View Source IO (Elixir v1.15.0)
Functions handling input/output (IO).
Many functions in this module expect an IO device as an argument.
An IO device must be a PID or an atom representing a process.
For convenience, Elixir provides :stdio and :stderr as
shortcuts to Erlang's :standard_io and :standard_error.
The majority of the functions expect chardata. In case another type is given,
functions will convert those types to string via the String.Chars protocol
(as shown in typespecs). For more information on chardata, see the
"IO data" section below.
io-devices
IO devices
An IO device may be an atom or a PID. In case it is an atom, the atom must be the name of a registered process. In addition, Elixir provides two shortcuts:
:stdio- a shortcut for:standard_io, which maps to the currentProcess.group_leader/0in Erlang:stderr- a shortcut for the named process:standard_errorprovided in Erlang
IO devices maintain their position, which means subsequent calls to any
reading or writing functions will start from the place where the device
was last accessed. The position of files can be changed using the
:file.position/2 function.
io-data
IO data
IO data is a data type that can be used as a more efficient alternative to binaries in certain situations.
A term of type IO data is a binary or a list containing bytes (integers within the 0..255 range)
or nested IO data. The type is recursive. Let's see an example of one of
the possible IO data representing the binary "hello":
[?h, "el", ["l", [?o]]]The built-in iodata/0 type is defined in terms of iolist/0. An IO list is
the same as IO data but it doesn't allow for a binary at the top level (but binaries
are still allowed in the list itself).
use-cases-for-io-data
Use cases for IO data
IO data exists because often you need to do many append operations on smaller chunks of binaries in order to create a bigger binary. However, in Erlang and Elixir concatenating binaries will copy the concatenated binaries into a new binary.
def email(username, domain) do
username <> "@" <> domain
endIn this function, creating the email address will copy the username and domain
binaries. Now imagine you want to use the resulting email inside another binary:
def welcome_message(name, username, domain) do
"Welcome #{name}, your email is: #{email(username, domain)}"
end
IO.puts(welcome_message("Meg", "meg", "example.com"))
#=> "Welcome Meg, your email is: meg@example.com"Every time you concatenate binaries or use interpolation (#{}) you are making
copies of those binaries. However, in many cases you don't need the complete
binary while you create it, but only at the end to print it out or send it
somewhere. In such cases, you can construct the binary by creating IO data:
def email(username, domain) do
[username, ?@, domain]
end
def welcome_message(name, username, domain) do
["Welcome ", name, ", your email is: ", email(username, domain)]
end
IO.puts(welcome_message("Meg", "meg", "example.com"))
#=> "Welcome Meg, your email is: meg@example.com"Building IO data is cheaper than concatenating binaries. Concatenating multiple
pieces of IO data just means putting them together inside a list since IO data
can be arbitrarily nested, and that's a cheap and efficient operation. Most of
the IO-based APIs, such as :gen_tcp and IO, receive IO data and write it
to the socket directly without converting it to binary.
One drawback of IO data is that you can't do things like pattern match on the
first part of a piece of IO data like you can with a binary, because you usually
don't know the shape of the IO data. In those cases, you may need to convert it
to a binary by calling iodata_to_binary/1, which is reasonably efficient
since it's implemented natively in C. Other functionality, like computing the
length of IO data, can be computed directly on the iodata by calling iodata_length/1.
chardata
Chardata
Erlang and Elixir also have the idea of chardata/0. Chardata is very
similar to IO data: the only difference is that integers in IO data represent
bytes while integers in chardata represent Unicode code points. Bytes
(byte/0) are integers within the 0..255 range, while Unicode code points
(char/0) are integers within the 0..0x10FFFF range. The IO module provides
the chardata_to_string/1 function for chardata as the "counter-part" of the
iodata_to_binary/1 function for IO data.
If you try to use iodata_to_binary/1 on chardata, it will result in an
argument error. For example, let's try to put a code point that is not
representable with one byte, like ?π, inside IO data:
IO.iodata_to_binary(["The symbol for pi is: ", ?π])
#=> ** (ArgumentError) argument errorIf we use chardata instead, it will work as expected:
iex> IO.chardata_to_string(["The symbol for pi is: ", ?π])
"The symbol for pi is: π"Link to this section Summary
Functions
Reads from the IO device. The operation is Unicode unsafe.
Returns a raw, line-based IO.Stream on :stdio. The operation is Unicode unsafe.
Converts the IO device into an IO.Stream. The operation is Unicode unsafe.
Writes iodata to the given device.
Converts chardata into a string.
Gets a number of bytes from IO device :stdio.
Gets a number of bytes from the IO device.
Reads a line from the IO device.
Inspects and writes the given item to the device.
Inspects item according to the given options using the IO device.
Returns the size of an IO data.
Converts IO data into a binary
Writes item to the given device, similar to write/2,
but adds a newline at the end.
Reads from the IO device.
Converts the IO device into an IO.Stream.
Writes a message to stderr, along with the current stacktrace.
Writes a message to stderr, along with the given stacktrace_info.
Writes chardata to the given device.
Link to this section Types
Link to this section Functions
@spec binread(device(), :eof | :line | non_neg_integer()) :: iodata() | nodata()
Reads from the IO device. The operation is Unicode unsafe.
The device is iterated as specified by the line_or_chars argument:
if
line_or_charsis an integer, it represents a number of bytes. The device is iterated by that number of bytes.if
line_or_charsis:line, the device is iterated line by line.if
line_or_charsis:eof, the device is iterated until:eof.line_or_charscan only be:eofsince Elixir 1.13.0.:eofreplaces the deprecated:all, with the difference that:allreturns""on end of file, while:eofreturns:eofitself.
It returns:
data- the output bytes:eof- end of file was encountered{:error, reason}- other (rare) error condition; for instance,{:error, :estale}if reading from an NFS volume
Note: do not use this function on IO devices in Unicode mode as it will return the wrong result.
Returns a raw, line-based IO.Stream on :stdio. The operation is Unicode unsafe.
This is equivalent to:
IO.binstream(:stdio, :line)@spec binstream(device(), :line | pos_integer()) :: Enumerable.t()
Converts the IO device into an IO.Stream. The operation is Unicode unsafe.
An IO.Stream implements both Enumerable and
Collectable, allowing it to be used for both read
and write.
The device is iterated by the given number of bytes or line by line if
:line is given. This reads from the IO device as a raw binary.
Note that an IO stream has side effects and every time you go over the stream you may get different results.
Finally, do not use this function on IO devices in Unicode mode as it will return the wrong result.
binstream/0 has been introduced in Elixir v1.12.0,
while binstream/2 has been available since v1.0.0.
Writes iodata to the given device.
This operation is meant to be used with "raw" devices
that are started without an encoding. The given iodata
is written as is to the device, without conversion. For
more information on IO data, see the "IO data" section in
the module documentation.
Use write/2 for devices with encoding.
Important: do not use this function on IO devices in Unicode mode as it will write the wrong data. In particular, the standard IO device is set to Unicode by default, so writing to stdio with this function will likely result in the wrong data being sent down the wire.
Converts chardata into a string.
For more information about chardata, see the "Chardata" section in the module documentation.
In case the conversion fails, it raises an UnicodeConversionError.
If a string is given, it returns the string itself.
examples
Examples
iex> IO.chardata_to_string([0x00E6, 0x00DF])
"æß"
iex> IO.chardata_to_string([0x0061, "bc"])
"abc"
iex> IO.chardata_to_string("string")
"string"@spec getn( device() | chardata() | String.Chars.t(), pos_integer() | :eof | chardata() | String.Chars.t() ) :: chardata() | nodata()
Gets a number of bytes from IO device :stdio.
If :stdio is a Unicode device, count implies
the number of Unicode code points to be retrieved.
Otherwise, count is the number of raw bytes to be retrieved.
See IO.getn/3 for a description of return values.
@spec getn(device(), chardata() | String.Chars.t(), pos_integer() | :eof) :: chardata() | nodata()
Gets a number of bytes from the IO device.
If the IO device is a Unicode device, count implies
the number of Unicode code points to be retrieved.
Otherwise, count is the number of raw bytes to be retrieved.
It returns:
data- the input characters:eof- end of file was encountered{:error, reason}- other (rare) error condition; for instance,{:error, :estale}if reading from an NFS volume
@spec gets(device(), chardata() | String.Chars.t()) :: chardata() | nodata()
Reads a line from the IO device.
It returns:
data- the characters in the line terminated by a line-feed (LF) or end of file (EOF):eof- end of file was encountered{:error, reason}- other (rare) error condition; for instance,{:error, :estale}if reading from an NFS volume
examples
Examples
To display "What is your name?" as a prompt and await user input:
IO.gets("What is your name?\n")@spec inspect( item, keyword() ) :: item when item: var
Inspects and writes the given item to the device.
It's important to note that it returns the given item unchanged.
This makes it possible to "spy" on values by inserting an
IO.inspect/2 call almost anywhere in your code, for example,
in the middle of a pipeline.
It enables pretty printing by default with width of
80 characters. The width can be changed by explicitly
passing the :width option.
The output can be decorated with a label, by providing the :label
option to easily distinguish it from other IO.inspect/2 calls.
The label will be printed before the inspected item.
See Inspect.Opts for a full list of remaining formatting options.
examples
Examples
IO.inspect(<<0, 1, 2>>, width: 40)Prints:
<<0, 1, 2>>We can use the :label option to decorate the output:
IO.inspect(1..100, label: "a wonderful range")Prints:
a wonderful range: 1..100The :label option is especially useful with pipelines:
[1, 2, 3]
|> IO.inspect(label: "before")
|> Enum.map(&(&1 * 2))
|> IO.inspect(label: "after")
|> Enum.sum()Prints:
before: [1, 2, 3]
after: [2, 4, 6]Inspects item according to the given options using the IO device.
See inspect/2 for a full list of options.
@spec iodata_length(iodata()) :: non_neg_integer()
Returns the size of an IO data.
For more information about IO data, see the "IO data" section in the module documentation.
Inlined by the compiler.
examples
Examples
iex> IO.iodata_length([1, 2 | <<3, 4>>])
4Converts IO data into a binary
The operation is Unicode unsafe.
Note that this function treats integers in the given IO data as
raw bytes and does not perform any kind of encoding conversion.
If you want to convert from a charlist to a UTF-8-encoded string,
use chardata_to_string/1 instead. For more information about
IO data and chardata, see the "IO data" section in the
module documentation.
If this function receives a binary, the same binary is returned.
Inlined by the compiler.
examples
Examples
iex> bin1 = <<1, 2, 3>>
iex> bin2 = <<4, 5>>
iex> bin3 = <<6>>
iex> IO.iodata_to_binary([bin1, 1, [2, 3, bin2], 4 | bin3])
<<1, 2, 3, 1, 2, 3, 4, 5, 4, 6>>
iex> bin = <<1, 2, 3>>
iex> IO.iodata_to_binary(bin)
<<1, 2, 3>>@spec puts(device(), chardata() | String.Chars.t()) :: :ok
Writes item to the given device, similar to write/2,
but adds a newline at the end.
By default, the device is the standard output. It returns :ok
if it succeeds.
examples
Examples
IO.puts("Hello World!")
#=> Hello World!
IO.puts(:stderr, "error")
#=> error@spec read(device(), :eof | :line | non_neg_integer()) :: chardata() | nodata()
Reads from the IO device.
The device is iterated by the given number of characters, line by line if
:line is given, or until :eof.
It returns:
data- the output characters:eof- end of file was encountered{:error, reason}- other (rare) error condition; for instance,{:error, :estale}if reading from an NFS volume
Returns a line-based IO.Stream on :stdio.
This is equivalent to:
IO.stream(:stdio, :line)@spec stream(device(), :line | pos_integer()) :: Enumerable.t()
Converts the IO device into an IO.Stream.
An IO.Stream implements both Enumerable and
Collectable, allowing it to be used for both read
and write.
The device is iterated by the given number of characters or line by line if
:line is given.
This reads from the IO as UTF-8. Check out
IO.binstream/2 to handle the IO as a raw binary.
Note that an IO stream has side effects and every time you go over the stream you may get different results.
stream/0 has been introduced in Elixir v1.12.0,
while stream/2 has been available since v1.0.0.
examples
Examples
Here is an example on how we mimic an echo server from the command line:
Enum.each(IO.stream(:stdio, :line), &IO.write(&1))Another example where you might want to collect a user input
every new line and break on an empty line, followed by removing
redundant new line characters (" "):
IO.stream(:stdio, :line)
|> Enum.take_while(&(&1 != ""))
|> Enum.map(&String.replace(&1, "", ""))
@spec warn(chardata() | String.Chars.t()) :: :ok
Writes a message to stderr, along with the current stacktrace.
It returns :ok if it succeeds.
Do not call this function at the tail of another function. Due to tail
call optimization, a stacktrace entry would not be added and the
stacktrace would be incorrectly trimmed. Therefore make sure at least
one expression (or an atom such as :ok) follows the IO.warn/1 call.
examples
Examples
IO.warn("variable bar is unused")
#=> warning: variable bar is unused
#=> (iex) evaluator.ex:108: IEx.Evaluator.eval/4@spec warn( chardata() | String.Chars.t(), Exception.stacktrace() | keyword() | Macro.Env.t() ) :: :ok
Writes a message to stderr, along with the given stacktrace_info.
The stacktrace_info must be one of:
a
__STACKTRACE__, where all entries in the stacktrace will be included in the error messagea
Macro.Envstructure (since v1.14.0), where a single stacktrace entry from the compilation environment will be useda keyword list with at least the
:fileoption representing a single stacktrace entry (since v1.14.0). The:line,:module,:functionoptions are also supported
This function also notifies the compiler a warning was printed
(in case --warnings-as-errors was enabled). It returns :ok
if it succeeds.
examples
Examples
stacktrace = [{MyApp, :main, 1, [file: 'my_app.ex', line: 4]}]
IO.warn("variable bar is unused", stacktrace)
#=> warning: variable bar is unused
#=> my_app.ex:4: MyApp.main/1@spec write(device(), chardata() | String.Chars.t()) :: :ok
Writes chardata to the given device.
By default, the device is the standard output.
examples
Examples
IO.write("sample")
#=> sample
IO.write(:stderr, "error")
#=> error