496 lines
13 KiB
Elixir
496 lines
13 KiB
Elixir
defmodule Elixir.SpecialForms do
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@moduledoc """
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In this module we define Elixir special forms. Those are called
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special forms because they cannot be overridden by the developer
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and sometimes have lexical scope (like refer, require, import, etc).
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"""
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@doc """
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Defines a new tuple.
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## Examples
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:{}.(1,2,3)
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{ 1, 2, 3 }
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"""
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defmacro :{}.(args)
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@doc """
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Defines a new list.
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## Examples
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:[].(1,2,3)
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[ 1, 2, 3 ]
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"""
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defmacro :[].(args)
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@doc """
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Defines a new bitstring.
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## Examples
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:<<>>.(1,2,3)
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<< 1, 2, 3 >>
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"""
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defmacro :<<>>.(args)
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@doc """
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`refer` is used to setup aliases between modules.
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## Examples
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`refer` can be used to setup an alias for any module:
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defmodule Math do
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refer MyKeyword, as: Keyword
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end
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In the example above, we have set up `MyOrdict` to be referenced
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as `Keyword`. So now, any reference to `Keyword` will be
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automatically replaced by `MyKeyword`.
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In case one wants to access the original `Keyword`, it can be done
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by accessing __MAIN__:
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Keyword.values #=> uses MyKeyword.values
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__MAIN__.Keyword.values #=> uses Keyword.values
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Notice that calling `refer` without the `as:` option automatically
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sets an alias based on the last part of the module. For example:
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refer Foo.Bar.Baz
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Is the same as:
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refer Foo.Bar.Baz, as: Baz
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## Lexical scope
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`import`, `require` and `refer` are called directives and all
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have lexical scope. This means you can set up aliases inside
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specific functions and it won't affect the overall scope.
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"""
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defmacro refer(module, opts)
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@doc """
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`require` is used to require the presence of external
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modules so macros can be invoked.
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## Examples
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Notice that usually modules should not be required before usage,
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the only exception is if you want to use the macros from a module.
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In such cases, you need to explicitly require them.
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Let's suppose you created your own `if` implementation in the module
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`MyMacros`. If you want to invoke it, you need to first explicitly
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require the `MyMacros`:
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defmodule Math do
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require MyMacros
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MyMacros.if do_something, it_works
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end
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An attempt to call a macro that was not loaded will raise an error.
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## Refer shortcut
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`require` also accepts `as:` as an option so it automatically sets
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up an alias. Please check `refer` for more information.
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"""
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defmacro require(module, opts)
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@doc """
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`import` allows one to easily access functions or macros from
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others modules without using the qualified name.
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## Examples
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If you want to use the `values` function from `Keyword` several times
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in your module and you don't want to always type `Keyword.values`,
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you can simply import it:
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defmodule Math do
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import Keyword, only: [values: 1]
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def some_function do
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# call values(orddict)
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end
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end
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In this case, we are importing only the function `values` (with arity 1)
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from `Keyword`. Although `only` is optional, its usage is recommended.
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`except` could also be given as an option. If no option is given, all
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functions and macros are imported.
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In case you want to import only functions or macros, you can pass a
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first argument selecting the scope:
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import :macros, MyMacros
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And you can then use `only` or `except` to filter the macros being
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included.
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## Lexical scope
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It is important to notice that `import` is lexical. This means you
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can import specific macros inside specific functions:
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defmodule Math do
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def some_function do
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# 1) Disable `if/2` from Elixir.Builtin
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import Elixir.Builtin, except: [if: 2]
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# 2) Require the new `if` macro from MyMacros
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import MyMacros
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# 3) Use the new macro
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if do_something, it_works
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end
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end
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In the example above, we imported macros from `MyMacros`, replacing
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the original `if/2` implementation by our own during that
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specific function. All other functions in that module will still
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be able to use the original one.
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## Refer/Require shortcut
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All imported modules are also required by default. `import`
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also accepts `as:` as an option so it automatically sets up
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an alias. Please check `refer` for more information.
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"""
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defmacro import(module, opts)
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@doc """
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Returns the current module name as an atom or nil otherwise.
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"""
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defmacro __MODULE__
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@doc """
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Returns the current file name as a binary.
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"""
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defmacro __FILE__
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@doc """
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Returns the current line number as an integer.
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"""
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defmacro __LINE__
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@doc """
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Returns the current function as a tuple,
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where the first element is the name as an atom
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and the second is the arity as an integer.
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"""
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defmacro __FUNCTION__
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@doc """
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Allows you to get the representation of any expression.
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## Examples
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quote do: sum(1, 2, 3)
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#=> { :sum, 0, [1, 2, 3] }
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## Homoiconicity
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Elixir is an homoiconic language. Any Elixir program can be
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represented using its own data structures. The building block
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of Elixir homoiconicity is a tuple with three elements, for example:
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{ :sum, 1, [1, 2, 3] }
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The tuple above represents a function call to sum passing 1, 2 and
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3 as arguments. The tuple elements are:
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* The first element of the tuple is always an atom or
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another tuple in the same representation;
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* The second element of the tuple is always an integer
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representing the line number;
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* The third element of the tuple are the arguments for the
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function call. The third argument may be an atom, meaning
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that it may be a variable.
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## Macro literals
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Besides the tuple described above, Elixir has a few literals that
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when quoted return themselves. They are:
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:sum #=> Atoms
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1 #=> Integers
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2.0 #=> Floats
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[1,2] #=> Lists
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"binaries" #=> Binaries
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{key, value} #=> Tuple with two elements
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## Hygiene
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Elixir macros are hygienic regarding to variables. This means
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a variable defined in a macro cannot affect the scope where
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the macro is included. Consider the following example:
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defmodule Hygiene do
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defmacro no_interference do
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quote do: a = 1
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end
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end
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require Hygiene
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a = 10
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Hygiene.no_interference
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a #=> 10
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In the example above, `a` returns 10 even if the macro
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is apparently setting it to 1 because the variables defined
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in the macro does not affect the context the macro is
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executed. If you want to set or get a variable, you can do
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it with the help of the `var!` macro:
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defmodule NoHygiene do
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defmacro interference do
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quote do: var!(a) = 1
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end
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end
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require NoHygiene
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a = 10
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NoHygiene.interference
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a #=> 11
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Notice that references are not hygienic in Elixir unless
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you explicitly access it via __MAIN__ to the reference name.
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quote do
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__MAIN__.Foo #=> Access the root Foo
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Foo #=> Access the Foo reference in the current
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module (if any is set), then fallback to root
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end
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## Options
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`quote` also accepts some options as arguments. For example,
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hygiene can be turned off via `hygiene: false` which is useful
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when one is generating a code that should be inserted into
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some function.
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"""
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defmacro quote(opts, do: contents)
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@doc """
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Unquotes the given expression from inside a macro.
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## Examples
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Imagine the situation you have a variable `name` and
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you want to inject it inside some quote. The first attempt
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would be:
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value = 13
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quote do: sum(1, value, 3)
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Which would then return:
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{ :sum, 0, [1, { :value, 0, quoted }, 3] }
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Which is not the expected result. For this, we use unquote:
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value = 13
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quote do: sum(1, unquote(value), 3)
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#=> { :sum, 0, [1, 13, 3] }
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"""
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defmacro unquote(expr)
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@doc """
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Unquotes the given list expanding its arguments. Similar
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to unquote.
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## Examples
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values = [2,3,4]
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quote do: sum(1, unquote_splicing(values), 5)
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#=> { :sum, 0, [1, 2, 3, 4, 5] }
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"""
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defmacro unquote_splicing(expr)
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@doc """
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Returns an anonymous function based on the given arguments.
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## Examples
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sum = fn(x, y, do: x + y)
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sum.(1, 2) #=> 3
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Notice that a function needs to be invoked using the dot between
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the function and the arguments.
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A function could also be defined using the `end` syntax, although
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it is recommend to use it only with the stab operator in order to
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avoid ambiguity. For example, consider this case:
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Enum.map [1,2,3], fn x ->
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x * 2
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end
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The example works fine because `->` binds to the closest function call,
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which is `fn`, but if we replace it by `do/end`, it will fail:
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Enum.map [1,2,3], fn(x) do
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x * 2
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end
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The reason it fails is because do/end always bind to the farthest
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function call.
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## Function with multiple clauses
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One may define a function which expects different clauses as long
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as all clauses expects the same number of arguments:
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fun = fn do:
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[x, y] when y < 0 ->
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x - y
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[x, y] ->
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x + y
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end
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fun.(10, -10) #=> 20
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fun.(10, 10) #=> 20
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"""
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defmacro fn(args)
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@doc """
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Handle annonymous recursive loops.
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## Examples
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list = [1,2,3]
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loop list, [] do
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[[h|t], acc] ->
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recur t, [h*2|acc]
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[[], acc] ->
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acc
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end
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#=> [6,4,2]
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Notice that all match clauses expects the same ammount
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of arguments. Guards can also be given.
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Recursion happens by calling recur with the same number
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of arguments of each match clause. `recur` does not guarantee
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that it will be tail recursive.
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"""
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defmacro loop(args)
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@doc """
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A function that forces the current loop to recur. See `loop/1`
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for more information.
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"""
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defmacro recur(args)
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@doc """
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List comprehensions allow you to quickly build a list from another list:
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lc n in [1,2,3,4], do: n * 2
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#=> [2,4,6,8]
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A comprehension accepts many generators and also filters. Filters must be given after the when clause:
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# A comprehension with a generator and a filter
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lc n in [1,2,3,4,5,6] when rem(n, 2) == 0, do: n
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#=> [2,4,6]
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# A comprehension with two generators
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lc x in [1,2], y in [2,3], do: x*y
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#=> [2,3,4,6]
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Elixir provides generators for both lists and bitstrings:
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# A list generator:
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lc n in [1,2,3,4], do: n * 2
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#=> [2,4,6,8]
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# A bit string generator:
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lc <<n>> in <<1,2,3,4>>, do: n * 2
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#=> [2,4,6,8]
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Bit string generators are quite useful when you need to organize bit string streams:
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iex> pixels = <<213,45,132,64,76,32,76,0,0,234,32,15>>
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iex> lc <<r:8,g:8,b:8>> in pixels, do: {r,g,b}
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[{213,45,132},{64,76,32},{76,0,0},{234,32,15}]
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Elixir does its best to hide the differences between list and bit string generators.
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However, there is a special case due to Erlang limitation where we need to explicitly
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tell Erlang that a list is being given as argument:
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# This will fail because when Elixir sees that the left side
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# of the in expression is a bit string, it expects the right side
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# to be a bit string as well:
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lc <<n>> in [<<1>>,<<2>>,<<3>>], do: n*2
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#=> ** (ErlangError) erlang error {:bad_generator,[<<1>>,<<2>>,<<3>>]}
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# You need to be explicit and use inlist:
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lc inlist(<<n>>, [<<1>>,<<2>>,<<3>>]), do: n*2
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#=> [2,4,6]
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# For consistency, inbin is also available:
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lc inbin(<<n>>, <<1,2,3>>), do: n*2
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#=> [2,4,6]
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Notice that although comprehensions uses `when` to specify filters, filters are not
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guards and therefore accept any expression (they are not limited as guards).
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"""
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defmacro lc(args)
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@doc """
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Defines a bit comprehension. It follows the same syntax as
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a list comprehension but expects each element returned to
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be a bitstring. For example, here is how to remove all
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spaces from a string:
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bc <<c>> in " hello world " when c != ?\s, do: <<c>>
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"helloworld"
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"""
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defmacro bc(args)
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@doc """
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Keeps one of the given expressions depending in the context
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of evaluation is a guard or not. This is useful when creating
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macro that should work both inside and outside guards but
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still hold some characteristics.
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## Example
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A good example is the `is_exception/1` macro defined in Elixir:
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defmacro is_exception(thing) do
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quote do
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quote do
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is_tuple(unquote(thing)) and elem(unquote(thing), 2) == :__exception__
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else
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result = unquote(thing)
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is_tuple(result) and elem(result, 2) == :__exception__
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end
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end
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end
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Notice that if inside a guard, we unquote the same element twice.
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This will cause the same element to be evaluted twice, but this is
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fine for guards since we cannot assign variables in guards and
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we cannot call expressions inside guards. However, when outside
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of a guard, evaluating the arguments twice can be harmful and
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unexpected, for this reason, we save the result in a variable.
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In the example above, `in_guard` is allowing us to customize
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the same macro to work inside and outside guards.
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"""
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defmacro in_guard(do: do_block, else: else_block)
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end |