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) : MacroId #

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

It is impossible to call this method with any regular call syntax. There is an associated literal type which calls the method with the literal content as command:

{{ `echo hi` }} # => "hi\n"

See Command literals in the language reference.

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def compare_versions(v1 : StringLiteral, v2 : StringLiteral) : NumberLiteral #

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

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def debug(format = true) : Nop #

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.

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def env(name) : StringLiteral | NilLiteral #

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

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def file_exists?(filename) : BoolLiteral #

Returns true if the given filename exists, false otherwise.

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def flag?(name) : BoolLiteral #

Returns whether a compile-time flag is set.

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

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def host_flag?(name) : BoolLiteral #

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

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def p(*expressions) : Nop #

Same as #puts.

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def p!(*expressions) : Nop #

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

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def parse_type(type_name : StringLiteral) : Path | Generic | ProcNotation | Metaclass #

Parses type_name into a Path, Generic (also used for unions), ProcNotation, or Metaclass.

The #resolve method could then be used to resolve the value into a TypeNode, if the type_name represents a type, otherwise the value of the constant.

A compile time error is raised if the type/constant does not actually exist, or if a required generic argument was not provided.

class Foo; end

struct Some::Namespace::Foo; end

module Bar(T); end

MY_CONST = 1234

{{ parse_type("Foo").resolve.class? }}                                   # => true
{{ parse_type("Some::Namespace::Foo").resolve.struct? }}                 # => true
{{ parse_type("Foo|Some::Namespace::Foo").resolve.union_types.size }}    # => 2
{{ parse_type("Bar(Int32)|Foo").resolve.union_types[0].type_vars.size }} # => 1
{{ parse_type("MY_CONST").resolve }}                                     # => 1234

{{ parse_type("MissingType").resolve }}   # Error: undefined constant MissingType
{{ parse_type("UNKNOWN_CONST").resolve }} # Error: undefined constant UNKNOWN_CONST

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def pp(*expressions) : Nop #

Same as #puts.

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def pp!(*expressions) : Nop #

Same as #p!

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def print(*expressions) : Nop #

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

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def puts(*expressions) : Nop #

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

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def raise(message) : NoReturn #

Gives a compile-time error with the given message.

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def read_file(filename) : StringLiteral #

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.

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def read_file?(filename) : StringLiteral | NilLiteral #

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

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def run(filename, *args) : MacroId #

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. 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:

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.

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def skip_file : Nop #

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.


{% skip_file unless flag?(:darwin) %}

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

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def system(command) : MacroId #

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

{{ system("echo hi") }} # => "hi\n"

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def warning(message : StringLiteral) : NilLiteral #

Emits a compile-time warning with the given message.

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