.. This file is part of Sympathy for Data.
..
..  Copyright (c) 2015 System Engineering Software Society
..
..     Sympathy for Data is free software: you can redistribute it and/or modify
..     it under the terms of the GNU General Public License as published by
..     the Free Software Foundation, either version 3 of the License, or
..     (at your option) any later version.
..
..     Sympathy for Data is distributed in the hope that it will be useful,
..     but WITHOUT ANY WARRANTY; without even the implied warranty of
..     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
..     GNU General Public License for more details.
..     You should have received a copy of the GNU General Public License
..     along with Sympathy for Data. If not, see <http://www.gnu.org/licenses/>.

.. _higher_order_functions:


.. _lambda_function:

Lambda
-------

A Lambda is like a subflow but when executing it the nodes inside are not
executed directly.  Instead they are packaged into a data type that is sent to
the output port on the outside of the Lambda to be executed at a later time.

Lambdas can be used to reduce duplication, to work with nested lists, and allow
for conditional execution of nodes. They are created by right-clicking on an
empty area of a workflow and choosing *Create Lambda*. To execute the contents
of the Lambda use either :ref:`Apply` or :ref:`Map`.

A Lambda has only a single output on the outside. The type of the output is a
function with argument types depending on the types connected to the input
ports on the inside. Like with any other data type the output from Lambda can
be used with any nodes that handle generic types, such as list or tuple
operations.

If input data is needed to properly configure the Lambda, the input ports can
be shown on the outside of the Lambda by right-clicking on the lambda and
selecting *Ports->Input->Create->Port[n]*. These ports can then be used to
connect data to be used for configuration. Note that this data is only used
when configuring and will not be used when the Lamba is used with Apply or Map.
To hide the port, right click on the port and choose *Delete*.

Some advanced use cases are also possible using Lambdas, for example putting
Lambdas in lists to be executed using a :ref:`Map`.

See also the nodes :ref:`Extract Lambdas` and :ref:`Extract Flows as Lambdas`
for a way to import Lambda functions from workflow files.