The Pony libraries, of course, want to abstract platform differences. Sometimes you may want a use command that only works under certain circumstances, most commonly only on a particular OS or only for debug builds. You can do this by specifying a condition for a use command:

use "foo" if linux
use "bar" if (windows and debug)

Use conditions can use any of the methods defined in builtin/Platform as conditions. There are currently the following booleans defined: freebsd, linux, osx, posix => (freebsd or linux or osx), windows, x86, arm, lp64, llp64, ilp32, native128, debug

They can also use the operators and, or, xor and not. As with other expressions in Pony, parentheses must be used to indicate precedence if more than one of and, or and xor is used.

Any use command whose condition evaluates to false is ignored.