Target conditionals

if const selects code by facts about the compilation target. It may appear at module scope, around declarations and imports, and in any function body, around statements:

if const target.os == "windows":
    import windows_console
    def separator() -> str:
        return "\\"
else:
    def separator() -> str:
        return "/"

def main() -> Result[(), IoError]:
    if const target.pointer_width == 64:
        limit = 1 << 40
    else:
        limit = 1 << 20
    print(separator())?
    print(limit)?
    Ok(())

On a 64-bit Linux or macOS target, it prints:

/
1099511627776

The compiler decides the condition while parsing and keeps only the first arm whose condition holds, or the else arm. Each continuation is written elif const; an elif without const is an error. Unselected arms must parse, but are not resolved or checked: they may import modules, call functions, and name declarations that exist only on another target. An unselected arm is not checked on the current target, so build on each target a program supports.

In a function body, the selected statements take the conditional’s place in the enclosing block; they open no new scope. A binding in the selected arm, such as limit above, is visible after the conditional. The last statement of a selected arm can be the function’s result.

Facts

The set is closed. An unknown fact, such as target.cpu, is an error, and so is a comparison with a value outside the known values, such as "linx", so a misspelling cannot silently select the else arm. The compiler currently builds natively for GNU/Linux on x86-64 and AArch64: target.os is "linux", target.arch is "x86_64" or "aarch64", pointer width is 64, and endian is "little". The facts describe the packaged runtime, and an installed compiler cannot select the other architecture with --target; see platform support.

Conditions

A condition compares one fact with a string or integer literal using == or !=, and combines comparisons with and, or, not, parentheses, True, and False. Any other expression is an error. A condition cannot name a module constant: conditions are decided while parsing, before names are resolved, so that unselected arms can name declarations that do not exist. A constant can instead be defined differently in each arm.