Odd error: pilar is a Mesh, and I want to change some fields, so I use the with-syntax. On the last line I get this error:

error.txt

Error	1	This expression was expected to have type
    unit    
but here has type
    Mesh	C:\dev\.Net\arealities\Arealities.Models\Veranda.fs	17	11	Arealities.Models

odd.fs

module Arealities.Models.Veranda

let windowWall(width:decimal, height:decimal, maxWidth: decimal , beamThickness:decimal) =
    let pilar = { geometry = CubeGeometry(Size(beamThickness,height, beamThickness));
                  position = Position(0M,0M,0M);
                  rotation = Rotation(0M,0M,0M);
                  material = ShinyStuff;
                  castsShadows = true;
                  receivesShadow = false;
                }
    let panelCount = (width - beamThickness)/maxWidth |> System.Math.Ceiling |> int
    let spaceBetweenBeams = (width-beamThickness) / decimal(panelCount) - beamThickness
    let halfWidth = width/2M
    let halfHeight = height/2M
    [1..panelCount] |> List.iter(fun i -> 
        let x = decimal(i-1)*(spaceBetweenBeams+beamThickness)-halfWidth;
        { pilar with position = Position(x, halfHeight, 0) })
    

let veranda(Size(x,y,z)) = 
    let beamSize = 1
    let panelDepth = 0.5
    let maxPanelWidth = 12
    windowWall(1M,1M,1M,1M)

types.fs

namespace Arealities.Models

type Size = Size of decimal * decimal * decimal
type Position = Position of decimal * decimal * decimal
type Rotation = Rotation of decimal * decimal * decimal
type Direction = Direction of decimal * decimal * decimal

type Intensity = float

type Color = Color of Intensity * Intensity * Intensity

type Material = Ground | Floor | ShinyStuff | Wall

type Geometry = CubeGeometry of Size

type Mesh = {
        geometry : Geometry;
        material : Material;  
        position : Position;  
        rotation : Rotation; 
        castsShadows: bool; 
        receivesShadow: bool
    }

type Object3D = Object3D of Mesh list * Position * Rotation

type Light = 
    | AmbientLight of Color
    | DirectionalLight of Color * Intensity * Direction

type Camera = Camera of Position * Direction

type SceneObject = 
    | Light
    | Object3D
    | Camera
          
type Scene = Scene of SceneObject list

doh; map, not iter