Space Lua is a custom dialect and implementation of the Lua programming language, embedded in SilverBullet. It aims to be a largely complete Lua implementation, but adds a few non-standard features while remaining syntactically compatible with “real” Lua.

Basics

In its essence, Space Lua adds two features to SilverBullet’s Markdown language:

  • Definitions: Code written in space-lua code blocks are enabled across your entire space.
  • Expressions: The ${expression} syntax will Live Preview to its evaluated value.

Definitions

Space Lua definitions are defined in fenced code blocks, in this case with the space-lua language. As follows:

-- adds two numbers
function adder(a, b)
  return a + b
end

Each space-lua block has its own local scope. However, following Lua semantics, when functions and variables are not explicitly defined as local they will be available globally across your Spaces|space. This means that the adder function above can be used in any other page.

Since there is a single global namespace, it is good practice to manually namespace functions using the following pattern:

-- This initializes the stuff variable with an empty table if it's not already defined
stuff = stuff or {}

function stuff.adder(a, b)
  return a + b
end

Load order

It is possible to control load order of space lua scripts using the special -- priority: <number> comment in Lua code.

Scripts are loaded in reverse priority order. When you set no priority (the default) your scripts will be run last.

The order used is determined by this Space Lua/Lua Integrated Query|query (also part of your ^Library/Std/Pages/Space Overview) page:

query[
  from index.tag "space-lua"
  order by priority desc
](
  from index.tag "space-lua"
  order by priority desc
)

This means that the higher the priority, the earlier the script is loaded. That also means that if you want to override previously defined definitions you need to a set a lower priority (or in most cases: simply omit the priority comment).

Here are the conventions used by the Library/Std library:

  • priority: 100 for config definitions (schemas)
  • priority: 50 for setting really core and root variables (like template.* APIs) that will be used by other scripts
  • priority: 10: for standard library definitions that may be overriden (by scripts with lower priority)

note Tip All your space-lua scripts are loaded on boot, to reload them without reloading the page, simply run the ${widgets.commandButton("System: Reload")} (Ctrl-Alt-r) command.

Expressions

A SilveBullet specific Markdown Markdown/Extensions is the ${lua expression} syntax that you can use in your pages. This syntax will Live Preview to the evaluation of that Lua expression.

For example: 10 + 2 = ${adder(10, 2)} (Alt-click, or select to see the expression) is using the just defined adder function.

Conversion of Strings to Numbers

Space Lua converts strings to numbers like standard Lua.

Syntax: * Leading and trailing whitespace (space, tab, newline, carriage return, form feed and vertical tab) is trimmed. * The entire string after trimming trailing whitespace must form a number. * Optional + or - signs are accepted before the number. * Decimal integers and decimal floats are supported with optional exponent (e.g., 42, -3.5, .5, 5., 1e3, -2.5E-2). * Hexadecimal integers and hexadecimal floats are also supported: * integers (e.g., 0x10, -0XFF), * floats require p or P exponent (e.g., 0x1.8p1, -0X10.3P-1).

Failure handling: * In arithmetic and unary minus expressions invalid strings cause an Lua exception: attempt to perform arithmetic on a non-number. * tonumber(s) returns nil on failure. * tonumber(s, base) parses signed integers in bases 2..36 (without decimal points or exponents) and returns nil on invalid input string or base.

Examples (with tonumber function):

Code Result Expected
tonumber(' 42 ') ${tonumber(' 42 ')} 42
tonumber('-0xFf ') ${tonumber('-0xFf ')} -255
tonumber('0x1.8p1') ${tonumber('0x1.8p1')} 3.0
tonumber('1e-2') ${tonumber('1e-2')} 0.01
tonumber('abc') ${tonumber('abc')} nil
tonumber('1010', 2) ${tonumber('1010', 2)} 10
tonumber(' +fF', 16) ${tonumber(' +fF', 16)} 255
tonumber('8', 8) ${tonumber('8', 8)} nil

Examples (with arithmetic operations):

Code Result Expected
'0xfFfFp1' ${'0xfFfFp1'} string: 0xfFfFp1
'0xffffP-3' + 0 ${'0xffffP-3' + 0} number: 8191.875
-'123E-12' ${-'123E-12'} number: -1.23e-10

Queries

Space Lua has a feature called Space Lua/Lua Integrated Query, which integrate SQL-like queries into Lua. Here’s a small example querying the last 3 modifies pages:

${query from index.tag "page" order by lastModified desc select name limit 3}

Widgets

See Space Lua/Widgets

Commands

Custom commands can be defined using API/command#command.define(commandDef):

command.define {
  name = "Hello World",
  run = function()
    editor.flashNotification "Hello world!"
    event.dispatch("my-custom-event", {name="Pete"})
  end
}

Try it: ${widgets.commandButton("Hello World")}

Slash commands

Custom slash commands can be defined using API/slashCommand#slashCommand.define(spec):

slashCommand.define {
  name = "hello",
  run = function()
    editor.insertAtCursor("Hello |^| world!", false, true)
  end
}

Event listeners

You can listen to events using API/event#event.listen(listenerDef):

event.listen {
  name = "my-custom-event";
  run = function(e)
    editor.flashNotification("Custom triggered: "
       .. e.data.name)
  end
}

Space Lua Extensions

Space Lua introduces a few new features on top core Lua:

  1. Space Lua/Lua Integrated Query, embedding a query language into Lua itself
  2. Thread locals

Thread locals

There's a magic _CTX global variable available from which you can access useful context-specific values. Currently the following keys are available:

  • _CTX.currentPage providing access (in the client only) to the currently open page (PageMeta object)
  • _CTX._GLOBAL providing access to the global scope

API

API

Lua implementation notes

Space Lua is intended to be a more or less complete implementation of Lua 5.4. However, a few features are (still) missing:

  • goto and labels (not planned, goto considered harmful)
  • coroutines (not planned, not useful in the SilverBullet context)
  • _ENV (planned)
  • Full metatable support (only partial now, planned)
  • Hexadecimal numeric constants with a fractional part, or binary exponents (not supported by JavaScript number parser either)

Frequently Asked Questions

Why Lua?

Lua is purpose-designed to be a simple, easy to learn, yet powerful language for extending existing applications. It is commonly used in the gaming industry, but to extend many other applications. If you know any other programming language, you will be able to learn Lua within hours or less.

Why a custom Lua runtime?

Rather than using a WebAssembly or other implementation of Lua that could run in the browser and server, we have opted for a custom implementation. This is achievable because Lua is a relatively simple and small language to implement and allows for deep integration in the custom Lua runtime. The thing that triggered a custom implementation was the need to call asynchronous (JavaScipt) APIs from Lua, without having to resort to ugly asynchronous callback-style API design (Lua does not support async-await). In SilverBullet’s Lua implementation, the differences between asynchronous and synchronous APIs is fully abstracted away, which makes for a very clean development experience.