The sequence: Duality of data and calculation

What makes the F# sequence so ambient and versatile is its dual nature. Being a strongly typed generic data collection, it exposes the contained data via two archetypal .NET interfaces of the System.Collections.Generic namespace, namely IEnumerable<T> ( and IEnumerator<T> (

These interfaces personify the classic data pull protocol, where a data consumer actively pulls data from the producer. Indeed, the type of seq<'T> in the F# is defined as the following abbreviation:

type seq<'T> = System.Collections.Generic.IEnumerable<'T> 

The preceding line of code means in practice ...

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