MapReduce.fsx

let genericMapReduce toOutput folder src = 
    src
    |> Seq.map toOutput
    |> Seq.reduce folder

let inline mapReduceAdd toOutput src = 
    src |> genericMapReduce toOutput (+)

type Person = 
    { Name   : string
      Salary : decimal }

type PersonReport = 
    { Names     : string list
      SalarySum : decimal }
      
    member x.AvgSalary =
            let cachedSum = x.SalarySum / decimal x.Names.Length
            fun () -> cachedSum
            
    static member (+) (a, b) =
            { Names     = a.Names @ b.Names 
              SalarySum = a.SalarySum + b.SalarySum }

let toReport person = 
    { Names     = [person.Name]
      SalarySum = person.Salary }
      
let names = ["John";"Pitt";"Mike"]

let persons = 
    names 
    |> Seq.mapi (fun i n -> 
        { Name = n; Salary = decimal (i * 100) })
        
let report =
    persons
    |> mapReduceAdd toReport

report.AvgSalary() |> printfn "%A"