@ -4,23 +4,39 @@ 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.
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*,
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.
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.
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]
@ -33,7 +49,8 @@ var2 += [4]
Numbers
--
Meson supports only integer numbers. They are declared simply by writing them out. Basic arithmetic operations are supported.
Meson supports only integer numbers. They are declared simply by
writing them out. Basic arithmetic operations are supported.
```meson
x = 1 + 2
@ -60,13 +77,15 @@ truth = true
Strings
--
Strings in Meson are declared with single quotes. To enter a literal single quote do it like this:
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.
Similarly `\n` gets converted to a newline and `\\\\` to a single
backslash.
#### String concatenation
@ -80,7 +99,8 @@ 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:
Strings running over multiple lines can be declared with three single
quotes, like this:
```meson
multiline_string = '''#include <foo.h>
@ -89,7 +109,8 @@ int main (int argc, char ** argv) {
}'''
```
This can also be combined with the string formatting functionality described below.
This can also be combined with the string formatting functionality
described below.
#### String formatting
@ -101,11 +122,13 @@ 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.
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.
Strings also support a number of other methods that return transformed
copies.
**.strip()**
@ -226,7 +249,9 @@ my_array += ['something']
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.
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.
#### Array methods
@ -239,7 +264,8 @@ The following methods are defined for all arrays:
Function calls
--
Meson provides a set of usable functions. The most common use case is creating build objects.
Meson provides a set of usable functions. The most common use case is
Objects can have methods, which are called with the dot operator. The exact methods it provides depends on the object.
Objects can have methods, which are called with the dot operator. The
exact methods it provides depends on the object.
```meson
myobj = some_function()
@ -279,7 +306,9 @@ 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.
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']],
@ -291,7 +320,9 @@ foreach p : progs
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.
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
--
@ -334,12 +365,20 @@ 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.
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.
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'
@ -349,4 +388,12 @@ 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).
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
Jussi Pakkanen
7 years ago