sequence
========
Sequential execute the functions.

Function Arguments
------------------
.. list-table::
   :widths: 20 10 60
   :header-rows: 1
   :align: left

   * - Name
     - Type
     - Description

   * - functions
     - list
     - The list of the function expression.

Description
-----------
The input of the next function is the output of the previous function.
The concept is::

  data = Func1(data)
  data = Func2(data)
  ...

This function is very useful when you want to union the outputs of a series
of :ref:`probe functions <AbstractProbeFunction>`.

Examples
--------
Assume that we want to design a probe statement to probe general
information from the device.  The expected probed data should contain two
fields:

- ``device_sku`` comes from the command ``cros_config /identity sku-id``
- ``device_version`` from the command ``get_version --board``.

Instead of implementing a new probe function, we can reuse the existing
:doc:`shell function <shell>`, which executes a single command, and writes
a probe statement like::

  {
    "eval": {
      "sequence": {
        "functions": [
          {
            "shell": {
              "command": "cros_config /identity sku-id",
              "key": "device_sku"
            }
          },
          {
            "shell": {
              "command": "get_version --board",
              "key": "device_version"
            }
          }
        ]
      }
    }
  }

The expected probed results is::

  [
    {
      "device_sku": <output_of_the_cros_config_command>,
      "device_version": <output_of_the_get_version_command>
    }
  ]

As this function is very common to use, the probe framework also supplies
a syntax sugar for it.  Above probe statement can be simplified to::

  {
    "eval": [  # A list of functions means to apply the `sequence`
               # function.
      {
        "shell": {
          "command": "cros_config /identity sku-id",
          "key": "device_sku"
        }
      },
      {
        "shell": {
          "command": "get_version --board",
          "key": "device_version"
        }
      }
    ]
  }