Creating ViewModel for WinRT app in F#.

MainPage.xaml

<Page
    x:Class="Win8App.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:Win8App"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:fSharpVm="using:FSharpVm"
    mc:Ignorable="d">

    <Page.Resources>
        <fSharpVm:MainPageViewModel x:Key="MainPageViewModel"/>
    </Page.Resources>
    
    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <StackPanel VerticalAlignment="Center" HorizontalAlignment="Center"
                    DataContext="{StaticResource MainPageViewModel}">
            <TextBlock Text="{Binding Count}" 
                       FontSize="72"
                       HorizontalAlignment="Center">0</TextBlock>
            <StackPanel Orientation="Horizontal">
                <Button Command="{Binding Increase}">Increase</Button>
                <Button Command="{Binding Decrease}">Decrease</Button>
            </StackPanel>
        </StackPanel>
    </Grid>
</Page>

MainPageViewModel.fs

namespace FSharpVm

type MainPageViewModel() =
    inherit ViewModelBase()
    let mutable _count = 0

    member x.Count 
        with get() = _count
        and set count = 
            _count <- count
            x.OnPropertyChanged "Count"

    member x.Increase = 
        new RelayCommand ((fun canExecute -> true), 
            (fun action -> x.Count <- x.Count + 1)) 

    member x.Decrease = 
        new RelayCommand ((fun canExecute -> true), 
            (fun action -> x.Count <- x.Count - 1))

RelayCommand.fs

namespace FSharpVm

open System
open System.Windows.Input
open System.ComponentModel

// from F# Windows App (WPF, MVVM) template by Daniel Mohl
// http://visualstudiogallery.msdn.microsoft.com/ad49fd5c-930c-4fe6-a30e-2d0d6778c565?SRC=VSIDE
type RelayCommand (canExecute:(obj -> bool), action:(obj -> unit)) =
    let event = new DelegateEvent<EventHandler>()
    interface ICommand with
        [<CLIEvent>]
        member x.CanExecuteChanged = event.Publish
        member x.CanExecute arg = canExecute(arg)
        member x.Execute arg = action(arg)

ViewModelBase.fs

namespace FSharpVm

open System.ComponentModel

// from F# Windows App (WPF, MVVM) template by Daniel Mohl
// http://visualstudiogallery.msdn.microsoft.com/ad49fd5c-930c-4fe6-a30e-2d0d6778c565?SRC=VSIDE
type ViewModelBase() =
    let propertyChangedEvent = new DelegateEvent<PropertyChangedEventHandler>()
    interface INotifyPropertyChanged with
        [<CLIEvent>]
        member x.PropertyChanged = propertyChangedEvent.Publish
    member x.OnPropertyChanged propertyName = 
        propertyChangedEvent.Trigger([| x; new PropertyChangedEventArgs(propertyName) |])