A sequence is a possibly infinite, ordered collection of elements seq [eo; e1;…]. The elements of a sequence are computed on demand only, as it would make no sense to actually compute an infinite sequence. Thus, at any stage in a computation, just a finite portion of the sequence has been computed.

The notion of a sequence provides a useful abstraction in a variety of applications where you are dealing with elements that should be processed one after the other. Sequences are supported by the collection library of F# and many of the library functions on lists presented in Chapter 5 have similar sequence variants. Furthermore, sequences can be defined in F# using sequence expressions, defining a process for generating the elements.

The type seq<′a> is a synonym for the .NET type IEnumerable<′a> and any .NET framework type that implements this interface can be used as a sequence. One consequence of this is, for the F# language, that lists and arrays (that are specializations of sequences) can be used as sequence arguments for the functions in the Seq library. Another consequence is that results from the Language-Integrated Query or LINQ component of the .NET framework can be viewed as F# sequences. LINQ gives query support for different kinds of sources like SQL databases and XML repositories.