What does this stream pipeline do? Trace the data flow step by step.

← Streams and Pipelines · Ref: Q987

This pipeline processes a list of order amounts in five steps:

1. cat orderAmounts — reads each Float from the list, one at a time
2. filter by isLargeOrder — keeps only amounts above the threshold, discards the rest
3. sort by compareDescending — orders the remaining amounts from largest to smallest
4. head 3 — takes only the first 3 sorted items, then stops the emission
5. > stdout — prints each of the 3 largest orders to standard output

The data flows left to right through each | operator. Each stage receives items from the previous stage and passes results to the next. The head operation acts like a limit — once 3 items have passed through, nothing more is pulled. A head stops the pull back to the nearest stage that must see everything: here that is the sort, which still has to read every large order to know which are biggest, so the filter runs on every amount; only the sorted items beyond the first 3 are never emitted. Without the sort, head 3 would stop the filter after the third large order.

Example

defines module qa.streams.explainpipeline

  defines function

    isLargeOrder() as pure
      -> amount as Float
      <- rtn as Boolean?

      threshold <- 100.0
      rtn: amount > threshold

    compareDescending() as pure
      ->
        left as Float
        right as Float
      <- rtn as Integer: right <=> left

  defines program

    ExplainPipelineDemo()
      stdout <- Stdout()

      orderAmounts <- [45.99, 250.00, 89.50, 175.00, 320.00, 15.00, 199.99]

      stdout.println("Top 3 large orders:")
      cat orderAmounts
        | filter by isLargeOrder
        | sort by compareDescending
        | head 3
        > stdout

Common mistakes

E50001 — EK9 uses Unix pipe syntax for streams, not Java method chaining. Write 'cat source | filter by fn | sort by fn | head N > stdout'.

Incorrect:

      orderAmounts.stream().filter(isLargeOrder).sorted(compareDescending).limit(3).forEach(System.out::println)

Correct:

      cat orderAmounts
        | filter by isLargeOrder
        | sort by compareDescending
        | head 3
        > stdout

Coming from another language?

EK9 stream pipelines read left-to-right like Unix pipes. Each | passes data to the next operation. The > terminal sends output to a sink.

Keywords: stream, data flow, trace, pipeline, investigate, explain