amosavian/target-platform-condition.md

## target-platform-condition.md

      
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              target-platform-condition.md
            
          
    Target platform condition


Proposal: SE-NNNN
Authors: Amir-Abbas Mousavian
Review Manager: TBD
Implementation: TBD
Status: Pending Review

Introduction

This proposal intends to indroduce a unified way to check building and evironment
conditions and allow to expand them in future. Another intnetion is to decrease os check
as all Apple platforms can be checked by #if target(kernel: Darwin) rather than
checking for 4 different OSes.
Motivation

A developer usually must check for environment to expose different behavior in different
building situations. In Swift, this usually is done with #if precompiling directive.
A flow with current approach is we should check every OS separately, this can lead to
confusion and increase maintanence cost for currnet codes as Swift is expanding to new
environments and a code should be checked against 12 OSes and architectures.
Proposed Solution

This proposal deprecates existing #if arch() & #if os() grammer and introduces new way:
#if (type: condition)
The type can be arch, os, kernel, config and env. This design allows to replace:
#if os(macOS) || os(ios) || os(tvOS) || os(watchOS)
simply with a more meaningful:
#if target(kernel: Darwin)
If a new operating system would be introduced by Apple based on Darwin, new code will be
compiled correctly without any modification.
Detailed Design

This will conditions available:
Architecture

For arch type, condition can be x86_64, arm, arm64, i386, powerpc64, powerpc64le and s390x
Operating system

For os type, condition can be macOS, iOS, watchOS, tvOS, GNU, Windows, FreeBSD, Android, PS4, Cygwin, Haiku, Fuchsia
Please note Linux is a kernel. GNU fits better as operating system.
Kernel

for kernel type, condition can be Darwin, Linux, Posix, Zircon.
Darwin will cover macOS, iOS, watchOS, tvOS.
Linux will cover GNU, Android, Cygwin.
BSD will cover FreeBSD, PS4.
Posix will cover all Darwin, BSD and Linux OSes.
This type simplifies importing in Foundation and StdLib to:
#if target(kernel: Darwin)
    import Darwin
#elseif target(kernel: Linux) || target(kernel: BSD)
// equal with `#elseif target(kernel: Posix)` as we checked Darwin already
    import Glibc
#endif
Environment

For env type, condition can be simulator, embedded, desktop.
Also we can define mobile, tablet, watch as environment, but these
conditions should be handled runtime rather than compile time.
This section covers proposal 190.
Configuration

For config type, condition can be debug, release. This will be affected by Xcode build configuration.
Endianness

For endian type, condition can be little, big. This will be mapped to corresponding arch types.
Other considerations

We may add support for Metal version by this approach. e.g. #if target(metal: 2).
Technically, Linux is a kernel and Gnu is userland of operating system.
In existing system it's a condition for os.
Cygwin is considered as operating system. We can define it as env with Windows
as operating system. But this may add complexity.
Source compatibility

This is an additive proposal, existing code will continue to work
but we can deprecate existing approach.
A warning and fixit may be provided for migrating recognizable cases
in existing code, but this will not be necessarily.
Effect on ABI stability

None
Effect on API resilience

None
Alternatives Considered

Some possible alternatives can be considered:

Keep current approach for architecture and os determination.
Add #if config() and #if kernel() to existing directives.
Using cpu as type instead of arch.