.. _sec-views:

*****************************
Views and the “``with``” rule
*****************************

Dependent pattern matching
==========================

Since types can depend on values, the form of some arguments can be
determined by the value of others. For example, if we were to write
down the implicit length arguments to ``(++)``, we’d see that the form
of the length argument was determined by whether the vector was empty
or not:

.. code-block:: idris

    (++) : Vect n a -> Vect m a -> Vect (n + m) a
    (++) {n=Z}   []        ys = ys
    (++) {n=S k} (x :: xs) ys = x :: xs ++ ys

If ``n`` was a successor in the ``[]`` case, or zero in the ``::``
case, the definition would not be well typed.

.. _sect-nattobin:

The ``with`` rule — matching intermediate values
================================================

Very often, we need to match on the result of an intermediate
computation. Idris provides a construct for this, the ``with``
rule, inspired by views in ``Epigram`` [1]_, which takes account of
the fact that matching on a value in a dependently typed language can
affect what we know about the forms of other values. In its simplest
form, the ``with`` rule adds another argument to the function being
defined, e.g. we have already seen a vector filter function, defined
as follows:

.. code-block:: idris

    filter : (a -> Bool) -> Vect n a -> (p ** Vect p a)
    filter p [] = ( _ ** [] )
    filter p (x :: xs) with (filter p xs)
      | ( _ ** xs' ) = if (p x) then ( _ ** x :: xs' ) else ( _ ** xs' )

Here, the ``with`` clause allows us to deconstruct the result of
``filter p xs``. Effectively, it adds this value as an extra argument,
which we place after the vertical bar.

If the intermediate computation itself has a dependent type, then the
result can affect the forms of other arguments — we can learn the form
of one value by testing another. For example, a ``Nat`` is either even
or odd. If it’s even it will be the sum of two equal ``Nat``.
Otherwise, it is the sum of two equal ``Nat`` plus one:

.. code-block:: idris

    data Parity : Nat -> Type where
       Even : Parity (n + n)
       Odd  : Parity (S (n + n))

We say ``Parity`` is a *view* of ``Nat``. It has a *covering function*
which tests whether it is even or odd and constructs the predicate
accordingly.

.. code-block:: idris

    parity : (n:Nat) -> Parity n

We’ll come back to the definition of ``parity`` shortly. We can use it
to write a function which converts a natural number to a list of
binary digits (least significant first) as follows, using the ``with``
rule:

.. code-block:: idris

    natToBin : Nat -> List Bool
    natToBin Z = Nil
    natToBin k with (parity k)
       natToBin (j + j)     | Even = False :: natToBin j
       natToBin (S (j + j)) | Odd  = True  :: natToBin j

The value of the result of ``parity k`` affects the form of ``k``,
because the result of ``parity k`` depends on ``k``. So, as well as
the patterns for the result of the intermediate computation (``Even``
and ``odd``) right of the ``|``, we also write how the results
affect the other patterns left of the ``|``. Note that there is
a function in the patterns (``+``) and repeated occurrences of
``j``—this is allowed because another argument has determined the form
of these patterns.

We will return to this function in the next section :ref:`sect-parity` to
complete the definition of ``parity``.

With and proofs
===============
To use a dependent pattern match for theorem proving, it is sometimes necessary
to explicitly construct the proof resulting from the pattern match.
To do this, you can postfix the with clause with ``proof p`` and the proof
generated by the pattern match will be in scope and named ``p``. For example:

.. code-block:: idris

    data Foo = FInt Int | FBool Bool

    optional : Foo -> Maybe Int
    optional (FInt x) = Just x
    optional (FBool b) = Nothing

    isFInt : (foo:Foo) -> Maybe (x : Int ** (optional foo = Just x))
    isFInt foo with (optional foo) proof p
      isFInt foo | Nothing = Nothing           -- here, p : Nothing = optional foo
      isFInt foo | (Just x) = Just (x ** Refl) -- here, p : Just x = optional foo


.. [1] Conor McBride and James McKinna. 2004. The view from the
       left. J. Funct. Program. 14, 1 (January 2004),
       69-111. DOI=10.1017/S0956796803004829
       http://dx.doi.org/10.1017/S0956796803004829ñ