Create "Getting started" tutorial.

[ci skip]

Cabal/doc/getting-started.rst

@@ -0,0 +1,159 @@
+Getting Started with Haskell and Cabal
+======================================
+
+Installing the Haskell toolchain
+--------------------------------
+
+To install the Haskell toolchain follow the `ghcup instructions
+<https://www.haskell.org/ghcup/>`__.
+
+
+Creating a new application
+--------------------------
+
+Let's start by creating a simple Haskell application from scratch where we'll
+learn about a Haskell package's directory structure, how to run the executable,
+and how to add external dependencies.
+
+
+Initializing the application
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Start by creating a ``myfirstapp`` directory to hold the project files:
+
+.. code-block:: console
+
+    $ mkdir myfirstapp && cd myfirstapp
+
+Once you have an empty directory we can initialize our package:
+
+.. code-block:: console
+
+    $ cabal init
+
+This will generate the following files:
+
+.. code-block:: console
+
+    $ ls
+    CHANGELOG.md
+    Main.hs
+    myfirstapp.cabal
+
+
+``Main.hs`` is where your package's code lives. By default ``cabal init``
+creates an executable with the same name as the package ``myfirstapp`` in this
+case, you can instruct ``cabal init`` to generate just a library (with
+``--lib``) or both a library and executable with (``--libandexe``); for the full
+set of options see ``cabal init --help``.
+
+``myfirstapp.cabal`` is Cabal's metadata file which describes your package and
+its dependencies. We'll be updating this file in a little bit when we add an
+external dependency to our package.
+
+.. note:: By default the name of the current directory will be used for the
+	  package name, you can change this with ``-p <package-name>``.
+
+
+Running the application
+^^^^^^^^^^^^^^^^^^^^^^^
+
+As mentioned above, ``cabal init`` with no arguments generates a package with a
+single executable that prints ``"Hello, Haskell!"`` to the terminal. To run the
+executable enter the following command:
+
+``cabal run :myfirstapp``
+
+You should see the following output in the terminal:
+
+.. code-block:: console
+
+     $ cabal run :myfirstapp
+     ...
+     Hello, Haskell!
+
+.. note:: The ``:`` prefix in ``:myfirstapp`` signifies that the the
+	  ``myfirstapp`` target is part of the current package.
+
+Notice that we didn't need to run a `build` command before ``cabal run``, this
+is because ``cabal run`` first determines if the code needs to be re-built
+before running the executable. If you just want to build a target you can do so
+with ``cabal build``:
+
+``cabal build :myfirstapp``
+
+
+Adding dependencies
+^^^^^^^^^^^^^^^^^^^
+
+Next we'll add an external dependency to our application. The unit of
+shareability in the Haskell ecosystem is a `package`, the database of available
+Haskell packages is called `Hackage <https://hackage.haskell.org/>`__.
+
+In our application, we'll use a package called `haskell-say
+<https://hackage.haskell.org/package/haskell-say>`__ to print text to the
+terminal with some embellishment.
+
+.. TIP::
+   If you installed ``cabal`` a while ago but haven't used it recently you may
+   need to update the package index, you can do this by running ``cabal
+   update``.
+
+In our ``myfirstapp.cabal`` file we'll update the ``build-depends`` attribute of
+the ``executable myfirstapp`` section to include ``haskell-say``:
+
+.. code-block:: cabal
+
+   executable myfirstapp
+       main-is: Main.hs
+       build-depends:
+           base >=4.11 && <4.12,
+           haskell-say ^>=1.0.0.0
+
+.. NOTE::
+   ``^>=1.0.0.0`` means use version 1.0.0.0 of the library or any more recent
+   minor release with the same major version.
+
+Next we'll update ``Main.hs`` to use the ``HaskellSay`` library:
+
+.. code-block:: haskell
+
+   module Main where
+
+   import HaskellSay (haskellSay)
+
+   main :: IO ()
+   main =
+     haskellSay "Hello, Haskell! You're using a function from another package!"
+
+``import HaskellSay (haskellSay)`` brings the ``haskellSay`` function from the
+module named ``HaskellSay`` into scope. The ``HaskellSay`` module is defined in
+the ``haskell-say`` packages that we added a dependency on above.
+
+Now you can build and re-run your code to see the new output:
+
+.. code-block:: console
+
+   $ cabal run
+       ________________________________________________________
+      /                                                        \
+     | Hello, Haskell! You're using a function from another     |
+     | package!                                                 |
+      \____       _____________________________________________/
+           \    /
+            \  /
+             \/
+       _____   _____
+       \    \  \    \
+        \    \  \    \
+         \    \  \    \
+          \    \  \    \  \-----------|
+           \    \  \    \  \          |
+            \    \  \    \  \---------|
+            /    /  /     \
+           /    /  /       \  \-------|
+          /    /  /    ^    \  \      |
+         /    /  /    / \    \  \ ----|
+        /    /  /    /   \    \
+       /____/  /____/     \____\
+

Cabal/doc/index.rst

@@ -6,6 +6,7 @@ Welcome to the Cabal User Guide
    :maxdepth: 2
    :numbered:
 
+   getting-started
    intro
    config-and-install
    concepts-and-development