F#, Split String and .Net methods

The function String.split is now defined in the F# Power Pack. You must add

#r "FSharp.PowerPack.dll";; 
#r "FSharp.PowerPack.Compatibility.dll";; 

See the errata for Expert F#: http://www.expert-fsharp.com/Updates/Expert-FSharp-Errata-Jan-27-2009.pdf

Get FSharp.PowerPack.dll here: http://fsharppowerpack.codeplex.com/

You call them as instance methods:

let Count (text : string) =
  let words = text.Split [|' '|]
  let nWords = words.Length
  (nWords)

(Note you need to use [| |] because Split takes an array not a list; or, as per Joel Mueller's comment, because Split takes a params array, you can just pass in the delimiters as separate arguments (e.g. text.Split(' ', '\n')).)