Itertools for F#

Question

I'm used to Python's itertools for doing functional things with iterators (F#: sequences) and wondered if there were equivalents in F# or a commonly used library since they're so handy.

The top tools for me are:

• product : cartesian product, equivalent to a nested for-loop
• combinations
• permutations
• takewhile
• dropwhile
• chain : chain multiple iterators together into a new longer iterator
• repeat* : repeat(5) -> 5, 5, 5...
• count* : count(10) -> 10, 11, 12...
• cycle* : cycle([1,2,3]) -> 1,2,3,1,2...

* I suppose these 3 would yield monads in F#? How do you make them infinite?

I'm prompted to ask because I saw this question on permutations in F# and was surprised it was not part of a library or built into the language.

Answer 1

I don't know if there's a commonly used library that contains functions like product, combinations and permutations, but the others you've mentioned are already in Seq and List modules or can be implemented without much trouble, and there are also useful methods in System.Linq.Enumerable.

• takewhile -> Seq.takeWhile
• dropwhile -> Seq.skipWhile
• chain -> Seq.concat
• repeat -> Seq.initInfinite
• count(10) -> Seq.initInfinite ((+) 10)
• cycle([1, 2, 3]) -> Seq.concat <| Seq.initInfinite (fun _ -> [1; 2; 3])

You also might want to check out the excellent FSharpx library -- it contains a lot of useful functions to work with collections and whatnot.