When to use a sequence in F# as opposed to a list?

Question

I understand that a list actually contains values, and a sequence is an alias for IEnumerable<T>. In practical F# development, when should I be using a sequence as opposed to a list?

Here's some reasons I can see when a sequence would be better:

• When interacting with other .NET languages or libraries that require IEnumerable<T>.
• Need to represent an infinite sequence (probably not really useful in practice).
• Need lazy evaluation.

Are there any others?

Solution 1

I think your summary for when to choose Seq is pretty good. Here are some additional points:

• Use Seq by default when writing functions, because then they work with any .NET collection
• Use Seq if you need advanced functions like Seq.windowed or Seq.pairwise

I think choosing Seq by default is the best option, so when would I choose different type?

• Use List when you need recursive processing using the head::tail patterns
  (to implement some functionality that's not available in standard library)

• Use List when you need a simple immutable data structure that you can build step-by-step
  (for example, if you need to process the list on one thread - to show some statistics - and concurrently continue building the list on another thread as you receive more values i.e. from a network service)

• Use List when you work with short lists - list is the best data structure to use if the value often represents an empty list, because it is very efficient in that scenario

• Use Array when you need large collections of value types
  (arrays store data in a flat memory block, so they are more memory efficient in this case)

• Use Array when you need random access or more performance (and cache locality)

Solution 2

Also prefer seq when:

• You don't want to hold all elements in memory at the same time.

• Performance is not important.

• You need to do something before and after enumeration, e.g. connect to a database and close connection.

• You are not concatenating (repeated Seq.append will stack overflow).

Prefer list when:

• There are few elements.

• You'll be prepending and decapitating a lot.

Neither seq nor list are good for parallelism but that does not necessarily mean they are bad either. For example, you could use either to represent a small bunch of separate work items to be done in parallel.

Solution 3

Just one small point: Seq and Array are better than List for parallelism.

You have several options: PSeq from F# PowerPack, Array.Parallel module and Async.Parallel (asynchronous computation). List is awful for parallel execution due to its sequential nature (head::tail composition).

Solution 4

list is more functional, math-friendly. when each element is equal, 2 lists are equal.

sequence is not.

let list1 =  [1..3]
let list2 =  [1..3]
printfn "equal lists? %b" (list1=list2)

let seq1 = seq {1..3}
let seq2 = seq {1..3}
printfn "equal seqs? %b" (seq1=seq2)

Solution 5

You should always expose Seq in your public APIs. Use List and Array in your internal implementations.

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