A "null" value is an empty value, it means some input wasn't specified or some tool produced
an empty value or was skipped entirely. This can be thought of as an NA in Excel or a None value
in Python. This expression tool can be fed multiple values - from parameter inputs or the
outputs or other tools or workflows - and pick one. This allows building workflows which can
skip steps and branch in complex ways.
There are subtle but important distinctions between the selection style - though they all
essentially pick the first value that is non-null (i.e. the first input with a real value).
- "Pick first value" is the default and will just return its own null value if none of the inputs
are non-null.
- "Pick first value (or provide default)" allows picking a default value that will be used if none of the
inputs are non-null.
- "Pick first value (or fail)" will cause the tool execution to fail if none of the selections are
non-null. This can be important because it provides a clearer indication that something went
wrong and can prevent the further evaluation of a workflow.
- "Pick only value" is like "First value (or fail)" in that it will cause an error if none of the
inputs are non-null but it will go further to ensure that exactly one value is non-null. When
building conditonal paths through a workflow this can ensure that at most one path is
taken.