Phillip Trelford's Array

Following up on the last Roslyn preview way back in 2012, this week saw the availability of a new preview with a more complete compiler along with a few new language features for C# and VB. A lot of inspiration for these features seems to have come from the F# language.

The C# interactive shell from 2012 appears to be missing, perhaps ScriptCS is expected to fill this space, or you could just use F# interactive which already exists in Visual Studio.

On the language features side, C# 6 gets primary constructors for classes, heavily inspired by F#, and using static which brings parity with Java and VB.Net.

For me VB.Net gets the most interesting new feature in the form of Select Case TypeOf. which provides the first steps towards pattern matching.

Shapes

Taking a hierarchy of shapes as an example:

Public MustInherit Class Shape
End Class

Public Class Rectangle
    Inherits Shape
    Public Property Width As Integer
    Public Property Height As Integer
End Class

Public Class Circle
    Inherits Shape
    Public Property Radius As Integer
End Class

Sub Main()
    Dim shape As Shape = New Rectangle With {.Width = 10, .Height = 10}
    Select Case shape
        Case r As Rectangle When r.Width = r.Height
            Console.WriteLine("Square of {0}", r.Width)
        Case r As Rectangle
            Console.WriteLine("Rectangle of {0},{1}", r.Width, r.Height)
        Case c As Circle
            Console.WriteLine("Circle of {0}", c.Radius)
    End Select
End Sub

The functionality is still quite limited and quite verbose in comparison to say F# or Scala, but I feel it’s definitely an interesting development for VB.Net.

For comparison here’s an equivalent F# version using discriminated unions:

type Shape =
    | Rectangle of width:int * height:int
    | Circle of radius:int

let shape = Rectangle(10,10)
match shape with
| Rectangle(w,h) when w=h -> printfn "Square %d" w
| Rectangle(w,h) -> printfn "Rectangle %d, %d" w h
| Circle(r) -> printfn "Circle %d" r

Eval

Pattern matching can be really useful when writing compilers, here’s a simple expression tree evaluator in F#:

type Expression =
   | Factor of value:int
   | Add of lhs:Expression * rhs:Expression

let rec eval e =
   match e with
   | Factor(x) -> x
   | Add(l,r) -> eval l + eval r

let onePlusOne = Add(Factor(1),Factor(1))

VB.Net vNext can approximate this, albeit in a rather more verbose way:

Public MustInherit Class Expression
End Class

Public Class Factor
    Inherits Expression
    Public Property Value As Integer
    Sub New(x As Integer)
        Value = x
    End Sub
End Class

Public Class Op
    Inherits Expression
    Public Property Lhs As Expression
    Public Property Rhs As Expression
End Class

Public Class Add
    Inherits Op
End Class

Function Eval(e As Expression) As Integer
    Select Case e
        Case x As Factor
            Return x.Value
        Case op As Add
            Return Eval(op.Lhs) + Eval(op.Rhs)
        Case Else
            Throw New InvalidOperationException
    End Select
End Function

Sub Main()
    Dim onePlusOne As Expression =
        New Add With {.Lhs = New Factor(1), .Rhs = New Factor(1)}
    Console.WriteLine(Eval(onePlusOne))
End Sub

Summary

It will be interesting to see how VB.Net vNext develops. I think first-class support for tuples could be an interesting next step for the language.