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 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 variables 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.
```
Strings can be converted to a number like this:
```meson
string_var = '42'
num = var1.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'
```
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:
```meson
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.
```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
-`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.
Meson provides a set of usable functions. The most common use case is creating build objects.
```meson
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.
```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
```
## 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.
```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
```
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.
```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.
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 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).
