Conventions and caveats

Things to watch out for!

Moontale attempts to make most of its internal operations straightforward and predictable, even if this is on the surface less convenient. The benefit is that as your game grows in complexity, there should (hopefully!) be much less scope for bugs and edge-cases.

🚧 Error handling

Expressions, script blocks, and Show() calls are 'protected'. If any of these throws an error part-way through execution, any output that was emitted so far is preserved, the error is logged, and the current expression/script terminates early. The rest of the passage (or content block) will continue to be rendered.

Errors are never shown 'inline' to the end user.

Block repetition

When a block or passage is executed, it is always actually executed - any code within will run, even if it has been run before. This is obviously undesirable if you want to perform a stateful action - for example, increasing the player's gold or experience count. In these cases you can use the Freeze or Once functions.


Once content is emitted to the host (with Push(), Pop(), Object(), and Text()), it stays there 'forever' and cannot be changed by any subsequent code. Think of the host as a typewriter: it can add letters, switch fonts and styles, but can't change what's on the page without tearing it up and starting over - in other words, calling Clear().

The advantage here is that the implementation of these 'emission' functions is much easier than if modification was allowed. Let's make a basic HTML emitter now:

tags = {}

function Push(tag)
    tags[#tags+1] = tag

function Pop()
    tags[#tags] = nil

function Text(text)

function Object(tag)
    print('<'..tag..' />')

function Clear()
    print('<hr />')

function Invalidate()

That's it! The upshot is that whether you're targeting HTML, Rich Text, or direct API calls, integration into your engine of choice should be 'stupid simple'.

Note that in this example, Clear() doesn't actually clear the screen; if our output is plain HTML, this is of course impossible. In your application you can use the appropriate APIs to clear any existing text.

Fortunately the immutability of the on-page text doesn't hinder our ability to modify the output of a passage after a user interaction - the Hover changer exists, after all! When something changes that would affect the output, we can simply call Reload() which will 're-render' the current passage. Lua is fast enough that you could do this every frame, meaning that smooth animations described entirely in a Moontale passage aren't out of the question.

It's possible to add custom emission functions to deal with on-page mutation if you really need it, but that's out of scope for this guide!

Text output is not re-parsed

When emitting text through code, e.g. <$ '*foo*$bar' $>, it is emitted 'verbatim', without any additional parsing. In the example shown, the literal string *foo*$bar will be printed.

The reason for this is that a second parsing step, in addition to placing a lot of implementation burden on the Lua runtime, host, and/or exporter, introduces some subtle and complex edge-cases relating to when exactly this parsing takes place (What does '$'..'foo' print? What about '$f'..'oo'? Or x = '$y'; Show('$x')?).

Fortunately, there are a number of options to achieve a very similar effect:

  • Use Lua directly, which is almost as terse as Markdown, e.g. <$ Style.em('foo'),bar $>

  • Wrap the content in a Name[] changer, and refer to it later, e.g. $Name.x[ *foo*$bar ] ... $x

  • Use the content directly in the passage, and add any code before/after, e.g. <$ code() $> *foo*$bar <$ moreCode() $>

The parser is dumb

The tokens <$, $>, {$, and $} were chosen because they are always invalid sequences in Lua and (probably) invalid in Markdown, so the parser just needs to scan for the matching end token and treat the whole match as the Lua code to execute.

However, if you have a Lua string literal that actually contains $} or $>, you'll run into problems! If for some reason you need to operate on these sequences or include them in your text, you'll have to elaborate the expression a bit: '$'..'}'. There are no 'escape sequences' or suchlike.

In a similar vein, the Twine editor will see any 'link-like' structure and assume it's a real passage link, even if it's within a Lua code block. We can use this to our advantage, for example ${-- [[Label->Passage]]$} will create a 'fake' link: Twine will draw a link line, but nothing will be visible in the output.

Variable scope

Moontale uses Lua's scoping rules for variables. A reference to $x will look for local variables in the current render function, then captured variables ('upvalues', in Lua terminology), then globals.

You can define a local variable using a script block: {$ local x = x $} will 'capture' the parent scope's value of x , for example. Note that such a variable is scoped to the render function (the current content block, or passage root) - not the passage as a whole. This means that local variables in nested content blocks will 'shadow' those in their parent.

Moontale has no special syntax to differentiate between local and global variables.

Infinite loops

Moontale does not guard against infinite loops (because of the Halting Problem this isn't possible to do without severe restrictions). If you call Jump or Display unconditionally, and it ends up back at the same passage you called it from, your app will lock up.

If this is a concern, a host implementation may deliberately limit the number of instructions executed as a result of any user interaction - perhaps using debug.sethook.

Render function reuse

Almost all values, including render functions, can be safely saved and reused in later passages. The exceptions are the results of Freeze, Once, and any values that depend on them. This is because these functions rely on an internal table that gets cleared when a new passage is loaded, so they may behave strangely if kept beyond their intended lifespan!

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