when

A when statement is similar to an if statement but its condition is evaluated at compile time. The condition must be a constant boolean.

when true {
    fmt.println("This is called");
}

A when condition can have an else clause but its contents does not get checked unless the condition is false.

when false {
    this_is_valid(syntax);
    but := "because";
    it = is_never_evaluated;
    it = never_gets_ran;
} else {
    fmt.println("Hello");
}

A when statement does create a new scope with its braces { } unlike other flow control statements.

This is very useful for platform specific code.

when condition

In many cases, a library may want to separate OS specific code into separate file. To only import that file depend on a constant condition, a when condition can be append to a hash directive.

#import win32 "sys/windows.odin" when ODIN_OS == "windows";
#import foo "foo_amd64.odin" when ODIN_ARCH == "amd64";
#import foo "foo_x86.odin"   when ODIN_ARCH == "x86";

#load "bar.odin" when ODIN_ENDIAN == "big";

#foreign_system_library gl32 "opengl32.lib" when ODIN_OS == "windows";