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+.. _filters:
+
+Filters
+=======
+
+Many places in `prompt_toolkit` require a boolean value that can change over
+time. For instance:
+
+- to specify whether a part of the layout needs to be visible or not;
+- or to decide whether a certain key binding needs to be active or not;
+- or the ``wrap_lines`` option of
+  :class:`~prompt_toolkit.layout.BufferControl`;
+- etcetera.
+
+These booleans are often dynamic and can change at runtime. For instance, the
+search toolbar should only be visible when the user is actually searching (when
+the search buffer has the focus). The ``wrap_lines`` option could be changed
+with a certain key binding. And that key binding could only work when the
+default buffer got the focus.
+
+In `prompt_toolkit`, we decided to reduce the amount of state in the whole
+framework, and apply a simple kind of reactive programming to describe the flow
+of these booleans as expressions. (It's one-way only: if a key binding needs to
+know whether it's active or not, it can follow this flow by evaluating an
+expression.)
+
+The (abstract) base class is :class:`~prompt_toolkit.filters.Filter`, which
+wraps an expression that takes no input and evaluates to a boolean. Getting the
+state of a filter is done by simply calling it.
+
+
+An example
+----------
+
+The most obvious way to create such a :class:`~prompt_toolkit.filters.Filter`
+instance is by creating a :class:`~prompt_toolkit.filters.Condition` instance
+from a function. For instance, the following condition will evaluate to
+``True`` when the user is searching:
+
+.. code:: python
+
+    from prompt_toolkit.application.current import get_app
+    from prompt_toolkit.filters import Condition
+
+    is_searching = Condition(lambda: get_app().is_searching)
+
+A different way of writing this, is by using the decorator syntax:
+
+.. code:: python
+
+    from prompt_toolkit.application.current import get_app
+    from prompt_toolkit.filters import Condition
+
+    @Condition
+    def is_searching():
+        return get_app().is_searching
+
+This filter can then be used in a key binding, like in the following snippet:
+
+.. code:: python
+
+    from prompt_toolkit.key_binding import KeyBindings
+
+    kb = KeyBindings()
+
+    @kb.add('c-t', filter=is_searching)
+    def _(event):
+        # Do, something, but only when searching.
+        pass
+
+If we want to know the boolean value of this filter, we have to call it like a
+function:
+
+.. code:: python
+
+    print(is_searching())
+
+
+Built-in filters
+----------------
+
+There are many built-in filters, ready to use. All of them have a lowercase
+name, because they represent the wrapped function underneath, and can be called
+as a function.
+
+- :class:`~prompt_toolkit.filters.app.has_arg`
+- :class:`~prompt_toolkit.filters.app.has_completions`
+- :class:`~prompt_toolkit.filters.app.has_focus`
+- :class:`~prompt_toolkit.filters.app.buffer_has_focus`
+- :class:`~prompt_toolkit.filters.app.has_selection`
+- :class:`~prompt_toolkit.filters.app.has_validation_error`
+- :class:`~prompt_toolkit.filters.app.is_aborting`
+- :class:`~prompt_toolkit.filters.app.is_done`
+- :class:`~prompt_toolkit.filters.app.is_read_only`
+- :class:`~prompt_toolkit.filters.app.is_multiline`
+- :class:`~prompt_toolkit.filters.app.renderer_height_is_known`
+- :class:`~prompt_toolkit.filters.app.in_editing_mode`
+- :class:`~prompt_toolkit.filters.app.in_paste_mode`
+
+- :class:`~prompt_toolkit.filters.app.vi_mode`
+- :class:`~prompt_toolkit.filters.app.vi_navigation_mode`
+- :class:`~prompt_toolkit.filters.app.vi_insert_mode`
+- :class:`~prompt_toolkit.filters.app.vi_insert_multiple_mode`
+- :class:`~prompt_toolkit.filters.app.vi_replace_mode`
+- :class:`~prompt_toolkit.filters.app.vi_selection_mode`
+- :class:`~prompt_toolkit.filters.app.vi_waiting_for_text_object_mode`
+- :class:`~prompt_toolkit.filters.app.vi_digraph_mode`
+
+- :class:`~prompt_toolkit.filters.app.emacs_mode`
+- :class:`~prompt_toolkit.filters.app.emacs_insert_mode`
+- :class:`~prompt_toolkit.filters.app.emacs_selection_mode`
+
+- :class:`~prompt_toolkit.filters.app.is_searching`
+- :class:`~prompt_toolkit.filters.app.control_is_searchable`
+- :class:`~prompt_toolkit.filters.app.vi_search_direction_reversed`
+
+
+Combining filters
+-----------------
+
+Filters can be chained with the ``&`` (AND) and ``|`` (OR) operators and
+negated with the ``~`` (negation) operator.
+
+Some examples:
+
+.. code:: python
+
+    from prompt_toolkit.key_binding import KeyBindings
+    from prompt_toolkit.filters import has_selection, has_selection
+
+    kb = KeyBindings()
+
+    @kb.add('c-t', filter=~is_searching)
+    def _(event):
+        " Do something, but not while searching. "
+        pass
+
+    @kb.add('c-t', filter=has_search | has_selection)
+    def _(event):
+        " Do something, but only when searching or when there is a selection. "
+        pass
+
+
+to_filter
+---------
+
+Finally, in many situations you want your code to expose an API that is able to
+deal with both booleans as well as filters. For instance, when for most users a
+boolean works fine because they don't need to change the value over time, while
+some advanced users want to be able this value to a certain setting or event
+that does changes over time.
+
+In order to handle both use cases, there is a utility called
+:func:`~prompt_toolkit.filters.utils.to_filter`.
+
+This is a function that takes
+either a boolean or an actual :class:`~prompt_toolkit.filters.Filter`
+instance, and always returns a :class:`~prompt_toolkit.filters.Filter`.
+
+.. code:: python
+
+    from prompt_toolkit.filters.utils import to_filter
+
+    # In each of the following three examples, 'f' will be a `Filter`
+    # instance.
+    f = to_filter(True)
+    f = to_filter(False)
+    f = to_filter(Condition(lambda: True))
+    f = to_filter(has_search | has_selection)