Listモナド

let (>>=) (m:'a list) f =
    List.concat (List.map f m) // concatMapがないのでconcatとmapで代用

type ListMonadBuilder() =
    member __.Return(x)  = [x]
    member __.Bind(m, f) = m >>= f
    member __.Zero() = []
let list = ListMonadBuilder()

//listモナドの利用
let test1 = list {
    let! a = [1; 2; 3]
    let! b = [10;20;30;40]
    return (a + b) }

//シーケンス式で代用
let test2 = [for i in [1;2;3] do for j in [10;20;30;40] do i+j]

//3段にしたリストモナド
let test3 = list {
    let! a = [1; 2; 3]
    let! b = [10;20;30]
    let! c = [100;200;300]
    return (a+b+c)
    }
let test4 = list {
    let! a = [1; 2; 3]
    let! b = [10;20;30]
    let! c = [100;200;300]
    match (a+b+c) with
       | x when (a + b/10 + c/100 = 5) -> return (a+b+c)
       | _ -> ()
       }

printfn "test1 = %A" test1
printfn "test2 = %A" test2
printfn "test3 = %A" test3
printfn "length of test3 = %d" <| List.length test3 // 27
printfn "test4 = %A" test4