Beginning Haskell
=================

This is a set of growing notes on Haskell programming. The note is mostly for myself at the moment.

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Hello World
***********

To respect the grand tradition of programming language tutorial, we will start with the example on how to print hello world to the screen:

.. code-block:: haskell

  main :: IO()
  main = do
    print "Hello world"

Put the snippet in the file ``hello-world.hs``, run ``runhaskell hello-world.hs``.

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Generalized Algebraic Data Types (GADTs)
****************************************

`Generalized Algebraic Data Types (GADTs)` allows data constructor to return different types. The feature is an extension of `GHC` included that can be enabled by `GADTs`. It's built upon another extension named `GADTSyntax`. It's sometimes confusing and intimating to have many extensions in GHC, but the separation can actually help understanding the feature. The syntax part allows you to provide the type signature of each value constructor:

.. code-block:: haskell

  data Data a where
    Nothing :: Data a
    Something :: a -> Data a

and with the ``newtype``:

.. code-block:: haskell

  newtype Data a where
    Data :: a -> Data a

When annotating the type explicitly, we changed the use of ``|`` to simple new lines. So far, we are still returning a single consistent type for each constructor. To have a true `GADT`, based on the syntax, we can write:

.. code-block:: haskell

  data Expr a b where
    Add :: Num n => Expr a n -> Expr a n -> Expr a n
    Negate :: Expr a Bool -> Expr a Bool

Notice that in the above snippet, two value constructors return different types, hence the name `generalized`.

.. toctree::
  :maxdepth: 2
  :caption: Contents:

  practical_haskell