Embedded Lua templating

$ luarocks install etlua

local etlua = require "etlua"
local template = etlua.compile([[
  Hello <%= name %>,
  Here are your items:
  <% for i, item in pairs(items) do %>
   * <%= item -%>
  <% end %>
]])

print(template({
  name = "leafo",
  items = { "Shoe", "Reflector", "Scarf" }
}))

The following tags are supported

• <% lua_code %> runs lua code verbatim
• <%= lua_expression %> writes result of expression to output, HTML escaped
• <%- lua_expression %> same as above but with no HTML escaping

Any of the embedded Lua tags can use the -%> closing tag to suppress a following newline if there is one, for example: <%= 'hello' -%>.

The module can be loaded by doing:

local etlua = require "etlua"

Compiles the template into a function, the returned function can be called to render the template. The function takes one argument: a table to use as the environment within the template. _G is used to look up a variable if it can't be found in the environment.

Compiles and renders the template in a single call. If you are concerned about high performance this should be avoided in favor of compile if it's possible to cache the compiled template.

If any of the methods fail they will return nil, followed by the error message.

• Templates are transparently translated into Lua code and then loaded as a function. Rendering a compiled template is very fast.
• Any compile time errors are rewritten to show the original source position in the template.
• The parser is aware of strings so you can put closing tags inside of a string literal without any problems.

The raw API is a bit more complicated but it lets you insert code between the compile stages in addition to exposing the internal buffer of the template.

All methods require a parser object:

local parser = etlua.Parser()

Parses a string of etlua code, returns the compiled Lua version as a string.

Here's an example of the generated Lua code:

local parser = etlua.Parser()
print(parser:compile_to_lua("hello<%= world %>"))

local _b, _b_i, _tostring, _concat, _escape = ...
_b_i = _b_i + 1
_b[_b_i] = "hello"
_b_i = _b_i + 1
--[[9]] _b[_b_i] = _escape(_tostring( world ))
_b_i = _b_i + 1
_b[_b_i] = ""
return _b

There are a few interesting things: there are no global variable references, all required values are passed in as arguments, and comments are inserted to annotate the positions of where code originated from. _b is expected to be a regular Lua table that is the buffer where chunks of the template are inserted as it's executed.

Converts the Lua code returned by parser.compile_to_lua into an actual function object. If there are any syntax errors then nil is returned along with the error message. At this stage syntax errors are rewritten to point to the original location in the etlua code and not the generated code.

Executes a loaded function returned by parser.load with the specified buffer and environment. Returns the result of fn, which is typically the buffer. The environment is applied to fn with setfenv (a version is included for Lua 5.2).

For example we can render multiple templates into the same buffer:

parser = etlua.Parser()

first_fn = parser:load(parser:compile_to_lua("Hello "))
second_fn = parser:load(parser:compile_to_lua("World"))

buffer = {}
parser:run(first_fn, nil, buffer, #buffer)
parser:run(second_fn, nil, buffer, #buffer)

print(table.concat(buffer)) -- print 'Hello World'

If you need to customize the Lua code that is generated by etlua to integrate with your own output buffers then you can provide a custom compiler.

You can extend etlua.Compiler and override it's methods to control the output. See https://github.com/leafo/etlua/blob/master/etlua.moon#L42 for the implementation of the default compiler

For an example we'll create a debug compiler that prints whenever a template is executed.

-- create a custom compiler
import Compiler from require "etlua"

class DebugCompiler extends Compiler
  header: =>
    @push 'print("Running template")\n'
    super!

-- try it out
import Parser from require "etlua"

print Parser!\compile_to_lua "hello", DebugCompiler

compile_to_lua takes an optional second argument of the compiler class to use.

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MIT, Copyright (C) 2014 by Leaf Corcoran