---
short-description: Syntax and structure of Meson files
...

# 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.

```meson
var1 = 'hello'
var2 = 102
```

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

```meson
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.

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

Hexadecimal literals are supported since version 0.45.0:

```meson
int_255 = 0xFF
```

Octal and binary literals are supported since version 0.47.0:

```meson
int_493 = 0o755
int_1365 = 0b10101010101
```

Strings can be converted to a number like this:

```meson
string_var = '42'
num = string_var.to_int()
```

Booleans
--

A boolean is either `true` or `false`.

```meson
truth = true
```

Strings
--

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

```meson
single quote = 'contains a \' character'
```

The full list of escape sequences is:

* `\\` Backslash
* `\'` Single quote
* `\a` Bell
* `\b` Backspace
* `\f` Formfeed
* `\n` Newline
* `\r` Carriage Return
* `\t` Horizontal Tab
* `\v` Vertical Tab
* `\ooo` Character with octal value ooo
* `\xhh` Character with hex value hh
* `\uxxxx` Character with 16-bit hex value xxxx
* `\Uxxxxxxxx` Character with 32-bit hex value xxxxxxxx
* `\N{name}` Character named name in Unicode database

As in python and C, up to three octal digits are accepted in `\ooo`.

#### String concatenation

Strings can be concatenated to form a new string using the `+` symbol.

```meson
str1 = 'abc'
str2 = 'xyz'
combined = str1 + '_' + str2 # combined is now abc_xyz
```

#### Strings running over multiple lines

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

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

These are raw strings that do not support the escape sequences listed
above.  These strings can also be combined with the string formatting
functionality described below.

#### String formatting

Strings can be built using the string formatting functionality.

```meson
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()**

```meson
# 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()**

```meson
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()**

```meson
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()**

```meson
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()**

```meson
# 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'

# Don't use join_paths for sources files, use files for that:
my_sources = files('foo.c')
...
my_sources += files('bar.c')
# This has the advantage of always calculating the correct relative path, even
# if you add files in another directory or use them in a different directory
# than they're defined in

# 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()**

```meson
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()**

```meson
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.

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

Accessing elements of an array can be done via array indexing:

```meson
my_array = [1, 2, 'string', some_obj]
second_element = my_array[1]
last_element = my_array[-1]
```

You can add more items to an array like this:

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

When adding a single item, you do not need to enclose it in an array:

```meson
my_array += ['something']
# This also works
my_array += 'else'
```

Note appending to an array will always create a new array object and
assign it to `my_array` instead of modifying the original since all
objects in Meson are immutable.

Since 0.49.0, you can check if an array contains an element like this:
```meson
my_array = [1, 2]
if 1 in my_array
# This condition is true
endif
if 1 not in my_array
# This condition is false
endif
```

#### 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. Provided for backwards-compatibility, it is identical to array indexing.

Dictionaries
--

Dictionaries are delimited by curly braces. A dictionary can contain an
arbitrary number of key value pairs. Keys are required to be literal
strings, values can be objects of any type.

```meson
my_dict = {'foo': 42, 'bar': 'baz'}
```

Keys must be unique:

```meson
# This will fail
my_dict = {'foo': 42, 'foo': 43}
```

Dictionaries are immutable.

Dictionaries are available since 0.47.0.

Visit the [Reference Manual](Reference-manual.md#dictionary-object) to read
about the methods exposed by dictionaries.

Since 0.49.0, you can check if a dictionary contains a key like this:
```meson
my_dict = {'foo': 42, 'foo': 43}
if 'foo' in my_dict
# This condition is true
endif
if 42 in my_dict
# This condition is false
endif
if 'foo' not in my_dict
# This condition is false
endif
```

Function calls
--

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

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

Most functions take only few positional arguments but several keyword
arguments, which are specified like this:

```meson
executable('progname',
  sources: 'prog.c',
  c_args: '-DFOO=1')
```

Starting with version 0.49.0 keyword arguments can be specified
dynamically. This is done by passing dictionary representing the
keywords to set in the `kwargs` keyword. The previous example would be
specified like this:

```meson
d = {'sources': 'prog.c',
  'c_args': '-DFOO=1'}

executable('progname',
  kwargs: d)
```

A single function can take keyword argumets both directly in the
function call and indirectly via the `kwargs` keyword argument. The
only limitation is that it is a hard error to pass any particular key
both as a direct and indirect argument.

```meson
d = {'c_args': '-DFOO'}
executable('progname', 'prog.c',
  c_args: '-DBAZ=1',
  kwargs: d) # This is an error!
```

Attempting to do this causes Meson to immediately exit with an error.

Method calls
--

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

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

If statements
--

If statements work just like in other languages.

```meson
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
```

Logical operations
--

Meson has the standard range of logical operations which can be used in
`if` statements.

```meson
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.

## Foreach statements

To do an operation on all elements of an iterable, use the `foreach`
command.

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

### Foreach with an array

Here's an example of how you could define two executables
with corresponding tests using arrays and foreach.

```meson
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
```

### Foreach with a dictionary

Here's an example of you could iterate a set of components that
should be compiled in according to some configuration. This uses
a [dictionary][dictionaries], which is available since 0.47.0.

```meson
components = {
  'foo': ['foo.c'],
  'bar': ['bar.c'],
  'baz': ['baz.c'],
}

# compute a configuration based on system dependencies, custom logic
conf = configuration_data()
conf.set('USE_FOO', 1)

# Determine the sources to compile
sources_to_compile = []
foreach name, sources : components
  if conf.get('USE_@0@'.format(name.to_upper()), 0) == 1
    sources_to_compile += sources
  endif
endforeach
```

### Foreach `break` and `continue`

Since 0.49.0 `break` and `continue` keywords can be used inside foreach loops.

```meson
items = ['a', 'continue', 'b', 'break', 'c']
result = []
foreach i : items
  if i == 'continue'
    continue
  elif i == 'break'
    break
  endif
  result += i
endforeach
# result is ['a', 'b']
```

Comments
--

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

```meson
some_function() # This is a comment
some_other_function()
```

Ternary operator
--

The ternary operator works just like in other languages.

```meson
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.

```meson
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](FAQ.md#why-is-meson-not-just-a-python-module-so-i-could-code-my-build-setup-in-python). If
because of this limitation you find yourself copying and pasting code
a lot you may be able to use a [`foreach` loop
instead](#foreach-statements).