E10020: Void cannot be used in stream pipelines

What went wrong

Void cannot be used in stream pipelines. Stream operations pass values from one stage to the next, but Void has no value to pass.

Worked examples

Compilable examples demonstrating this error and the correct fix:

• ek9 -q 928 (Why can't I use a void function in a stream pipeline i...)

Stream operation requirements

• 'map' - function must have return value
• 'filter' - function must return Boolean
• 'tee' - takes a target that keeps or writes the items (a collection, 'stdout'), not a function
• 'collect' - must specify collection type

How to fix it

1. If using 'map', ensure the function has a returning variable
2. If the function is for side effects only, it does not belong in a pipeline: iterate with 'for item in values' and call it for each item (a void function is not a pipeline sink or 'tee' target either)
3. Check that mapping function signature matches stream type

The [EXAMPLE] section below shows a void function mapped and collected 'as Void', and a function that returns the upper-cased String mapped and collected as a List.

See also

• E10020: void in stream pipeline (this error)
• E10010: void result in assignment

Examples

Every example is compiled by the EK9 compiler: the first fails exactly as captioned, the others fix it.

Fails with E10020: Void cannot be used in stream pipelines

#!ek9
defines module error.e10020.bad

  defines function
    voidFunction()
      -> item as String
      require item?
      //No return mainValue - returns Void

    testStreamWithVoid()
      values <- ["a", "b", "c"]
      result <- cat values | map voidFunction | collect as Void

Fixed: Give each stage a function that returns a value

#!ek9
defines module error.e10020.good

  defines function

    shout() as pure
      -> item as String
      <- rtn as String: item.upperCase()

    testStream()
      <- rtn as List of String: List()
      values <- ["a", "b", "c"]
      rtn: cat values | map shout | collect as List of String