Generating Project Files

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Now let's extend our new action to actually output some workspace and project files so we can begin to get a sense for how things work.

First we need to know what we want to generate. Let's start with a very simple Premake project script, then we'll figure out how we want it to appear when we translate it to Lua.

workspace "Greetings"
	configurations { "Debug", "Release" }

project "HelloApp"
	kind "ConsoleApp"
	language "C++"
	files { "hello.h", "hello.cpp" }

There are, of course, many ways we could express this in Lua. For the purposes of this tutorial, we'll start by creating two files, starting with Greetings.wks.lua:

workspace = {
	name = "Greetings",
	projects = {
		["HelloApp"] = {
			path = "HelloApp.prj.lua",
		},
	},
}

Workspaces generally are used to manage a list of projects, so we'll try to do the same with our Lua version.

We'll also generate a second file named HelloApp.prj.lua, containing some of the easily accessible project information.

project = {
	name = "MyConsoleApp",
	uuid = "B19F86AA-524E-4260-B200-243C70F2DA04",
	kind = "ConsoleApp",
	language = "C++",
}

This is just to get things started; we'll come back to the configurations and the source code files and all of the other settings later.

Creating the Files

Creating these files is easy: Premake has a built-in function to do just that, which we can leverage in our action's onWorkspace() and onProject() callbacks.

-- lua/lua.lua

premake.modules.lua = {}
local m = premake.modules.lua

local p = premake

newaction {
	trigger = "lua",
	description = "Export project information as Lua",

	onStart = function ()
	end,

	-- create a new file with a ".wks.lua" extension and
	-- then call our generateWorkspace() function.
	onWorkspace = function(wks)
		p.generate(wks, ".wks.lua", m.generateWorkspace)
	end,

	-- create a new file with a ".prj.lua" extension and
	-- then call our generateProject() function.
	onProject = function(prj)
		p.generate(prj, ".prj.lua", m.generateProject)
	end,
}


function m.generateWorkspace(wks)
	p.w('This is a Lua "workspace" file.')
end


function m.generateProject(prj)
	p.w('This is a Lua "project" file.')
end


return m

The premake.generate() function uses the information contained in the workspace or project to figure out the right name of location for the file, and then creates it on the disk. Once the file has been successfully created and opened, it then calls the function we provide as the last argument in the call (generateWorkspace() and generateProject() respectively) and passes it the corresponding workspace or project object to be exported.

The p.w() function, which stands for "Premake write", simply outputs a text string to the currently open file. You'll be seeing much more of this one.

If you go ahead and generate that project (i.e. run premake5 lua again), you will see the files get created on disk, each containing the corresponding "This is a..." message.

Populating the Workspace

Now we can begin to fill in our workspace and project files. Let's begin with the easy parts of the workspace.

function m.generateWorkspace(wks)
	p.push('workspace = {')
	p.w('name = "%s",', wks.name)

	p.push('projects = {')
	p.pop('},')

	p.pop('}')
end

A couple of new functions here: p.push() writes the provided string to the output file, and increments an internal indentation level. p.pop() decrements the indentation level, and then writes its provided string to the output. p.w(), which we saw earlier, outputs its string at the current indentation level as set by push() and pop().

So between all that pushing and popping, we end up with a nicely indented workspace file with an empty list of projects.

workspace = {
	name = "Greetings",
	projects = {
	},
}

Let's tackle that project list next. Premake has an entire API for working with workspaces, which you can find by browsing the src/base/workspace.lua script in Premake's source code.

(Coming soon, just need to make a few code changes...)

Populating the Project

Since we're only exporting a few of the simple fields, generating our project file is quite easy:

function m.generateProject(prj)
	p.push('project = {')
	p.w('name = "%s",', prj.name)
	p.w('uuid = "%s",', prj.uuid)
	p.w('kind = "%s"', prj.kind)
	p.pop('}')
end

Which gives us a project file like:

project = {
	name = "MyConsoleApp",
	uuid = "B19F86AA-524E-4260-B200-243C70F2DA04",
	kind = "ConsoleApp",
	language = "C++",
}

Escapes and Indents and EOLs

For the sake of completeness, a few last points.

First, indentation. By default, Premake will uses tab characters to indent the output. If your target format uses a different character sequence, two spaces for instances, you can adjust that using Premake's p.indent() function.

p.indent("  ")

Similarly, Premake will output Unix-style "\n" line endings by default, which can be changed with the p.eol() function.

p.eol("\r\n")

If you wish to change these values for both your generated workspaces and projects, you can place them in your action's onStart() function. If the values are different between workspaces and projects, put then in onWorkspace() and onProject() instead.

onStart = function()
	p.indent("  ")
	p.eol("\r\n")
end

Finally, before we go we should consider string escaping. If, for example, someone were to name their project Joe's "Lucky" Diner, we would try to generate this Lua script...

	name = "Joe's "Lucky" Diner",

...which would fail to load in a Lua interpreter, since the double quotes aren't properly matched. Instead, we ought to be generating:

	name = "Joe's \"Lucky\" Diner",

Premake allows exporters to define an "escaper", a function which is used to transform values before they are written to the output. For our Lua exporter, we want to escape those double quotes with a backslash, and we should also escape backslashes while we're at it, which we can do by adding this function to our module:

function m.esc(value)
	value = value:gsub('\\', '\\\\')
	value = value:gsub('"', '\\"')
	return value
end

We can then tell Premake to use this function for both our workspaces and our project by registering our escaper in our action's onStart().

onStart = function()
	p.escaper(m.escaper)
end

One more step: since we don't always want to escape values, Premake provides a separate call p.x() for those times when we do. For our example case, we really only need to worry about the workspace and solution names right now, since the other fields are limited to values which do not contain special characters (while there is no harm in using p.x() on values that do not contain special characters, there is a small performance hit which can add up for large projects).

So our final script looks like this:

-- lua/lua.lua

premake.modules.lua = {}
local m = premake.modules.lua

local p = premake

newaction {
	trigger = "lua",
	description = "Export project information as Lua",

	onStart = function ()
		p.escaper(m.esc)
	end,

	onWorkspace = function(wks)
		p.generate(wks, ".wks.lua", m.generateWorkspace)
	end,

	onProject = function(prj)
		p.generate(prj, ".prj.lua", m.generateProject)
	end,
}


function m.generateWorkspace(wks)
	p.push('workspace = {')
	p.x('name = "%s",', wks.name)

	p.push('projects = {')
	p.pop('},')

	p.pop('}')
end


function m.generateProject(prj)
	p.push('project = {')
	p.x('name = "%s",', prj.name)
	p.w('uuid = "%s",', prj.uuid)
	p.w('kind = "%s"', prj.kind)
	p.pop('}')
end


function m.esc(value)
	value = value:gsub('\\', '\\\\')
	value = value:gsub('"', '\\"')
	return value
end


return m