select_first

X select_first(Array[X?]+)

Selects the first - i.e. left-most - non-None value from an Array of optional values. It is an error if the array is empty, or if the array only contains None values.

Parameters

1. Array[X?]+: Non-empty Array of optional values.

Returns: The first non-None value in the input array.

Example: test_select_first.wdl

version 1.1

workflow test_select_first {
  input {
    Int? maybe_five = 5
    Int? maybe_four_but_is_not = None
    Int? maybe_three = 3
  }

  output {
    # both of these statements evaluate to 5
    Int five1 = select_first([maybe_five, maybe_four_but_is_not, maybe_three])
    Int five2 = select_first([maybe_four_but_is_not, maybe_five, maybe_three])
  }
}

{}

{
  "test_select_first.five1": 5,
  "test_select_first.five2": 5
}

Example: select_first_only_none_fail.wdl

version 1.1

workflow select_first_only_none_fail {
  Int? maybe_four_but_is_not = None
  Int result = select_first([maybe_four_but_is_not])  # error! array contains only None values
}

{}

{}

{
  "fail": true
}

Example: select_first_empty_fail.wdl

version 1.1

workflow select_first_empty_fail {
  Int check = select_first([])  # error! array is empty
}

{}

{}

{
  "fail": true
}