module Crystal::Macros


The Macros module is a fictitious module used to document macros and macro methods.

You can invoke a fixed subset of methods on AST nodes at compile-time. These methods are documented on the classes in this module. Additionally, methods of the Macros module are top-level methods that you can invoke, like #puts and #run.

Defined in:


Instance Method Summary

Instance Method Detail

def `(command) : MacroIdSource

Executes a system command and returns the output as a MacroId. Gives a compile-time error if the command failed to execute.

def compare_versions(v1 : StringLiteral, v2 : StringLiteral) : NumberLiteralSource

Compares two semantic versions.

Returns -1, 0 or 1 depending on whether v1 is lower than v2, equal to v2 or greater than v2.

{{ compare_versions("1.10.0", "1.2.0") }} # => 1

def debug(format = true) : NopSource

Outputs the current macro's buffer to the standard output. Useful for debugging a macro to see what's being generated.

By default, the output is tried to be formatted using Crystal's formatter, but you can disable this by passing false to this method.

def env(name) : StringLiteral | NilLiteralSource

Gets the value of an environment variable at compile-time, or nil if it doesn't exist.

def flag?(name) : BoolLiteralSource

Returns whether a compile-time flag is set.

{{ flag?(:x86_64) }} # true or false

def host_flag?(name) : BoolLiteralSource

Returns whether a compile-time flag is set for the host platform, which can differ from the target platform (#flag?) during cross-compilation.

{{ host_flag?(:win32) }} # true or false

def p(*expressions) : NopSource

Same as #puts.

def p!(*expressions) : NopSource

Prints macro expressions together with their values at compile-time. Useful for debugging macros.

def pp(*expressions) : NopSource

Same as #puts.

def pp!(*expressions) : NopSource

Same as #p!

def puts(*expressions) : NopSource

Prints AST nodes at compile-time. Useful for debugging macros.

def raise(message) : NoReturnSource

Gives a compile-time error with the given message.

def read_file(filename) : StringLiteralSource

Reads a file and returns a StringLiteral with its contents.

Gives a compile-time error if the file doesn't exist or if reading the file fails.

To read a file relative to where the macro is defined, use:


NOTE Relative paths are resolved to the current working directory.

def read_file?(filename) : StringLiteral | NilLiteralSource

Same as #read_file, except that nil is returned on any I/O failure instead of issuing a compile-time failure.

def run(filename, *args) : MacroIdSource

Compiles and execute a Crystal program and returns its output as a MacroId.

The file denoted by filename must be a valid Crystal program. This macro invocation passes args to the program as regular program arguments. The program must output a valid Crystal expression. This output is the result of this macro invocation, as a MacroId.

The #run macro is useful when the subset of available macro methods are not enough for your purposes and you need something more powerful. With #run you can read files at compile time, connect to the internet or to a database.

A simple example:

# read.cr
puts File.read(ARGV[0])
# main.cr
macro read_file_at_compile_time(filename)
  {{ run("./read", filename).stringify }}

puts read_file_at_compile_time("some_file.txt")

The above generates a program that will have the contents of some_file.txt. The file, however, is read at compile time and will not be needed at runtime.

NOTE the compiler is allowed to cache the executable generated for filename and only recompile it if any of the files it depends on changes (their modified time). This is why it's strongly discouraged to use a program for #run that changes in subsequent compilations (for example, if it executes shell commands at compile time, or other macro run programs). It's also strongly discouraged to have a macro run program take a lot of time, because this will slow down compilation times. Reading files is OK, opening an HTTP connection at compile-time will most likely result in very slow compilations.

def skip_file : NopSource

Skips the rest of the file from which it is executed. Typical usage is to skip files that have platform specific code, without having to surround the most relevant code in {% if flag?(...) %} ... {% end %} macro blocks.


# sth_for_osx.cr
{% skip_file unless flag?(:darwin) %}

# Class FooForMac will only be defined if we're compiling on OS X
class FooForMac

def system(command) : MacroIdSource

Executes a system command and returns the output as a MacroId. Gives a compile-time error if the command failed to execute.

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Licensed under the Apache License, Version 2.0.