Back to Questions
1.0k
views4
comments
Records as Objects
f#book

Hi there,

l have been trying to understand an example from Robert Pickering's book "Beginning F#", but the syntax makes no sense to me. In the following code, there is a function makeShape :

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24

#light
open System.Drawing

// a Shape record that will act as our object
type Shape =
    { Reposition: Point -> unit;
      Draw: unit -> unit }

// create a new instance of Shape
let makeShape initPos draw =
    // currPos is the internal state of the object
    let currPos = ref initPos
    { Reposition = 
        // the Reposition member updates the internal state
        (fun newPos -> currPos := newPos);
      Draw =
        // draw the shape passing the current position
        // to given draw function
        (fun () -> draw !currPos); }

// "draws" a shape, prints out the shapes name and position
let draw shape (pos: Point) =
    printfn "%s, with x = %i and y = %i"
        shape pos.X pos.Y

Now, the type Shape has a Reposition member that takes a Point as an input and a unit as the output.
But when used in the function makeShape, it has a function on the left. A function is not the same as a Point.

On the right-hand side, it is updating the variable currPos with a variable newPos. Where does newPos get its value from? In addition to that, how can the expression currPos := newPos have the type unit?

This example has utterly confused me. Can anyone please help explain it.

.

I have figured out the answer to my first question. The whole function syntax thing.

But I am still baffled by the other questions i.e. where does newPos get its value etc.

By on 5/15/2010 7:12 PM ()

OK. I have finally figured this out.

The penny dropped with this line fun s -> s.Reposition (new Point (40,40)) , which was a bit further on in the example.

So newPos gets its value from the Point parameter passed in to that method.
The example just had a few too many "new things" which had not been presented before in the book. Like setting a function as one of the members of a record. That was a leap.

By on 5/16/2010 12:14 AM ()

Sometimes the best way to understand something is to ask a question about it.

A business partner of mine and I used to do that. When one of us was stuck on a problem, we'd call the other and start off with something like: "Let ask you if you can solve this problem I'm having. You won't have to actually solve it, because by the time I finish asking the question, I'll have solved the problem myself." Lol.

My only explanation is that asking the question somehow helps you zero in on what information you need to answer it.

-Neil

By on 5/16/2010 7:19 AM ()

There's definitely wisdom in that.
Thanks!

By on 5/17/2010 3:23 AM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper