Back to Questions
1.7k
views2
comments
F# newbie Expert F# example page 148
f#

The example code shown in Ch 6 on page 148 does not compile. I've tried adding the AbstractClass attribute which makes the code correct but I'm not sure if that was the intent. The original code is shown below.

[<AbstractClassAttribute>]
type TextOutputSink() =
abstract WriteChar: char -> unit
abstract WriteString: string -> unit
override this.WriteString(s) = s |> String.iter this.WriteChar

open System.Text

/// This type definition does not compile.
type ByteOutputSink() =
inherit TextOutputSink()
abstract WriteByte: byte -> unit
abstract WriteBytes: byte[]->unit
default sink.WriteChar(c) = sink.WriteBytes(Encoding.UTF8.GetBytes([|c|]))
override sink.WriteString(s) = sink.WriteBytes(Encoding.UTF8.GetBytes(s))
default sink.WriteBytes(b) = b|> Array.iter (fun c -> sink.WriteByte(c))

Any input ?

Haroon

.

btw

what 'default' means here?

By on 9/8/2009 2:05 AM ()

If you want to have abstract members, you need to explicitly add the [<AbstractClass>] attribute (I believe that previously there was just a warning, not sure though) unless there are no implemented members. This is meant to avoid "missing" unintendedly left unimplemented abstract members.

1
2
3
4
5
6
7
8
9
10
11
12
13
14

//will compile
type Foo =
  abstract member Run : unit -> unit

//won't compile
type Bar =
  abstract member Run : unit -> unit
  member x.SayHi() = ()

//will compile
[<AbstractClass>]
type Bar =
  abstract member Run : unit -> unit
  member x.SayHi() = ()

As for the default keyword, it means it is the default implementation in abstract classes if the children classes don't implement it.

Cheers

By on 9/8/2009 3:14 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