Syntax

Syntax and structure of Meson files

The syntax of Meson's specification language has been kept as simple as possible. It is strongly typed so no object is ever converted to another under the covers. Variables have no visible type which makes Meson dynamically typed (also known as duck typed).

The main building blocks of the language are variables, numbers, booleans, strings, arrays, function calls, method calls, if statements and includes.

Usually one Meson statement takes just one line. There is no way to have multiple statements on one line as in e.g. C. Function and method calls' argument lists can be split over multiple lines. Meson will autodetect this case and do the right thing. Apart from line ending whitespace has no syntactical meaning.

Variables

Variables in Meson work just like in other high level programming languages. A variable can contain a value of any type, such as an integer or a string. Variables don't need to be predeclared, you can just assign to them and they appear. Here's how you would assign values to two different variables.

var1 = 'hello'
var2 = 102

Numbers

Meson supports only integer numbers. They are declared simply by writing them out.

Strings can be converted to a number like this:

string_var = '42'
num = var1.to_int()

Booleans

A boolean is either true or false.

truth = true

Strings

Strings in Meson are declared with single quotes. To enter a literal single quote do it like this:

single quote = 'contains a \' character'

Similarly \n gets converted to a newline and \\ to a single backslash.

Strings running over multiple lines

Strings running over multiple lines can be declared with three single quotes, like this:

multiline_string = '''#include <foo.h>
                      int main (int argc, char ** argv) {
                        return FOO_SUCCESS;
                      }'''

This can also be combined with the string formatting functionality described below.

String formatting

Strings can be built using the string formatting functionality.

template = 'string: @0@, number: @1@, bool: @2@'
res = template.format('text', 1, true)
# res now has value 'string: text, number: 1, bool: true'

As can be seen, the formatting works by replacing placeholders of type @number@ with the corresponding argument.

Arrays

Arrays are delimited by brackets. An array can contain an arbitrary number of objects of any type.

my_array = [1, 2, 'string', some_obj]

You can add additional items to an array like this:

my_array += [ 'foo', 3, 4, another_obj ]

Function calls

Meson provides a set of usable functions. The most common use case is creating build objects.

executable('progname', 'prog.c')

Method calls

Objects can have methods, which are called with the dot operator. The exact methods it provides depends on the object.

myobj = some_function()
myobj.do_something('now')

If statements

If statements work just like in other languages.

var1 = 1
var2 = 2
if var1 == var2 # Evaluates to false
  something_broke()
elif var3 == var2
  something_else_broke()
else
  everything_ok()
endif

opt = get_option('someoption')
if opt == 'foo'
  do_something()
endif

Foreach statements

To do an operation on all elements of an array, use the foreach command. As an example, here's how you would define two executables with corresponding tests.

progs = [['prog1', ['prog1.c', 'foo.c']],
         ['prog2', ['prog2.c', 'bar.c']]]

foreach p : progs
  exe = executable(p.get(0), p.get(1))
  test(p.get(0), exe)
endforeach

Note that Meson variables are immutable. Trying to assign a new value to progs inside a foreach loop will not affect foreach's control flow.

Logical operations

Meson has the standard range of logical operations.

if a and b
  # do something
endif
if c or d
  # do something
endif
if not e
  # do something
endif
if not (f or g)
  # do something
endif

Logical operations work only on boolean values.

Comments

A comment starts with the # character and extends until the end of the line.

some_function() # This is a comment
some_other_function()

Includes

Most source trees have multiple subdirectories to process. These can be handled by Meson's subdir command. It changes to the given subdirectory and executes the contents of meson.build in that subdirectory. All state (variables etc) are passed to and from the subdirectory. The effect is roughly the same as if the contents of the subdirectory's Meson file would have been written where the include command is.

test_data_dir = 'data'
subdir('tests')

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