Functional programming style APIs in Go - with concurrency support

Concepts

Pipeline - allows for the definition of a data pipeline with a sequence of PreFilters, Transformers and PostFilters
PipelineComponents - utility structure to define the pipeline components. Including concurrency.
FilterOperation - allows determination of whether an element should stay or be filtered out during pipeline processing
TransformOperation - transforms element. additionally supports filtering. additionally supports returning errors
FoldOperation - folds / reduces elements down

The Pipeline is the overarching concept that provides the executable behviours of:

Apply - takes a slice and applies the data pipeline to it
ApplyAndFold - applies data pipelien to slice and then folds it down to a single element
Stream - continuously streams data through the pipeline until closure

Set the Concurrency in PipelineComponents to greater than 1 to get transparent concurrency when processing the data pipeline.
Data is smartly distributed between independent jobs that execute the data pipeline across the filters, transforms and even the folding operation.
During streaming this may naturally result in out of sequence data handling, so beware!
During slice processing (Apply) the result is sorted by original sequence allowing sequence to be maintained.
Goes without saying that when using concurrency all functions must be thread-safe.