.. _sect-starting:

***************
Getting Started
***************

Prerequisites
=============

You need a C compiler (default is clang), and optionally Idris 1.3.2 to build
from source. You will also need:

- `Chez Scheme <https://cisco.github.io/ChezScheme/>`_
- The `GNU Multiple Precision Arithmetic Library <https://gmplib.org/>`_ (GMP) 

Both are available from MacPorts/Homebrew and all major Linux distributions.

**Note**: If you install ChezScheme from source files, building it locally, make sure
you run ``./configure --threads`` to build multithreading support in.

Downloading and Installing
==========================

You can download the Idris 2 source from the `Idris web site
<https://www.idris-lang.org/pages/download.html>`_. 
This includes the Idris 2 source code (written in Idris 1) and the C
code generated from that.  Once you have unpacked the source, you can
install it as follows:

* If you have Idris 1.3.2 installed::

    make install

* If not, you can install directly from the generated C::

    make install-fromc

This will, by default, install into ``${HOME}/.idris2``. You can change this
by editing the options in ``config.mk``. For example,
to install into ``/usr/local``, you can edit the ``PREFIX`` as follows::

    PREFIX ?= /usr/local

To check that installation has succeeded, and to write your first
Idris program, create a file called ``hello.idr`` containing the
following text:

.. code-block:: idris

    module Main

    main : IO ()
    main = putStrLn "Hello world"

If you are familiar with Haskell, it should be fairly clear what the
program is doing and how it works, but if not, we will explain the
details later. You can compile the program to an executable by
entering ``idris2 hello.idr -o hello`` at the shell prompt. This will,
by default, create an executable called ``hello``, which invokes a generated
and compiled Chez Schem program, in the destination directory ``build/exec``
which you can run:

::

    $ idris2 hello.idr -o hello
    compiling hello.ss with output to hello.so
    $ ./build/exec/hello.so
    Hello world

Please note that the dollar sign ``$`` indicates the shell prompt!
Some useful options to the Idris command are:

- ``-o prog`` to compile to an executable called ``prog``.

- ``--check`` type check the file and its dependencies without starting the interactive environment.

- ``--package pkg`` add package as dependency, e.g. ``--package contrib`` to make use of the contrib package.

- ``--help`` display usage summary and command line options.

You can find out more about compiling to executables in
Section :ref:`sect-execs`.

The Interactive Environment
===========================

Entering ``idris2`` at the shell prompt starts up the interactive
environment. You should see something like the following:

.. literalinclude:: ../listing/idris-prompt-start.txt

This gives a ``ghci`` style interface which allows evaluation of, as
well as type checking of, expressions; theorem proving, compilation;
editing; and various other operations. The command ``:?`` gives a list
of supported commands. Below, we see an example run in
which ``hello.idr`` is loaded, the type of ``main`` is checked and
then the program is compiled to the executable file ``hello``,
available in the destination directory ``build/exec/``. Type
checking a file, if successful, creates a bytecode version of the file (in this
case ``build/ttc/hello.ttc``) to speed up loading in future. The bytecode is
regenerated if the source file changes.

.. _run1:
.. literalinclude:: ../listing/idris-prompt-helloworld.txt