Compile-time flags

Compile-time flags are boolean values provided through the compiler via a macro method. They allow to conditionally include or exclude code based on compile time conditions.

There are several default flags provided by the compiler with information about compiler options and the target platform. User-provided flags are passed to the compiler, which allow them to be used as feature flags.

Querying flags

A flag is just a named identifier which is either set or not. The status can be queried from code via the macro method flag?. It receives the name of a flag as a string or symbol literal and returns a bool literal indicating the flag’s state.

The following program shows the use of compile-time flags by printing the target OS family.

{% if flag?(:unix) %}
  puts "This program is compiled for a UNIX-like operating system"
{% elsif flag?(:windows) %}
  puts "This program is compiled for Windows"
{% else %}
  # Currently, all supported targets are either UNIX or Windows platforms, so
  # this branch is practically unreachable.
  puts "Compiling for some other operating system"
{% end %}

There’s also the macro method host_flag? which returns whether a flag is set for the host platform, which can differ from the target platform (queried by flag?) during cross-compilation.

Compiler-provided flags

The compiler defines a couple of implicit flags. They describe either the target platform or compiler options.

Target platform flags

Platform-specific flags derive from the target triple. See Platform Support for a list of supported target platforms.

crystal --version shows the default target triple of the compiler. It can be changed with the --target option.

The flags in each of the following tables are mutually exclusive, except for those marked as (derived).

Architecture

The target architecture is the first component of the target triple.

Vendor

The vendor is the second component of the target triple. This is typically unused, so the most common vendor is unknown.

Operating System

The operating system is derived from the third component of a the target triple.

ABI

The ABI is derived from the last component of the target triple.

Compiler options

The compiler sets these flags based on compiler configuration.

User-provided flags

User-provided flags are not defined automatically. They can be passed to the compiler via the --define or -D command line options.

These flags usually enable certain features which activate breaking new or legacy functionality, a preview for a new feature, or entirely alternative behaviour (e.g. for debugging purposes).

$ crystal eval -Dfoo 'puts {{ flag?(:foo) }}'
true

Stdlib features

These flags enable or disable features in the standard library when building a Crystal program.

Compiler features

These flags enable or disable compiler features when building a Crystal program.

Compiler build features

These flags enable or disable features when building the Crystal compiler.

User code features

Custom flags can be freely used in user code as long as they don’t collide with compiler-provided flags or other user-defined flags. When using a flag specific to a shard, it’s recommended to use the shard name as a prefix.