`IO.puts "Hello world!"

接触elixir时, 学会的第一行代码是 IO.puts "Hello world!". 出于好奇, 我观察了一下 IO.puts 函数的实现.

IO

我们先到elixir源文件中的IO模块里看看.

IO 模块中 IO.puts 的定义

  @doc """Writes `item` to the given `device`, similar to `write/2`,but adds a newline at the end."""@spec puts(device, chardata | String.Chars.t) :: :okdef puts(device \\ :stdio, item) do:io.put_chars map_dev(device), [to_chardata(item), ?\n]end

首先看它的类型, 这里我们需要先了解一点, 在erlang和elixir中, 字符串的类型是不同的.
比如 erlang 中 [97] 等同于 "a", 而 elixir 中 <<97>> 等同于 "a".

类型 device, chardata 的定义

  @type device :: atom | pid@type chardata() :: :unicode.chardata()

定义内联函数 map_dev/1, to_chardata/1.

内联函数的作用是在编译时, 调用内联函数的地方会被替换成该函数的函数体.

  @compile {:inline, map_dev: 1, to_chardata: 1}# Map the Elixir names for standard IO and error to Erlang namesdefp map_dev(:stdio),  do: :standard_iodefp map_dev(:stderr), do: :standard_errordefp map_dev(other) when is_atom(other) or is_pid(other) or is_tuple(other), do: otherdefp to_chardata(list) when is_list(list), do: listdefp to_chardata(other), do: to_string(other)

Kernel

Kernel 模块中 to_string/1 函数的定义

  @doc """Converts the argument to a string according to the`String.Chars` protocol.This is the function invoked when there is string interpolation.## Examplesiex> to_string(:foo)"foo""""defmacro to_string(arg) doquote do: String.Chars.to_string(unquote(arg))end

String.Chars protocol 对不同类型的参数的实现

import Kernel, except: [to_string: 1]defprotocol String.Chars do@moduledoc ~S"""The `String.Chars` protocol is responsible forconverting a structure to a binary (only if applicable).The only function required to be implemented is`to_string` which does the conversion.The `to_string/1` function automatically importedby `Kernel` invokes this protocol. Stringinterpolation also invokes `to_string` in itsarguments. For example, `"foo#{bar}"` is the sameas `"foo" <> to_string(bar)`."""def to_string(term)
enddefimpl String.Chars, for: Atom dodef to_string(nil) do""enddef to_string(atom) doAtom.to_string(atom)end
enddefimpl String.Chars, for: BitString dodef to_string(term) when is_binary(term) dotermenddef to_string(term) doraise Protocol.UndefinedError,protocol: @protocol,value: term,description: "cannot convert a bitstring to a string"end
enddefimpl String.Chars, for: List dodef to_string(charlist), do: List.to_string(charlist)
enddefimpl String.Chars, for: Integer dodef to_string(term) doInteger.to_string(term)end
enddefimpl String.Chars, for: Float dodef to_string(term) doIO.iodata_to_binary(:io_lib_format.fwrite_g(term))end
end

注意这里使用了一个神奇的 erlang 函数 -- :io_lib_format.fwrite_g 用来转换 Float, 我看了一下 erlang 的源代码, 发现了一段注释

%%  Writes the shortest, correctly rounded string that converts
%%  to Float when read back with list_to_float/1.
%%
%%  See also "Printing Floating-Point Numbers Quickly and Accurately"
%%  in Proceedings of the SIGPLAN '96 Conference on Programming
%%  Language Design and Implementation.

具体的实现挺复杂的, 有兴趣的朋友可以去看看 https://github.com/erlang/otp...

IO.iodata_to_binary/1 的定义

  @doc """Converts iodata (a list of integers representing bytes, listsand binaries) into a binary.The operation is Unicode unsafe.Notice that this function treats lists of integers as raw bytesand does not perform any kind of encoding conversion. If you wantto convert from a charlist to a string (UTF-8 encoded), pleaseuse `chardata_to_string/1` instead.If this function receives a binary, the same binary is returned.Inlined by the compiler.## Examplesiex> 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 iodata_to_binary(iodata) :: binarydef iodata_to_binary(item) do:erlang.iolist_to_binary(item)end

总结

• 内联函数的作用是在编译时, 调用内联函数的地方会被替换成该函数的函数体, 适用于短小的函数. 定义的内联函数的方法是使用模块属性 @compile {:inline, fun_a: 1, fun_b: 1}.

• 使用 defprotocal 来定义协议. 使用 defimpl .., for: .. 来为某种数据类型实现协议.

• quote do:中间没有逗号