Syntax

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. In other cases you can get multi-line statements by ending the line with a \. Apart from line ending whitespace has no syntactic 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

One important difference in how variables work in Meson is that all variables are immutable. This is different from, for example, how Python works.

var1 = [1, 2, 3]
var2 = var1
var2 += [4]
# var2 is now [1, 2, 3, 4]
# var1 is still [1, 2, 3]

Numbers

Meson supports only integer numbers. They are declared simply by writing them out. Basic arithmetic operations are supported.

x = 1 + 2
y = 3 * 4
d = 5 % 3 # Yields 2.

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.

String methods

Strings also support a number of other methods that return transformed copies.

.strip()

# Similar to the Python str.strip(). Removes leading/ending spaces and newlines
define = ' -Dsomedefine '
stripped_define = define.strip()
# 'stripped_define' now has the value '-Dsomedefine'

.to_upper(), .to_lower()

target = 'x86_FreeBSD'
upper = target.to_upper() # t now has the value 'X86_FREEBSD'
lower = target.to_lower() # t now has the value 'x86_freebsd'

.to_int()

version = '1'
# Converts the string to an int and throws an error if it can't be
ver_int = version.to_int()

.contains(), .startswith(), .endswith()

target = 'x86_FreeBSD'
is_fbsd = target.to_lower().contains('freebsd')
# is_fbsd now has the boolean value 'true'
is_x86 = target.startswith('x86') # boolean value 'true'
is_bsd = target.to_lower().endswith('bsd') # boolean value 'true'

.split(), .join()

# Similar to the Python str.split()
components = 'a b   c d '.split()
# components now has the value ['a', 'b', 'c', 'd']
components = 'a b   c d '.split(' ')
# components now has the value ['a', 'b', '', '', 'c', 'd', '']

# Similar to the Python str.join()
output = ' '.join(['foo', 'bar'])
# Output value is 'foo bar'
pathsep = ':'
path = pathsep.join(['/usr/bin', '/bin', '/usr/local/bin'])
# path now has the value '/usr/bin:/bin:/usr/local/bin'

# For joining paths, you should use join_paths()
# This has the advantage of being cross-platform
path = join_paths(['/usr', 'local', 'bin'])
# path now has the value '/usr/local/bin'

# Example to set an API version for use in library(), install_header(), etc
project('project', 'c', version: '0.2.3')
version_array = meson.project_version().split('.')
# version_array now has the value ['0', '2', '3']
api_version = '.'.join([version_array[0], version_array[1]])
# api_version now has the value '0.2'

# We can do the same with .format() too:
api_version = '@0@.@1@'.format(version_array[0], version_array[1])
# api_version now (again) has the value '0.2'

.underscorify()

name = 'Meson Docs.txt#Reference-manual'
# Replaces all characters other than `a-zA-Z0-9` with `_` (underscore)
# Useful for substituting into #defines, filenames, etc.
underscored = name.underscorify()
# underscored now has the value 'Meson_Docs_txt_Reference_manual'

.version_compare()

version = '1.2.3'
# Compare version numbers semantically
is_new = version.version_compare('>=2.0')
# is_new now has the boolean value false
# Supports the following operators: '>', '<', '>=', '<=', '!=', '==', '='

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 ]

Array methods

The following methods are defined for all arrays:

  • length, the size of the array
  • contains, returns true if the array contains the object given as argument, false otherwise
  • get, returns the object at the given index, negative indices count from the back of the array, indexing out of bounds is a fatal error

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[0], p[1])
  test(p[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()

Ternary operator

The ternary operator works just like in other languages.

x = condition ? true_value : false_value

The only exception is that nested ternary operators are forbidden to improve legibility. If your branching needs are more complex than this you need to write an if/else construct.

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')

User-defined functions and methods

Meson does not currently support user-defined functions or methods. The addition of user-defined functions would make Meson Turing-complete which would make it harder to reason about and more difficult to integrate with tools like IDEs. More details about this are in the FAQ. If because of this limitation you find yourself copying and pasting code a lot you may be able to use a foreach loop instead.

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