fsharp comonad monad isomorphism

cmarray.fsx

module CArray =
    type CArray<'a> = CA of 'a array  * int
    let fmap f (CA(a, i)) = CA(Array.map f a, i)
    let extract (CA(a, i)) = a.[i]
    let extend f (CA(a, i)) = 
        let es' = Array.mapi (fun i _ -> f (CA(a,i))) a 
        in CA(es',i)

module MArray =
    type CA<'a> = CA of 'a array  * int

    type MArray<'a,'r> = CA<'a> -> 'r
    let runCA m (CA(arr,i)) = 
        let r = Array.mapi (fun i _ ->  m (CA(arr, i))) arr
        CA(r,i)
                
    type MArrayBuilder internal () =
        member __.Bind(m : MArray<'a,'r> , f : 'r -> MArray<'a,'r2> ) : MArray<'a,'r2> = 
            fun c -> 
                let r = m c   
                f r c
        member __.Return(v)  = fun _ -> v

    let ma = MArrayBuilder()

    let get i' (CA(arr,i))  = 
        try
            arr.[i+i']
        with ex ->
            0

    

open MArray

let test =
        ma{
            let! a = get 1
            let! b = get 0
            return a + b
        }

printfn "%A" << runCA test <| CA([|1; 2; 3; 4; 5; 6; 7; 8; 9; 10|], 0) 
//CA ([|3; 5; 7; 9; 11; 13; 15; 17; 19; 10|],0)

let test2 = ma{
        let! a = test
        let! b = test
        return a + b
   }
        
printfn "%A" << runCA test2 <| CA([|1; 2; 3; 4; 5; 6; 7; 8; 9; 10|], 0) 
//CA ([|6; 10; 14; 18; 22; 26; 30; 34; 38; 20|],0)