackish, you could do this:

I thought about that, but it doesn't work: the default should be true, not false. Therefore, passing nil should be the same as passing true, instead of passing false.

I like slime's solution best. Simple, elegant, non-obfuscating.