********************************
Layout of a Solidity Source File
********************************

Source files can contain an arbitrary number of contract definitions and include directives.

.. index:: source file, ! import

.. _import:

Importing other Source Files
============================

Syntax and Semantics
--------------------

Solidity supports import statements that are very similar to those available in JavaScript
(from ES6 on), although Solidity does not know the concept of a "default export".

At a global level, you can use import statements of the following form:

::

  import "filename";

...will import all global symbols from "filename" (and symbols imported there) into the
current global scope (different than in ES6 but backwards-compatible for Solidity).

::

  import * as symbolName from "filename";

...creates a new global symbol ``symbolName`` whose members are all the global symbols from ``"filename"``.

::

  import {symbol1 as alias, symbol2} from "filename";

...creates new global symbols ``alias`` and ``symbol2`` which reference ``symbol1`` and ``symbol2`` from ``"filename"``, respectively.

Another syntax is not part of ES6, but probably convenient:

::

  import "filename" as symbolName;

...is equivalent to ``import * as symbolName from "filename";``.

Paths
-----

In the above, ``filename`` is always treated as a path with ``/`` as directory separator,
``.`` as the current and ``..`` as the parent directory. Path names that do not start
with ``.`` are treated as absolute paths.

To import a file ``x`` from the same directory as the current file, use ``import "./x" as x;``.
If you use ``import "x" as x;`` instead, a different file could be referenced
(in a global "include directory").

It depends on the compiler (see below) how to actually resolve the paths.
In general, the directory hierarchy does not need to strictly map onto your local
filesystem, it can also map to resources discovered via e.g. ipfs, http or git.

Use in actual Compilers
-----------------------

When the compiler is invoked, it is not only possible to specify how to
discover the first element of a path, but it is possible to specify path prefix
remappings so that e.g. ``github.com/ethereum/dapp-bin/library`` is remapped to
``/usr/local/dapp-bin/library`` and the compiler will read the files from there. If
remapping keys are prefixes of each other, the longest is tried first. This
allows for a "fallback-remapping" with e.g. ``""`` maps to
``"/usr/local/include/solidity"``. Furthermore, these remappings can
depend on the context, which allows you to configure packages to
import e.g. different versions of a library of the same name.

**solc**:

For solc (the commandline compiler), these remappings are provided as
``context:prefix=target`` arguments, where both the ``context:`` and the
``=target`` parts are optional (where target defaults to prefix in that
case). All remapping values that are regular files are compiled (including
their dependencies). This mechanism is completely backwards-compatible (as long
as no filename contains = or :) and thus not a breaking change. All imports
in files in or below the directory ``context`` that import a file that
starts with ``prefix`` are redirected by replacing ``prefix`` by ``target``.

So as an example, if you clone
``github.com/ethereum/dapp-bin/`` locally to ``/usr/local/dapp-bin``, you can use
the following in your source file:

::

  import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol" as it_mapping;

and then run the compiler as

.. code-block:: bash

  solc github.com/ethereum/dapp-bin/=/usr/local/dapp-bin/ source.sol

As a more complex example, suppose you rely on some module that uses a
very old version of dapp-bin. That old version of dapp-bin is checked
out at ``/usr/local/dapp-bin_old``, then you can use 

.. code-block:: bash

  solc module1:github.com/ethereum/dapp-bin/=/usr/local/dapp-bin/ \
       module2:github.com/ethereum/dapp-bin/=/usr/local/dapp-bin_old/ \
       source.sol

so that all imports in ``module2`` point to the old version but imports
in ``module1`` get the new version.

Note that solc only allows you to include files from certain directories:
They have to be in the directory (or subdirectory) of one of the explicitly
specified source files or in the directory (or subdirectory) of a remapping
target. If you want to allow direct absolute includes, just add the
remapping ``=/``.

If there are multiple remappings that lead to a valid file, the remapping
with the longest common prefix is chosen.

**browser-solidity**:

The `browser-based compiler <https://ethereum.github.io/browser-solidity>`_
provides an automatic remapping for github and will also automatically retrieve
the file over the network:
You can import the iterable mapping by e.g.
``import "github.com/ethereum/dapp-bin/library/iterable_mapping.sol" as it_mapping;``.

Other source code providers may be added in the future.


.. index:: ! comment, natspec

Comments
========

Single-line comments (``//``) and multi-line comments (``/*...*/``) are possible.

::

  // This is a single-line comment.

  /*
  This is a
  multi-line comment.
  */


Additionally, there is another type of comment called a natspec comment,
for which the documentation is not yet written. They are written with a
triple slash (``///``) or a double asterisk block(``/** ... */``) and
they should be used directly above function declarations or statements.
You can use Doxygen-style tags inside these comments to document
functions, annotate conditions for formal verification, and provide a
**confirmation text** which is shown to users when they attempt to invoke a
function.