# Top Level Namespace

##
Included Modules

- Spec::Expectations
- Spec::Methods

##
Extended Modules

- Spec::Expectations
- Spec::Methods

##
Defined in:

##
Constant Summary

ARGF = `IO::ARGF.new(ARGV, STDIN)`

  An `IO` for reading files from `ARGV`.

  Usage example:

  `program.cr`:

      puts ARGF.gets_to_end

  A file to read from: (`file`)

      123

      $ crystal build program.cr
      $ ./program file
      123
      $ ./program file file
      123123
      $ # If ARGV is empty, ARGF reads from STDIN instead:
      $ echo "hello" | ./program
      hello
      $ ./program unknown
      Unhandled exception: Error opening file with mode 'r': 'unknown': No such file or directory (File::NotFoundError)
      ...

  After a file from `ARGV` has been read, it's removed from `ARGV`.

  You can manipulate `ARGV` yourself to control what `ARGF` operates on. If you remove a file from `ARGV`, it is ignored by `ARGF`; if you add files to `ARGV`, `ARGF` will read from it.

      ARGV.replace ["file1"]
      ARGF.gets_to_end # => Content of file1
      ARGV             # => []
      ARGV << "file2"
      ARGF.gets_to_end # => Content of file2

ARGV = `Array.new(ARGC_UNSAFE - 1) do |i| String.new(ARGV_UNSAFE[1 + i]) end`

  An array of arguments passed to the program.

EOL = `{% if flag?(:windows) %} "\r\n" {% else %} "\n" {% end %}`

  The newline constant

PROGRAM_NAME = `String.new(ARGV_UNSAFE.value)`

  The name, the program was called with.

  The result may be a relative or absolute path (including symbolic links), just the command name or the empty string.

  See `Process.executable_path` for a more convenient alternative that always returns the absolute real path to the executable file (if it exists).

STDERR = `IO::FileDescriptor.from_stdio(2)`

  The standard error file descriptor.

  Typically used to output error messages and diagnostics.

  At the start of the program STDERR is configured like this:

  - if it's a TTY device (like the console) then `sync` is `true`, meaning that output will be outputted as soon as it is written to STDERR. This is so users can see real time output data.
  - if it's not a TTY device (like a file, or because the output was piped to a file) then `sync` is `false` but `flush_on_newline` is `true`. This is so that if you pipe the output to a file, and, for example, you `tail -f`, you can see data on a line-per-line basis. This is convenient but slower than with `flush_on_newline` set to `false`. If you need a bit more performance and you don't care about near real-time output you can do `STDERR.flush_on_newline = false`.

  On Unix systems, if the file descriptor is a TTY, the runtime duplicates it. So `STDERR.fd` might not be `2`. The reason for this is to enable non-blocking writes for concurrency. Other fibers can run while waiting on IO. The original file descriptor is inherited from the parent process. Setting it to non-blocking mode would reflect back which can cause problems.

  On Windows, `STDERR` is always blocking.

STDIN = `IO::FileDescriptor.from_stdio(0)`

  The standard input file descriptor. Contains data piped to the program.

  On Unix systems, if the file descriptor is a TTY, the runtime duplicates it. So `STDIN.fd` might not be `0`. The reason for this is to enable non-blocking reads for concurrency. Other fibers can run while waiting on user input. The original file descriptor is inherited from the parent process. Setting it to non-blocking mode would reflect back, which can cause problems.

  On Windows, `STDIN` is always blocking.

STDOUT = `IO::FileDescriptor.from_stdio(1)`

  The standard output file descriptor.

  Typically used to output data and information.

  At the start of the program STDOUT is configured like this:

  - if it's a TTY device (like the console) then `sync` is `true`, meaning that output will be outputted as soon as it is written to STDOUT. This is so users can see real time output data.
  - if it's not a TTY device (like a file, or because the output was piped to a file) then `sync` is `false` but `flush_on_newline` is `true`. This is so that if you pipe the output to a file, and, for example, you `tail -f`, you can see data on a line-per-line basis. This is convenient but slower than with `flush_on_newline` set to `false`. If you need a bit more performance and you don't care about near real-time output you can do `STDOUT.flush_on_newline = false`.

  On Unix systems, if the file descriptor is a TTY, the runtime duplicates it. So `STDOUT.fd` might not be `1`. The reason for this is to enable non-blocking writes for concurrency. Other fibers can run while waiting on IO. The original file descriptor is inherited from the parent process. Setting it to non-blocking mode would reflect back which can cause problems.

  On Windows, `STDOUT` is always blocking.

##
Method Summary

- `(command) : String

  Returns the standard output of executing command in a subshell.

- abort(message = nil, status = 1) : NoReturn

  Terminates execution immediately, printing message to `STDERR` and then calling `exit(status)`.

- alignof(type : Class) : Int32

  Returns the alignment of the given type as number of bytes.

- at_exit(&handler : Int32, Exception | Nil -> ) : Nil

  Registers the given `Proc` for execution when the program exits regularly.

- caller : Array(String)

  Returns the current execution stack as an array containing strings usually in the form file:line:column or file:line:column in 'method'.

- exit(status = 0) : NoReturn

  Terminates execution immediately, returning the given status code to the invoking environment.

- gets(*args, **options)

  Reads a line from `STDIN`.

- instance_alignof(type : Class) : Int32

  Returns the instance alignment of the given class as number of bytes.

- instance_sizeof(type : Class) : Int32

  Returns the instance size of the given class as number of bytes.

- loop(&)

  Repeatedly executes the block.

- main(argc : Int32, argv : Pointer(Pointer(UInt8))) : Int32

  Main function that acts as C's main function.

- offsetof(type : Class, offset) : Int32

  Returns the byte offset of an instance variable in a struct or class type.

- p(object)

  Inspects object to `STDOUT` followed by a newline.

- p(*objects)

  Inspects each object in objects to `STDOUT`, followed by a newline.

- p(**objects)

  Inspects objects to `STDOUT`, followed by a newline.

- pointerof(variable : T) : Pointer(T) forall T

  Returns a `Pointer` to the contents of a variable.

- pp(object)

  Pretty prints object to `STDOUT` followed by a newline.

- pp(*objects)

  Pretty prints each object in objects to `STDOUT`, followed by a newline.

- pp(**objects)

  Pretty prints objects to `STDOUT`, followed by a newline.

- print(*objects : _) : Nil

  Prints objects to `STDOUT` and then invokes `STDOUT.flush`.

- printf(format_string, args : Array | Tuple) : Nil

  Prints a formatted string to `STDOUT`.

- printf(format_string, *args) : Nil

  Prints a formatted string to `STDOUT`.

- puts(*objects) : Nil

  Prints objects to `STDOUT`, each followed by a newline character unless the object is a `String` and already ends with a newline.

- raise(exception : Exception) : NoReturn

  Raises the exception.

- raise(message : String) : NoReturn

  Raises an Exception with the message.

- rand(x)

  See `Random#rand(x)`.

- rand : Float64

  See `Random#rand`.

- read_line(*args, **options)

  Reads a line from `STDIN`.

- sizeof(type : Class) : Int32

  Returns the size of the given type as number of bytes.

- sleep(seconds : Number) : Nil

  Blocks the current fiber for the specified number of seconds.

  DEPRECATED Use `::sleep(Time::Span)` instead

- sleep(time : Time::Span) : Nil

  Blocks the current Fiber for the specified time span.

- sleep : Nil

  Blocks the current fiber forever.

- spawn(*, name : String | Nil = nil, same_thread = false, &block)

  Spawns a new fiber.

- sprintf(format_string, args : Array | Tuple) : String

  Returns a formatted string.

- sprintf(format_string, *args) : String

  Returns a formatted string.

- system(command : String, args = nil) : Bool

  Executes the given command in a subshell.

- timeout_select_action(timeout : Time::Span) : Channel::TimeoutAction

  Timeout keyword for use in `select`.

- typeof(*expression) : Class

  Returns the type of an expression.

##
Macro Summary

- debugger

  Invokes an execution trap to catch the attention of a debugger.

- p!(*exps)

  Prints a series of expressions together with their inspected values.

- pp!(*exps)

  Prints a series of expressions together with their pretty printed values.

- record(__name name, *properties, **kwargs)

  Defines a `Struct` type called name with the given properties.

- spawn(call, *, name = nil, same_thread = false, &block)

  Spawns a fiber by first creating a `Proc`, passing the call's expressions to it, and letting the `Proc` finally invoke the call.

### Instance methods inherited from module `Spec::Methods`

after_all(&block) after_all, after_each(&block) after_each, around_all(&block : ExampleGroup::Procsy -> ) around_all, around_each(&block : Example::Procsy -> ) around_each, before_all(&block) before_all, before_each(&block) before_each, context(description = nil, file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil, &block) context, describe(description = nil, file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil, &block) describe, fail(msg, file = __FILE__, line = __LINE__) fail, it(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil, &block) it, pending(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil, &)
pending(description = "assert", file = __FILE__, line = __LINE__, end_line = __END_LINE__, focus : Bool = false, tags : String | Enumerable(String) | Nil = nil) pending, pending!(msg = "Cannot run example", file = __FILE__, line = __LINE__) pending!

### Macros inherited from module `Spec::Methods`

assert_iterates_iterator(expected, method, *, infinite = false) assert_iterates_iterator, assert_iterates_yielding(expected, method, *, infinite = false, tuple = false) assert_iterates_yielding, assert_prints(call, *, should expectation, file = __FILE__, line = __LINE__)
assert_prints(call, str, *, file = __FILE__, line = __LINE__) assert_prints, it_iterates(description, expected, method, *, infinite = false, tuple = false, file = __FILE__, line = __LINE__) it_iterates

### Instance methods inherited from module `Spec::Expectations`

be(value)
be be, be_close(expected, delta) be_close, be_empty be_empty, be_false be_false, be_falsey be_falsey, be_nil be_nil, be_true be_true, be_truthy be_truthy, contain(expected) contain, end_with(expected) end_with, eq(value) eq, expect_raises(klass : T.class, message : String | Regex | Nil = nil, file = __FILE__, line = __LINE__, &) forall T expect_raises, match(value) match, start_with(expected) start_with

### Macros inherited from module `Spec::Expectations`

be_a(type) be_a

##
Method Detail

def `(command) : String #

Returns the standard output of executing command in a subshell. Standard input, and error are inherited. The special `$?` variable is set to a `Process::Status` associated with this execution.

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"
    $?.success? # => true

See `Command` literals in the language reference.

def abort(message = nil, status = 1) : NoReturn #

Terminates execution immediately, printing message to `STDERR` and then calling `exit(status)`.

def alignof(type : Class) : Int32 #

Returns the alignment of the given type as number of bytes.

type must be a constant or `typeof()` expression. It cannot be evaluated at runtime.

    alignof(Int32)        # => 4
    alignof(Float64)      # usually 4 or 8
    alignof(typeof(true)) # => 1

    For `Reference` types, the alignment is the same as the alignment of a pointer:

#
On a 64 bits machine

alignof(Pointer(Int32)) # => 8 alignof(String) # => 8

    This is because a `Reference`'s memory is allocated on the heap and a pointer
    to it is passed around. The alignment of a class on the heap can be determined
    using `#instance_alignof`.

    NOTE: This is a pseudo-method provided directly by the Crystal compiler.
    It cannot be redefined nor overridden.

def at_exit(&handler : Int32, Exception | Nil -> ) : Nil #

Registers the given `Proc` for execution when the program exits regularly.

A regular exit happens when either

- the main fiber reaches the end of the program,
- the main fiber rescues an unhandled exception, or
- `::exit` is called.

`Process.exit` does not trigger `at_exit` handlers, nor does external process termination (see `Process.on_terminate` for handling that).

If multiple handlers are registered, they are executed in reverse order of registration.

    def do_at_exit(str1)
      at_exit { print str1 }
    end

    at_exit { puts "cruel world" }
    do_at_exit("goodbye ")
    exit

Produces:

    goodbye cruel world

The exit status code that will be returned by this program is passed to the block as its first argument. In case of any unhandled exception, it is passed as the second argument to the block, if the program terminates normally or `exit(status)` is called explicitly, then the second argument will be `nil`.

NOTE If `at_exit` is called inside an `at_exit` handler, it will be called right after the current `at_exit` handler ends, and then other handlers will be invoked.

def caller : Array(String) #

Returns the current execution stack as an array containing strings usually in the form file:line:column or file:line:column in 'method'.

def exit(status = 0) : NoReturn #

Terminates execution immediately, returning the given status code to the invoking environment.

Registered `at_exit` procs are executed.

def gets(*args, **options) #

Reads a line from `STDIN`.

See also: `IO#gets`.

def instance_alignof(type : Class) : Int32 #

Returns the instance alignment of the given class as number of bytes.

type must be a constant or `typeof()` expression. It cannot be evaluated at runtime.

    instance_alignof(String)    # => 4
    instance_alignof(Exception) # => 8

See `alignof` for determining the size of value types.

NOTE This is a pseudo-method provided directly by the Crystal compiler. It cannot be redefined nor overridden.

def instance_sizeof(type : Class) : Int32 #

Returns the instance size of the given class as number of bytes.

type must be a constant or `typeof()` expression. It cannot be evaluated at runtime.

    instance_sizeof(String)    # => 16
    instance_sizeof(Exception) # => 48

See `sizeof` for determining the size of value types.

NOTE This is a pseudo-method provided directly by the Crystal compiler. It cannot be redefined nor overridden.

def loop(&) #

Repeatedly executes the block.

    loop do
      line = gets
      break unless line
      # ...
    end

def main(argc : Int32, argv : Pointer(Pointer(UInt8))) : Int32 #

Main function that acts as C's main function. Invokes `Crystal.main`.

Can be redefined. See `Crystal.main` for examples.

On Windows the actual entry point is `wmain`, but there is no need to redefine that. See the file required below for details.

def offsetof(type : Class, offset) : Int32 #

Returns the byte offset of an instance variable in a struct or class type.

type must be a constant or `typeof()` expression. It cannot be evaluated at runtime. offset must be the name of an instance variable of type, prefixed by `@`, or the index of an element in a Tuple, starting from 0, if type is a `Tuple`.

    offsetof(String, @bytesize)       # => 4
    offsetof(Exception, @message)     # => 8
    offsetof(Time, @location)         # => 16
    offsetof({Int32, Int8, Int32}, 0) # => 0
    offsetof({Int32, Int8, Int32}, 1) # => 4
    offsetof({Int32, Int8, Int32}, 2) # => 8

NOTE This is a pseudo-method provided directly by the Crystal compiler. It cannot be redefined nor overridden.

def p(object) #

Inspects object to `STDOUT` followed by a newline. Returns object.

See also: `Object#inspect(io)`.

def p(*objects) #

Inspects each object in objects to `STDOUT`, followed by a newline. Returns objects.

See also: `Object#inspect(io)`.

def p(**objects) #

Inspects objects to `STDOUT`, followed by a newline. Returns objects.

    p foo: 23, bar: 42 # => {foo: 23, bar: 42}

See `Object#inspect(io)`

def pointerof(variable : T) : Pointer(T) forall T #

Returns a `Pointer` to the contents of a variable.

variable must be a variable (local, instance, class or library).

    a = 1
    ptr = pointerof(a)
    ptr.value = 2

    a # => 2

NOTE This is a pseudo-method provided directly by the Crystal compiler. It cannot be redefined nor overridden.

def pp(object) #

Pretty prints object to `STDOUT` followed by a newline. Returns object.

See also: `Object#pretty_print(pp)`.

def pp(*objects) #

Pretty prints each object in objects to `STDOUT`, followed by a newline. Returns objects.

See also: `Object#pretty_print(pp)`.

def pp(**objects) #

Pretty prints objects to `STDOUT`, followed by a newline. Returns objects.

    p foo: 23, bar: 42 # => {foo: 23, bar: 42}

See `Object#pretty_print(pp)`

def print(*objects : _) : Nil #

Prints objects to `STDOUT` and then invokes `STDOUT.flush`.

See also: `IO#print`.

def printf(format_string, args : Array | Tuple) : Nil #

Prints a formatted string to `STDOUT`.

For details on the format string, see `sprintf`.

def printf(format_string, *args) : Nil #

Prints a formatted string to `STDOUT`.

For details on the format string, see `sprintf`.

def puts(*objects) : Nil #

Prints objects to `STDOUT`, each followed by a newline character unless the object is a `String` and already ends with a newline.

See also: `IO#puts`.

def raise(exception : Exception) : NoReturn #

Raises the exception.

This will set the exception's callstack if it hasn't been already. Re-raising a previously caught exception won't replace the callstack.

def raise(message : String) : NoReturn #

Raises an Exception with the message.

def rand(x) #

See `Random#rand(x)`.

def rand : Float64 #

See `Random#rand`.

def read_line(*args, **options) #

Reads a line from `STDIN`.

See also: `IO#read_line`.

def sizeof(type : Class) : Int32 #

Returns the size of the given type as number of bytes.

type must be a constant or `typeof()` expression. It cannot be evaluated at runtime.

    sizeof(Int32)        # => 4
    sizeof(Int64)        # => 8
    sizeof(typeof(true)) # => 1

For `Reference` types, the size is the same as the size of a pointer:

    # On a 64 bits machine
    sizeof(Pointer(Int32)) # => 8
    sizeof(String)         # => 8

This is because a `Reference`'s memory is allocated on the heap and a pointer to it is passed around. The size of a class on the heap can be determined using `#instance_sizeof`.

NOTE This is a pseudo-method provided directly by the Crystal compiler. It cannot be redefined nor overridden.

def sleep(seconds : Number) : Nil #

Blocks the current fiber for the specified number of seconds.

While this fiber is waiting this time, other ready-to-execute fibers might start their execution.

DEPRECATED Use `::sleep(Time::Span)` instead

def sleep(time : Time::Span) : Nil #

Blocks the current Fiber for the specified time span.

While this fiber is waiting this time, other ready-to-execute fibers might start their execution.

def sleep : Nil #

Blocks the current fiber forever.

Meanwhile, other ready-to-execute fibers might start their execution.

def spawn(*, name : String | Nil = nil, same_thread = false, &block) #

Spawns a new fiber.

When using execution contexts, the fiber spawns into the current execution context (`Fiber::ExecutionContext.current`).

NOTE The newly created fiber doesn't run as soon as spawned.

Example:

    # Write "1" every 1 second and "2" every 2 seconds for 6 seconds.

    require "wait_group"

    wg = WaitGroup.new 2

    spawn do
      6.times do
        sleep 1.second
        puts 1
      end
    ensure
      wg.done
    end

    spawn do
      3.times do
        sleep 2.seconds
        puts 2
      end
    ensure
      wg.done
    end

    wg.wait

def sprintf(format_string, args : Array | Tuple) : String #

Returns a formatted string. The string is produced according to the format_string with format specifiers being replaced by values from args formatted according to the specifier.

Within the format string, any characters other than format specifiers (specifiers beginning with `%`) are copied to the result. The formatter supports the following kinds of format specifiers:

- Sequential (`%.1f`). The first `%` consumes the first argument, the second consumes the second argument, and so on.
- Positional (`%3$.1f`). The one-based argument index is specified as part of the flags.
- Named substitution (`%<name>.1f`, `%{name}`). The angle bracket form accepts flags and the curly bracket form doesn't. Exactly one `Hash` or `NamedTuple` must be passed as the argument.

Mixing of different kinds of format specifiers is disallowed, except that the two named forms may be used together.

A simple format specifier consists of a percent sign, followed by optional flags, width, and precision indicators, then terminated with a field type character.

    %[flags][width][.precision]type

A formatted substitution is similar but after the percent sign follows the mandatory name of the substitution wrapped in angle brackets.

    %<name>[flags][width][.precision]type

The percent sign itself is escaped by `%%`. No format modifiers are allowed, and no arguments are consumed.

The field type controls how the corresponding argument value is to be interpreted, while the flags modify that interpretation.

The field type characters are:

    Field | Integer Format
    ------+------------------------------------------------------------------
      b   | Formats argument as a binary number.
      d   | Formats argument as a decimal number.
      i   | Same as d.
      o   | Formats argument as an octal number.
      x   | Formats argument as a hexadecimal number using lowercase letters.
      X   | Same as x, but uses uppercase letters.

    Field | Float Format
    ------+---------------------------------------------------------------
      e   | Formats floating point argument into exponential notation
          | with one digit before the decimal point as [-]d.dddddde[+-]dd.
          | The precision specifies the number of digits after the decimal
          | point (defaulting to six).
      E   | Equivalent to e, but uses an uppercase E to indicate
          | the exponent.
      f   | Formats floating point argument as [-]ddd.dddddd,
          | where the precision specifies the number of digits after
          | the decimal point.
      g   | Formats a floating point number using exponential form
          | if the exponent is less than -4 or greater than or
          | equal to the precision, or in dd.dddd form otherwise.
          | The precision specifies the number of significant digits.
      G   | Equivalent to g, but use an uppercase E in exponent form.
      a   | Formats floating point argument as [-]0xh.hhhhp[+-]dd,
          | which consists of an optional sign, "0x", fraction part
          | as hexadecimal, "p", and exponential part as decimal.
      A   | Equivalent to a, but uses uppercase X and P.

    Field | Other Format
    ------+------------------------------------------------------------
      c   | Argument is a single character itself.
      s   | Argument is a string to be substituted. If the format
          | sequence contains a precision, at most that many characters
          | will be copied.

Flags modify the behavior of the format specifiers:

    Flag     | Applies to    | Meaning
    ---------+---------------+----------------------------------------------------
    space    | bdiouxX       | Add a leading space character to
             | aAeEfgG       | non-negative numbers. Has no effect if the plus
             | (numeric fmt) | flag is also given.
    ---------+---------------+----------------------------------------------------
    #        | boxX          | Use an alternative format.
             | aAeEfgG       | For x, X, b, prefix any non-zero result with
             |               | 0x, 0X, or 0b respectively.
             |               | For o, prefix any non-zero result with 0o. Note
             |               | that this prefix is different from C and Ruby.
             |               | For a, A, e, E, f, g, G,
             |               | force a decimal point to be added,
             |               | even if no digits follow.
             |               | For g and G, do not remove trailing zeros.
    ---------+---------------+----------------------------------------------------
    +        | bdiouxX       | Add a leading plus sign to non-negative
             | aAeEfgG       | numbers.
             | (numeric fmt) |
    ---------+---------------+----------------------------------------------------
    -        | all           | Left-justify the result of this conversion.
    ---------+---------------+----------------------------------------------------
    0 (zero) | bdiouxX       | Pad with zeros, not spaces. Has no effect if the
             | aAeEfgG       | number is left-justified, or a precision is given
             | (numeric fmt) | to an integer field type.
    ---------+---------------+----------------------------------------------------
    *        | all           | Use the next argument as the field width or
             |               | precision. If width is negative, set the minus flag
             |               | and use the argument's absolute value as field
             |               | width. If precision is negative, it is ignored.
    ---------+---------------+----------------------------------------------------
    1$ 2$ 3$ | all           | As a flag, specifies the one-based argument index
    ...      |               | for a positional format specifier.
             |               | Immediately after *, use the argument at the given
             |               | one-based index as the width or precision, instead
             |               | of the next argument. The entire format string must
             |               | also use positional format specifiers throughout.

Examples of flags:

Decimal number conversion:

    sprintf "%d", 123  # => "123"
    sprintf "%+d", 123 # => "+123"
    sprintf "% d", 123 # => " 123"

Octal number conversion:

    sprintf "%o", 123   # => "173"
    sprintf "%+o", 123  # => "+173"
    sprintf "%o", -123  # => "-173"
    sprintf "%+o", -123 # => "-173"

Hexadecimal number conversion:

    sprintf "%x", 123   # => "7b"
    sprintf "%+x", 123  # => "+7b"
    sprintf "%x", -123  # => "-7b"
    sprintf "%+x", -123 # => "-7b"
    sprintf "%#x", 0    # => "0"
    sprintf "% x", 123  # => " 7b"
    sprintf "% x", -123 # => "-7b"
    sprintf "%X", 123   # => "7B"
    sprintf "%#X", -123 # => "-0X7B"

Binary number conversion:

    sprintf "%b", 123    # => "1111011"
    sprintf "%+b", 123   # => "+1111011"
    sprintf "%+b", -123  # => "-1111011"
    sprintf "%b", -123   # => "-1111011"
    sprintf "%#b", 0     # => "0"
    sprintf "% b", 123   # => " 1111011"
    sprintf "%+ b", 123  # => "+1111011"
    sprintf "% b", -123  # => "-1111011"
    sprintf "%+ b", -123 # => "-1111011"
    sprintf "%#b", 123   # => "0b1111011"

Floating point conversion:

    sprintf "%a", 123 # => "0x1.ecp+6"
    sprintf "%A", 123 # => "0X1.ECP+6"

Exponential form conversion:

    sprintf "%g", 123.4          # => "123.4"
    sprintf "%g", 123.4567       # => "123.457"
    sprintf "%20.8g", 1234.56789 # => "           1234.5679"
    sprintf "%20.8g", 123456789  # => "       1.2345679e+08"
    sprintf "%20.8G", 123456789  # => "       1.2345679E+08"
    sprintf "%20.8g", -123456789 # => "      -1.2345679e+08"
    sprintf "%20.8G", -123456789 # => "      -1.2345679E+08"

The field width is an optional integer, followed optionally by a period and a precision. The width specifies the minimum number of characters that will be written to the result for this field.

Examples of width:

    sprintf "%20d", 123   # => "                 123"
    sprintf "%+20d", 123  # => "                +123"
    sprintf "%020d", 123  # => "00000000000000000123"
    sprintf "%+020d", 123 # => "+0000000000000000123"
    sprintf "% 020d", 123 # => " 0000000000000000123"
    sprintf "%-20d", 123  # => "123                 "
    sprintf "%-+20d", 123 # => "+123                "
    sprintf "%- 20d", 123 # => " 123                "
    sprintf "%020x", -123 # => "-000000000000000007b"
    sprintf "%020X", -123 # => "-000000000000000007B"

For numeric fields, the precision controls the number of decimal places displayed.

For string fields, the precision determines the maximum number of characters to be copied from the string.

Examples of precisions:

Precision for `d`, `o`, `x` and `b` is minimum number of digits:

    sprintf "%20.8d", 123   # => "            00000123"
    sprintf "%020.8d", 123  # => "            00000123"
    sprintf "%20.8o", 123   # => "            00000173"
    sprintf "%020.8o", 123  # => "            00000173"
    sprintf "%20.8x", 123   # => "            0000007b"
    sprintf "%020.8x", 123  # => "            0000007b"
    sprintf "%20.8b", 123   # => "            01111011"
    sprintf "%20.8d", -123  # => "           -00000123"
    sprintf "%020.8d", -123 # => "           -00000123"
    sprintf "%20.8o", -123  # => "           -00000173"
    sprintf "%20.8x", -123  # => "           -0000007b"
    sprintf "%20.8b", -11   # => "           -00001011"

Precision for `e` is number of digits after the decimal point:

    sprintf "%20.8e", 1234.56789 # => "      1.23456789e+03"

Precision for `f` is number of digits after the decimal point:

    sprintf "%20.8f", 1234.56789 # => "       1234.56789000"

Precision for `g` is number of significant digits:

    sprintf "%20.8g", 1234.56789 # => "           1234.5679"
    sprintf "%20.8g", 123456789  # => "       1.2345679e+08"
    sprintf "%-20.8g", 123456789 # => "1.2345679e+08       "

Precision for `s` is maximum number of characters:

    sprintf "%20.8s", "string test" # => "            string t"

Additional examples:

    sprintf "%d %04x", 123, 123                 # => "123 007b"
    sprintf "%08b '%4s'", 123, 123              # => "01111011 ' 123'"
    sprintf "%+g:% g:%-g", 1.23, 1.23, 1.23     # => "+1.23: 1.23:1.23"
    sprintf "%-3$*1$.*2$s", 10, 5, "abcdefghij" # => "abcde     "

def sprintf(format_string, *args) : String #

Returns a formatted string. The string is produced according to the format_string with format specifiers being replaced by values from args formatted according to the specifier.

Within the format string, any characters other than format specifiers (specifiers beginning with `%`) are copied to the result. The formatter supports the following kinds of format specifiers:

- Sequential (`%.1f`). The first `%` consumes the first argument, the second consumes the second argument, and so on.
- Positional (`%3$.1f`). The one-based argument index is specified as part of the flags.
- Named substitution (`%<name>.1f`, `%{name}`). The angle bracket form accepts flags and the curly bracket form doesn't. Exactly one `Hash` or `NamedTuple` must be passed as the argument.

Mixing of different kinds of format specifiers is disallowed, except that the two named forms may be used together.

A simple format specifier consists of a percent sign, followed by optional flags, width, and precision indicators, then terminated with a field type character.

    %[flags][width][.precision]type

A formatted substitution is similar but after the percent sign follows the mandatory name of the substitution wrapped in angle brackets.

    %<name>[flags][width][.precision]type

The percent sign itself is escaped by `%%`. No format modifiers are allowed, and no arguments are consumed.

The field type controls how the corresponding argument value is to be interpreted, while the flags modify that interpretation.

The field type characters are:

    Field | Integer Format
    ------+------------------------------------------------------------------
      b   | Formats argument as a binary number.
      d   | Formats argument as a decimal number.
      i   | Same as d.
      o   | Formats argument as an octal number.
      x   | Formats argument as a hexadecimal number using lowercase letters.
      X   | Same as x, but uses uppercase letters.

    Field | Float Format
    ------+---------------------------------------------------------------
      e   | Formats floating point argument into exponential notation
          | with one digit before the decimal point as [-]d.dddddde[+-]dd.
          | The precision specifies the number of digits after the decimal
          | point (defaulting to six).
      E   | Equivalent to e, but uses an uppercase E to indicate
          | the exponent.
      f   | Formats floating point argument as [-]ddd.dddddd,
          | where the precision specifies the number of digits after
          | the decimal point.
      g   | Formats a floating point number using exponential form
          | if the exponent is less than -4 or greater than or
          | equal to the precision, or in dd.dddd form otherwise.
          | The precision specifies the number of significant digits.
      G   | Equivalent to g, but use an uppercase E in exponent form.
      a   | Formats floating point argument as [-]0xh.hhhhp[+-]dd,
          | which consists of an optional sign, "0x", fraction part
          | as hexadecimal, "p", and exponential part as decimal.
      A   | Equivalent to a, but uses uppercase X and P.

    Field | Other Format
    ------+------------------------------------------------------------
      c   | Argument is a single character itself.
      s   | Argument is a string to be substituted. If the format
          | sequence contains a precision, at most that many characters
          | will be copied.

Flags modify the behavior of the format specifiers:

    Flag     | Applies to    | Meaning
    ---------+---------------+----------------------------------------------------
    space    | bdiouxX       | Add a leading space character to
             | aAeEfgG       | non-negative numbers. Has no effect if the plus
             | (numeric fmt) | flag is also given.
    ---------+---------------+----------------------------------------------------
    #        | boxX          | Use an alternative format.
             | aAeEfgG       | For x, X, b, prefix any non-zero result with
             |               | 0x, 0X, or 0b respectively.
             |               | For o, prefix any non-zero result with 0o. Note
             |               | that this prefix is different from C and Ruby.
             |               | For a, A, e, E, f, g, G,
             |               | force a decimal point to be added,
             |               | even if no digits follow.
             |               | For g and G, do not remove trailing zeros.
    ---------+---------------+----------------------------------------------------
    +        | bdiouxX       | Add a leading plus sign to non-negative
             | aAeEfgG       | numbers.
             | (numeric fmt) |
    ---------+---------------+----------------------------------------------------
    -        | all           | Left-justify the result of this conversion.
    ---------+---------------+----------------------------------------------------
    0 (zero) | bdiouxX       | Pad with zeros, not spaces. Has no effect if the
             | aAeEfgG       | number is left-justified, or a precision is given
             | (numeric fmt) | to an integer field type.
    ---------+---------------+----------------------------------------------------
    *        | all           | Use the next argument as the field width or
             |               | precision. If width is negative, set the minus flag
             |               | and use the argument's absolute value as field
             |               | width. If precision is negative, it is ignored.
    ---------+---------------+----------------------------------------------------
    1$ 2$ 3$ | all           | As a flag, specifies the one-based argument index
    ...      |               | for a positional format specifier.
             |               | Immediately after *, use the argument at the given
             |               | one-based index as the width or precision, instead
             |               | of the next argument. The entire format string must
             |               | also use positional format specifiers throughout.

Examples of flags:

Decimal number conversion:

    sprintf "%d", 123  # => "123"
    sprintf "%+d", 123 # => "+123"
    sprintf "% d", 123 # => " 123"

Octal number conversion:

    sprintf "%o", 123   # => "173"
    sprintf "%+o", 123  # => "+173"
    sprintf "%o", -123  # => "-173"
    sprintf "%+o", -123 # => "-173"

Hexadecimal number conversion:

    sprintf "%x", 123   # => "7b"
    sprintf "%+x", 123  # => "+7b"
    sprintf "%x", -123  # => "-7b"
    sprintf "%+x", -123 # => "-7b"
    sprintf "%#x", 0    # => "0"
    sprintf "% x", 123  # => " 7b"
    sprintf "% x", -123 # => "-7b"
    sprintf "%X", 123   # => "7B"
    sprintf "%#X", -123 # => "-0X7B"

Binary number conversion:

    sprintf "%b", 123    # => "1111011"
    sprintf "%+b", 123   # => "+1111011"
    sprintf "%+b", -123  # => "-1111011"
    sprintf "%b", -123   # => "-1111011"
    sprintf "%#b", 0     # => "0"
    sprintf "% b", 123   # => " 1111011"
    sprintf "%+ b", 123  # => "+1111011"
    sprintf "% b", -123  # => "-1111011"
    sprintf "%+ b", -123 # => "-1111011"
    sprintf "%#b", 123   # => "0b1111011"

Floating point conversion:

    sprintf "%a", 123 # => "0x1.ecp+6"
    sprintf "%A", 123 # => "0X1.ECP+6"

Exponential form conversion:

    sprintf "%g", 123.4          # => "123.4"
    sprintf "%g", 123.4567       # => "123.457"
    sprintf "%20.8g", 1234.56789 # => "           1234.5679"
    sprintf "%20.8g", 123456789  # => "       1.2345679e+08"
    sprintf "%20.8G", 123456789  # => "       1.2345679E+08"
    sprintf "%20.8g", -123456789 # => "      -1.2345679e+08"
    sprintf "%20.8G", -123456789 # => "      -1.2345679E+08"

The field width is an optional integer, followed optionally by a period and a precision. The width specifies the minimum number of characters that will be written to the result for this field.

Examples of width:

    sprintf "%20d", 123   # => "                 123"
    sprintf "%+20d", 123  # => "                +123"
    sprintf "%020d", 123  # => "00000000000000000123"
    sprintf "%+020d", 123 # => "+0000000000000000123"
    sprintf "% 020d", 123 # => " 0000000000000000123"
    sprintf "%-20d", 123  # => "123                 "
    sprintf "%-+20d", 123 # => "+123                "
    sprintf "%- 20d", 123 # => " 123                "
    sprintf "%020x", -123 # => "-000000000000000007b"
    sprintf "%020X", -123 # => "-000000000000000007B"

For numeric fields, the precision controls the number of decimal places displayed.

For string fields, the precision determines the maximum number of characters to be copied from the string.

Examples of precisions:

Precision for `d`, `o`, `x` and `b` is minimum number of digits:

    sprintf "%20.8d", 123   # => "            00000123"
    sprintf "%020.8d", 123  # => "            00000123"
    sprintf "%20.8o", 123   # => "            00000173"
    sprintf "%020.8o", 123  # => "            00000173"
    sprintf "%20.8x", 123   # => "            0000007b"
    sprintf "%020.8x", 123  # => "            0000007b"
    sprintf "%20.8b", 123   # => "            01111011"
    sprintf "%20.8d", -123  # => "           -00000123"
    sprintf "%020.8d", -123 # => "           -00000123"
    sprintf "%20.8o", -123  # => "           -00000173"
    sprintf "%20.8x", -123  # => "           -0000007b"
    sprintf "%20.8b", -11   # => "           -00001011"

Precision for `e` is number of digits after the decimal point:

    sprintf "%20.8e", 1234.56789 # => "      1.23456789e+03"

Precision for `f` is number of digits after the decimal point:

    sprintf "%20.8f", 1234.56789 # => "       1234.56789000"

Precision for `g` is number of significant digits:

    sprintf "%20.8g", 1234.56789 # => "           1234.5679"
    sprintf "%20.8g", 123456789  # => "       1.2345679e+08"
    sprintf "%-20.8g", 123456789 # => "1.2345679e+08       "

Precision for `s` is maximum number of characters:

    sprintf "%20.8s", "string test" # => "            string t"

Additional examples:

    sprintf "%d %04x", 123, 123                 # => "123 007b"
    sprintf "%08b '%4s'", 123, 123              # => "01111011 ' 123'"
    sprintf "%+g:% g:%-g", 1.23, 1.23, 1.23     # => "+1.23: 1.23:1.23"
    sprintf "%-3$*1$.*2$s", 10, 5, "abcdefghij" # => "abcde     "

def system(command : String, args = nil) : Bool #

Executes the given command in a subshell. Standard input, output and error are inherited. Returns `true` if the command gives zero exit code, `false` otherwise. The special `$?` variable is set to a `Process::Status` associated with this execution.

If command contains no spaces and args is given, it will become its argument list.

If command contains spaces and args is given, command must include `"${@}"` (including the quotes) to receive the argument list.

No shell interpretation is done in args.

Example:

    system("echo *")

Produces:

    LICENSE shard.yml Readme.md spec src

def timeout_select_action(timeout : Time::Span) : Channel::TimeoutAction #

Timeout keyword for use in `select`.

    select
    when x = ch.receive
      puts "got #{x}"
    when timeout(1.seconds)
      puts "timeout"
    end

NOTE It won't trigger if the `select` has an `else` case (i.e.: a non-blocking select).

def typeof(*expression) : Class #

Returns the type of an expression.

    typeof(1) # => Int32

It accepts multiple arguments, and the result is the union of the expression types:

    typeof(1, "a", 'a') # => (Int32 | String | Char)

The expressions passed as arguments to `typeof` do not evaluate. The compiler only analyzes their return type.

NOTE This is a pseudo-method provided directly by the Crystal compiler. It cannot be redefined nor overridden.

##
Macro Detail

macro debugger #

Invokes an execution trap to catch the attention of a debugger. This has the same effect as placing a breakpoint in debuggers or IDEs supporting them.

Execution is allowed to continue if the debugger instructs so. If no debugger is attached, usually the current process terminates with a status that corresponds to `Process::ExitReason::Breakpoint`.

Inside an interpreter session, this drops into the REPL's pry mode instead of a system debugger.

macro p!(*exps) #

Prints a series of expressions together with their inspected values. Useful for print style debugging.

    a = 1
    p! a # => "a # => 1"

    p! [1, 2, 3].map(&.to_s) # => "[1, 2, 3].map(&.to_s) # => ["1", "2", "3"]"

See also: `p`, `Object#inspect`.

macro pp!(*exps) #

Prints a series of expressions together with their pretty printed values. Useful for print style debugging.

    a = 1
    pp! a # => "a # => 1"

    pp! [1, 2, 3].map(&.to_s) # => "[1, 2, 3].map(&.to_s) # => ["1", "2", "3"]"

See also: `pp`, `Object#pretty_inspect`.

macro record(__name name, *properties, **kwargs) #

Defines a `Struct` type called name with the given properties.

The generated struct has a constructor with the given properties in the same order as declared. The struct only provides getters, not setters, making it immutable by default.

    record Point, x : Int32, y : Int32

    p = Point.new 1, 2 # => #<Point(@x=1, @y=2)>
    p.x                # => 1
    p.y                # => 2

The properties are a sequence of type declarations (`x : Int32`, `x : Int32 = 0`) or assigns (`x = 0`). They declare instance variables and respective getter methods of their name with optional type restrictions and default value.

When passing a block to this macro its body is inserted inside the struct definition. This allows to define additional methods or include modules into the record type (reopening the type would work as well).

    record Person, first_name : String, last_name : String do
      def full_name
        "#{first_name} #{last_name}"
      end
    end

    person = Person.new "John", "Doe"
    person.full_name # => "John Doe"

An example with type declarations and default values:

    record Point, x : Int32 = 0, y : Int32 = 0

    Point.new      # => #<Point(@x=0, @y=0)>
    Point.new y: 2 # => #<Point(@x=0, @y=2)>

An example with assignments (in this case the compiler must be able to infer the types from the default values):

    record Point, x = 0, y = 0

    Point.new      # => #<Point(@x=0, @y=0)>
    Point.new y: 2 # => #<Point(@x=0, @y=2)>

This macro also provides a `#copy_with` method which returns a copy of the record with the provided properties altered.

    record Point, x = 0, y = 0

    p = Point.new y: 2 # => #<Point(@x=0, @y=2)>
    p.copy_with x: 3   # => #<Point(@x=3, @y=2)>
    p                  # => #<Point(@x=0, @y=2)>

macro spawn(call, *, name = nil, same_thread = false, &block) #

Spawns a fiber by first creating a `Proc`, passing the call's expressions to it, and letting the `Proc` finally invoke the call.

When using execution contexts, the fiber spawns into the current execution context (`Fiber::ExecutionContext.current`).

Compare this:

    i = 0
    while i < 5
      spawn { print(i) }
      i += 1
    end
    Fiber.yield
    # Output: 55555

To this:

    i = 0
    while i < 5
      spawn print(i)
      i += 1
    end
    Fiber.yield
    # Output: 01234

This is because in the first case all spawned fibers refer to the same local variable, while in the second example copies of i are passed to a `Proc` that eventually invokes the call.