[solved] Storing a test condition as a string

[white_haired_uncle]

I have a table of preferences. For some preferences, I only want them to be visible if certain game events have occurred. So I'm looking to store my test condition as a string in the table for later execution. For example, the discovered field for the blood_rain item:

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table.insert(new_prefs,{ id = "show_map5", default = true , menu = "chapter5", discovered = true, type = "bool", value = true,
                description = _"Show world map on entering scenario (chapter 5)"})
table.insert(new_prefs,{ id = "show_map9", default = true , menu = "chapter9", discovered = true, type = "bool", value = true,
                description = _"Show world map on entering scenario (chapter 9)" })
table.insert(new_prefs,{ id = "blood_rain", default = true , menu = "chapter9", discovered = "return wml.variables.quests.seen_blood_rain == 1", type = "bool", value = true,
                        description = _"Show effects of blood rain" })

I'm not in love with that syntax, it's just what I have almost working, while under the assumption the "load" is the tool I want to use here.

Let's assume I want to determine whether or not to display the blood_rain preference. I can do this:

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a=(load(new_prefs[3].discovered))
if(a()) then ...

or

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status,value = pcall(load(new_prefs[3].discovered))
if status and value then ...

But what I can't figure out is how to do that in one line. Just execute the block and check the return value without the intermediate step of assigning said value to something first. I'm probably just being stubborn here, but I think I'm also displaying a fundamental lack of understanding of lua functions here and I'd like to fix that.

[Celtic_Minstrel]

I'll answer your direct question in a moment, but before that I want to question why you're storing the condition as a string of Lua code. Is new_prefs a general Lua table, or does it need to be valid WML?

Note: All these will assume you set the $quests.seen_blood_rain variable to yes instead of 1, which you probably should do anyway if it's meant to be a boolean value. That's not required for the examples to work, you just need to adjust them slightly if you have a good reason for it to be a number.

1. First, let me suppose it doesn't have to be WML. In that case, you're better off just putting a function in the table, like this:

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   table.insert(new_prefs, {
   	id = "blood_rain",
   	default = true,
   	menu = "chapter9",
   	discovered = function() return wml.variables.quests.seen_blood_rain end,
   	type = "bool",
   	value = true
   })
   

   And then when using it you can just call it:

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   local discovered = new_prefs[3].discovered
   if type(discovered) == 'function' then discovered = discovered() end
   

2. But, maybe it does have to be valid WML. If you need to serialize this data into the saved game value, it certainly needs to be WML-compatible, which means it can't contain functions. In that case, my personal recommendation would be to use WFL for the condition. The table would then look like this:

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   table.insert(new_prefs, {
   	id = "blood_rain",
   	default = true,
   	menu = "chapter9",
   	-- Enclosing parenthesis not technically required, but I recommend them
   	-- as an easy way to distinguish a formula from a yes/no
   	discovered = "(quests.seen_blood_rain)"
   	type = "bool",
   	value = true
   })
   

   When using this version, you would need to evaluate the formula:

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   local discovered = new_prefs[3].discovered
   if type(discovered) == 'string' then discovered = wesnoth.eval_formula(discovered, wml.all_variables) end
   

3. There is another option you could take if it's required to be WML – you could use ConditionalWML. That method would look like this:

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   local always_discovered = wml.tag.discovered{wml.tag['true']{}}
   table.insert(new_prefs,{ id = "show_map5", default = true , menu = "chapter5", always_discovered, type = "bool", value = true})
   table.insert(new_prefs, {
   	id = "blood_rain",
   	default = true,
   	menu = "chapter9",
   	wml.tag.discovered{ wml.tag.variable{ name="quest.seen_blood_rain", boolean_equals=true }}
   	type = "bool",
   	value = true
   })
   

   When using this version, you would need to evaluate the conditional:

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   local discovered = wml.get_child(new_prefs[3], "discovered")
   if discovered then discovered = wml.eval_conditional(discovered) end
   

4. Lastly, the answer to your question, as best I understand. As far as I can, your first example is, in fact, correct, but it will fail for items 1 and 2 because true is not a valid Lua statement. To make the table structure you showed work, calling it like this should be fine:

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   local discovered = new_prefs[3].discovered
   if type(discovered) == 'string' then discovered = load(discovered)() end
   

   Though, that will raise an error if the string has a syntax error. If you'd prefer to ignore syntax errors and move on, something like this should work:

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   local discovered = new_prefs[3].discovered
   if type(discovered) == 'string' then
   	local f, error = load(discovered)
   	if type(f) == 'function' then
   		discovered = f()
   		-- or, slightly safer:
   		local status, result = pcall(f)
   		if stats then
   			discovered = result
   		else
   			-- at this point, the result variable contains what went wrong, in case you want to do something with it
   			discovered = false
   		end
   	else
   		-- at this point, the error variable contains what went wrong, in case you want to do something with it
   		discovered = false
   	end
   end
   

   But what I can't figure out is how to do that in one line.

   I can't think of any way to make a one-liner that will never throw an error. If you really want a one-liner though, the following should work:

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   local discovered = load(new_prefs[3].discovered)()
   

   However, for that code to work, all your cases of discovered = true would need to be changed to discovered = "return true".

[white_haired_uncle]

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load(new_prefs[3].discovered)()

Thank you, that's the syntax I was looking for. As I rather suspected, it may not be the right way to accomplish the task, but it's the piece that was eluding me.

I stored the condition as lua code because that's what I could get to work. I'd prefer to just store the condition itself.

I assume this needs to be WML, since prefs.value at least will be accessed by WML and it will need to be saved between games (and perhaps even persist?).

quests.seen_blood_rain is 1, not yes, and that is more or less out of my control, unfortunately. Working around that should be the easiest part.

Solution #2 looks like the cleanest, though I'm unclear how it would handle errors. Solution #4 has my attention, mostly because the other day I was looking for a try/catch solution for lua and read about pcall and now I feel like I should be doing a lot more to trap errors as a habit (or I found a new toy and I'm looking for an excuse to play with it).

Thanks

[Celtic_Minstrel]

it will need to be saved between games (and perhaps even persist?).

This means it will need to be WML, yes, so version #1 will not work for you.

Solution #2 looks like the cleanest, though I'm unclear how it would handle errors.

Broadly speaking, it more or less doesn't, but it shouldn't crash in the event of an error. If there's an error in the formula string, you'll get nil back as the result, which would effectively mean that the item is never discovered.

Solution #4 has my attention, mostly because the other day I was looking for a try/catch solution for lua and read about pcall and now I feel like I should be doing a lot more to trap errors as a habit (or I found a new toy and I'm looking for an excuse to play with it).

Indeed, pcall is Lua's try-catch equivalent. I would say that the closest way to attain a try-catch idiom in Lua would be like this:

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local status, error = pcall(function()
	-- do some stuff here
end)
if not status then
	-- do stuff with error here
end

Though more commonly pcall is used to call a function and get back its results if it's successful. If the function is defined like this:

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function f(a, b, c, d) return a + b, c .. d end

then the safe way to call it is like this:

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local status, result1, result2 = pcall(f, a, b, 123, 'stuff')
if status then
	-- do stuff with result1 and result2
else
	local error = result1
	-- do stuff with error
end

Side note: It's a bad idea to call your variables error in Lua, since that's the name of a function (and functions as Lua's throw-equivalent, in fact). Oh well.

[gnombat]

I think there's a 5th solution that could be used here. You would need to create a global function:

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-- This function needs to be global (we will see why later).
function blood_rain_discovered()
    return wml.variables.quests.seen_blood_rain == 1
end

Then store the name of this function (as a string) in the preferences table:

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table.insert(new_prefs,{ id = "blood_rain", default = true , menu = "chapter9", discovered = "blood_rain_discovered", type = "bool", value = true,
                        description = _"Show effects of blood rain" })

Finally, when you want to determine whether or not to display the blood_rain preference:

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-- We are looking for the function in the global variables table _G (this is why the function has to be global).
if _G[new_prefs[3].discovered]() then
    ...

I haven't tested any of this, so it's possible this might not work for some reason, but it seems like this would be better than using load()...

[Celtic_Minstrel]

That's another good solution, yes. However, instead of a global function, I'd recommend namespacing it:

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-- I'm starting the global variable with a capital letter only because the linting systems I use
-- consider lowercase names to be a probable error
PrefFunctions = {}
function PrefFunctions.blood_rain_discovered()
	return wml.variables.quests.seen_blood_rain == 1
end

Store it the same as what gnombat posted, then use it like this:

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if PrefFunctions[new_prefs[3].discovered]() then