mirror of
https://github.com/idris-lang/Idris2.git
synced 2024-12-17 16:21:46 +03:00
88 lines
2.9 KiB
ReStructuredText
88 lines
2.9 KiB
ReStructuredText
Exceptions and State
|
|
====================
|
|
|
|
``Control.App`` is primarily intended to make it easier to manage the common
|
|
cases of applications with exceptions and state. We can throw and catch
|
|
exceptions listed in the list of errors (the ``es`` parameter to ``App``) and
|
|
introduce new global state.
|
|
|
|
Exceptions
|
|
----------
|
|
|
|
The ``List Error`` is a list of error types, usable via the
|
|
``Exception`` interface defined in ``Control.App``:
|
|
|
|
.. code-block:: idris
|
|
|
|
interface Exception err e where
|
|
throw : err -> App e a
|
|
catch : App e a -> (err -> App e a) -> App e a
|
|
|
|
We can use ``throw`` and ``catch`` for some exception type
|
|
``err`` as long as the exception type exists in the list of errors. This is
|
|
checked with the ``HasErr`` predicate, also defined in ``Control.App``:
|
|
|
|
.. code-block:: idris
|
|
|
|
data HasErr : Error -> List Error -> Type where
|
|
Here : HasErr e (e :: es)
|
|
There : HasErr e es -> HasErr e (e' :: es)
|
|
|
|
HasErr err es => Exception err es where ...
|
|
|
|
Note the ``HasErr`` constraint on ``Exception``: this is one place
|
|
where it is notationally convenient that the ``auto`` implicit mechanism
|
|
and the interface resolution mechanism are identical in Idris 2.
|
|
Finally, we can introduce new exception types via ``handle``, which runs a
|
|
block of code which might throw, handling any exceptions:
|
|
|
|
.. code-block:: idris
|
|
|
|
handle : App (err :: e) a ->
|
|
(onok : a -> App e b) ->
|
|
(onerr : err -> App e b) -> App e b
|
|
|
|
Adding State
|
|
------------
|
|
|
|
Applications will typically need to keep track of state, and we support
|
|
this primitively in ``App`` using a ``State`` type, defined in
|
|
``Control.App``:
|
|
|
|
.. code-block:: idris
|
|
|
|
data State : (tag : a) -> Type -> List Error -> Type
|
|
|
|
The ``tag`` is used purely to distinguish between different states,
|
|
and is not required at run-time, as explicitly stated in the types of
|
|
``get`` and ``put``, which are used to access and update a state:
|
|
|
|
.. code-block:: idris
|
|
|
|
get : (0 tag : _) -> State tag t e => App {l} e t
|
|
put : (0 tag : _) -> State tag t e => (1 val : t) -> App {l} e ()
|
|
|
|
These use an ``auto``-implicit to pass around
|
|
a ``State`` with the relevant ``tag`` implicitly, so we refer
|
|
to states by tag alone. In ``helloCount`` earlier, we used an empty type
|
|
``Counter`` as the tag:
|
|
|
|
.. code-block:: idris
|
|
|
|
data Counter : Type where -- complete definition
|
|
|
|
The list of errors ``e`` is used to ensure that
|
|
states are only usable in the list of errors in which they are introduced.
|
|
States are introduced using ``new``:
|
|
|
|
.. code-block:: idris
|
|
|
|
new : t -> (1 p : State tag t e => App {l} e a) -> App {l} e a
|
|
|
|
Note that the type tells us ``new`` runs the program with the state
|
|
exactly once.
|
|
Rather than using ``State`` and ``Exception`` directly, however,
|
|
we typically use interfaces to constrain the operations which are allowed
|
|
in a list of errors. Internally, ``State`` is implemented via an
|
|
``IORef``, primarily for performance reasons.
|