Did it with .NET

Writing F# Type Extensions for Nullable

Dustin Campbell — Sat, 18 Feb 2012 23:44:30 GMT

A type extension is F#’s syntax for augmenting an existing type with new members – similar in spirit to C# and VB’s extensions methods. I employ type extensions to make life a bit easier when working with .NET libraries from F#. For example, I might make System.IO.Stream.ReadByte() a bit more natural to use from F# by writing a type extension like so:


          type Stream with

    match x.ReadByte() with

    | -1 -> None

    |  b -> Some(byte b)

The function above captures the semantic of the int returned by Stream.ReadByte(), which might be either an unsigned byte or -1 when at the end of the stream. In F#, this semantic is easily represented as an Option and then more easily consumed by other F# code.

So yes, type extensions are dead useful. However, if you’re writing a type extension for a generic type, you’ll need to include all of the type parameters and there constraints. Normally this isn’t a big deal, but I just spent several minutes tearing (more) of my hair out trying to determine the correct incantation to write a type extension for Nullable.

In the documentation, Nullable is declared with two generic constraints: a struct constraint and a default constructor constraint. However, that’s not quite enough to make the F# compiler happy. F# expects a type constraint to System.ValueType as well. So, three constraints are needed to declare a type extension for Nullable .


          type Nullable<'a when 'a
: struct

                  and 'a
: (new : unit -> 'a)

                  and 'a
:> System.ValueType> with

    member x.AsOption() =

        match x.HasValue with

        | true  -> Some(x.Value)

        | false -> None

Of course, once you’ve written that, you shouldn’t need to write it again and you can use the extension to handle Nullable a bit more naturally in F#.


          match someNullable.AsOption() with

| Some(n) -> printfn "%d" n

| None    -> ()

Using Sublime Text 2 to Edit Git Commit Messages on Windows

Dustin Campbell — Sat, 18 Feb 2012 20:25:58 GMT

These days, I’m using the Sublime Text 2 Beta as my text editor of choice on Windows. I searched around to find the magical incantation to use that would allow Git to use it as my commit message editor and came up empty. It turns out not to be a big deal. Just create a batch file that launches sublime_text.exe with --wait and --new-window arguments. Something like the following:

"C:\Program
Files\Sublime Text 2\sublime_text.exe" --wait --new-window "%1"

Then it’s just a matter of setting the batch file as the core editor used by git:

git
--config core.editor C:/Scripts/sublime.bat

Update (2/25/2012): The –multiinstance command will do a much better job when launching sublime_text.exe from the command prompt, especially when you already have an instance of sublime_text.exe open. Update your batch file to look like so, and you should be good to go.

"C:\Program
Files\Sublime Text 2\sublime_text.exe" --wait --multiinstance "%1"

Post-CodeMash Resolutions

Dustin Campbell — Mon, 16 Jan 2012 00:25:23 GMT

I failed to make any New Year’s resolutions this year. I guess I’d decided that things were going “OK” and I really didn’t need to make any changes. However, after spending the last several days at CodeMash engaging with old friends and making new ones, I feel seriously challenged to step up my game. Below is my list of resolutions for the coming year.

Get Back to Blogging
Some of you may have noticed that I haven’t blogged in awhile. In fact, I missed 2011 entirely.
At CodeMash, I met several very cool people for the first time who said that they knew me from my blog. It simply amazed (and flattered) me that, with no new content, this little programming blog was still making an impact. It’s time to dust things off around here.
Start a New F# Pet Project
I have neglected my love of F# for a long time, but conversations at CodeMash convinced me that I need to rekindle my romance.
Return to Twitter
My activity has really slowed to a crawl over the past few years, and I intend to correct that. I’m tired of feeling out of the loop.
Be at CodeMash Next Year
How did I allow myself to miss CodeMash for the last two years? I resolve to never let that to happen again.

F# Contracts on the Cheap

Dustin Campbell — Sun, 31 Oct 2010 18:26:02 GMT

In my current F# project, I’ve employed a very simple library to support basic contract checking. It’s not fantastically clever, but I thought I’d post it here in case others find it useful.

To use the library, pipe values into the various functions of the “Is” module. For example…


          let jz
: OpcodeRoutine = fun (i, proc) ->

  i.Operands.Length |> Is.EqualTo
1

  i.HasBranchOffset |> Is.True

  let x = i.Operands.[0] |> proc.GetOperandValue

  let res = x = 0us

  if i.BranchOffset.Condition = res then

    proc.Branch i.BranchOffset

If either of the two preconditions highlighted above fail, a ContractFailureException will be thrown.

The full module is below. Again, the following code isn’t exactly rocket science, but I’ve found it dead useful, and you might, too.


          exception ContractFailureException of string

[<RequiredModuleAccess>]
module Is =

  let inline private fail
message =

    raise <| ContractFailureException(message)

  let inline private failf
fmt =

    Printf.ksprintf fail fmt

  let inline True
condition =

    if condition <> true then

      fail "condition should
be true."

  let inline False
condition =

    if condition = true then

      fail "condition should
be false."

  let inline Null
obj =

    if obj <> null then

      fail "obj should be null."

  let inline Some
obj =

    match obj with

    | Some(_) -> ()

    | None -> fail "obj
should be Some."

  let inline None
obj =

    match obj with

    | Some(_) -> fail "obj
should be None."

    | None -> ()

  let inline NotNull
obj =

    if obj = null then

      fail "obj should not be
null."

  let inline EqualTo
expected value =

    if value <> expected then

      failf "value should be
equal to %A." expected

  let inline NotEqualTo
expected value =

    if value = expected then

      failf "value should not
be equal to %A." expected

  let inline LessThan
high value =

    if value >= high then

      failf "value should be
less than %A." high

  let inline LessThanOrEqualTo
high value =

    if value > high then

      failf "value should be
less than or equal to %A." high

  let inline GreaterThan
low value =

    if value <= low then

      failf "value should be
greater than %A." low

  let inline GreaterThanOrEqualTo
low value =

    if value < low then

      failf "value should be
greater than or equal to %A." low

  let inline InRange
low high value =

    if value < low || value >
high then

      failf "value should be
in range %A to %A." low high

  let inline NotInRange
low high value =

    if value >= low &&
value <= high then

      failf "value should not
be in range %A to %A." low high

  let inline Empty
(value : seq<_>) =

    if not (value |> Seq.isEmpty) then

      fail "value should be empty."

  let inline NotEmpty
(value : seq<_>) =

    if value |> Seq.isEmpty then

      fail "value should not
be empty."

  let inline OfType<'a>
(value : obj) =

    match value with

    | :? 'a -> ()

    | _ -> failf "value
should be of type %s." typeof<'a>.FullName

  let inline NotOfType<'a>
(value : obj) =

    match value with

    | :? 'a -> failf "value
should not be of type %s." typeof<'a>.FullName

    | _ -> ()

Enjoy!

Goings On

Dustin Campbell — Wed, 05 May 2010 15:00:52 GMT

The last several weeks have been pretty hectic for me. First, Visual Studio 2010 and .NET Framework 4 shipped. Of course, only those living in caves and under rocks missed that bit of news. However, that event forced me to leave my own cave and make a few public appearances.

April 12-15 - DevConnections, Las Vegas

One thing that I love about the Bellagio is how they go out of their way to make me comfortable by naming their convention center rooms after the Teenage Mutant Ninja Turtles. Let’s see, there’s Michelangelo… Raphael… Donatello… Huh? What do you mean the rooms were named after Renaissance painters?
April 26 - .NET Rocks Road Trip, Houston

Hanging out with Carl and Richard is always a blast. In the past, I’ve been left with stories that I can’t really share in mixed company. This time they turned on the microphones and pressed “record.”
June 7-10 – Tech Ed 2010, New Orleans

I’ll be there. Who else is coming?

Empty Nothings

Dustin Campbell — Thu, 04 Mar 2010 15:42:19 GMT

A few weeks ago, some of my colleagues and I were discussing the idiosyncrasies of various programming languages (as we often find ourselves doing—we’re kind of geeky that way), when one of us pointed out that the following code is completely valid C++0x syntax:

[](){}();

The “operator soup”¹ above defines a C++ lambda expression (denoted by the square brackets) which declares no parameters (the first empty parentheses) or body (the empty curly braces) and is immediately invoked (the final parentheses). Conceptually, this is a nop—an empty lambda that is immediately invoked.

We found ourselves fascinated by this idea of a do-nothing lambda, and went ahead to define the same thing in our respective languages. Our first attempt was C#.

()
=> { }();

While the code above looks quite pretty, it’s not exactly legal. In C#, lambdas must always have an explicit delegate type, so an ugly type-cast is required in order to compile:

((Action)(()
=> { }))();

Sigh, so close, yet so dissatisfying!

The stronger notion of type inference in F# allows for much more succinctness.²

(fun () -> ())() 

However, my favorite version is written in Visual Basic 10.


          Call (Sub() Exit Sub)()

What it lacks in succinctness,³ it makes up for with human-readable clarity.

How do you write a do-nothing lambda in your language?

¹One could also declare the square brackets with either an = or & operator inside to define how variables that are declared in the same scope as the lambda are captured within the lambda function’s closure. It’s amazing how much one can do without typing a single identifier character!

[&](){}();

²Note that the F# example contains a subtle difference from the others in that it returns a value of type Unit. This implies that the entire F# expression could be passed as an argument to another function, but that is not true of the other examples.

³Though it’s the same size as the C# version when unnecessary whitespace characters are removed.

Random Crash

Dustin Campbell — Wed, 03 Mar 2010 21:04:58 GMT


          Module RandomCrash

    Sub Main()

        Try

            Throw New Exception

        Catch ex As Exception When DateTime.Now.Ticks Mod 2
= 0

        End Try

    End Sub

End Module

Naming Anonymous Types with Generate from Usage

Dustin Campbell — Sat, 24 Oct 2009 18:01:40 GMT

Now that Visual Studio 2010 Beta 2 is finally out the door, I’ve had a bit more time to spend coding on some of my personal projects. Yesterday, I happened upon a cool trick while using the new Generate from Usage feature. It was so helpful to me that I thought others might benefit, so I’m sharing it here.

The Anonymous Type Problem

When you need to project some data from a LINQ expression, anonymous types can be enormously convenient.

Because anonymous types are… well… anonymous, they don’t have names that can be expressed in code. This is problematic if you want to expose an anonymous type as the return type of a function. I have run into this problem many times. When refactoring code, it’s easy to get into a situation like the one below.

So, how can you get around this problem? Well, there a few possibilities.

You could replace ??? with object and use reflection to get at the properties. (Yuck!)
You could make the function generic and add a parameter to “mumble” the anonymous type.¹ (Awkward!)
Assuming C# 4.0, you could replace ??? with dynamic.² (No compiler errors!)

Because none of these solutions is particularly savory, most of us are forced to create a new named type to replace the anonymous type. Thankfully, there are some fantastic third-party refactoring tools out there that can automate this tedious process, but if you don’t use one of these tools you’re stuck writing the code by hand.

Actually, no, that’s not quite true.

Generate from Usage to The Rescue!

In Visual Studio 2010, the new Generate from Usage feature makes the task of coding up new classes a snap! Just type a new name for the anonymous type in the editor, making the code look like a type constructor followed by an object initializer. Then, press Ctrl+. to expand the smart tag that immediately appears and choose the first suggestion to generate a new class.

Next, expand each smart tag in the object initializer to generate each property.

When you’re finished, you should have a brand new class containing each property, declared as auto-implemented properties. Cool!

For Visual Basic coders, Generate from Usage is even easier. Let’s start with the same LINQ expression in VB. (Notice the lack of the “_” line continuation characters. Hooray for VB10 implicit line continuation!)

Just like before, type the name of the new type that you wish to generate and press Ctrl+. to expand the smart tag. After choosing the first suggestion from the smart tag, you’re finished. The VB Generate Class feature will drill into the object initializer and generate all of the necessary properties at the same time that the class is generated.

Wrapping Up

Of course, this technique is not without flaws.

The resulting type is not immutable like the anonymous type that you’re replacing. To address this, you can easily modify the generated properties to be read-only.
The new type does not have the same structural equality semantics that anonymous types have. In practice, I’ve rarely run into an bug caused by anonymous type structural equality, but if this is a concern for you, use one of the excellent third-party tools that account for these differences.

¹See Wes Dyer's excellent article for an example of this clever trick.
²Check out Bill Wagner's post for details.

A Lover of Monkeys

Dustin Campbell — Fri, 26 Jun 2009 04:07:05 GMT

The other day, I caught a quick snapshot of Bethany playing with her favorite new toy:

A special thanks to my friend Joseph Hill for providing her favorite monkey!

CodeRush Xpress for C# and VB is Live!

Dustin Campbell — Fri, 15 May 2009 00:59:29 GMT

Last October, DevExpress released a massively powerful FREE tool specifically for C# developers called CodeRush Xpress. Today, in partnership with Microsoft, DevExpress has outdone themselves with a brand new version of CodeRush Xpress containing full support for both C# and Visual Basic. This release merges CodeRush Xpress with Refactor! for Visual Basic, creating one giant über-tool with an amazing features:

Duplicate Line
Highlight All References
Increase or Reduce Selection
Smart Clipboard Operations
Generate from Using (TDD)
Quick Navigation Window
Quick File Navigation
60+ Refactorings!

Get it while it’s hot! You can download your FREE copy of CodeRush Xpress for Visual Studio 2008 from http://devexpress.com/crx.

YAPES: Problem Ten

Dustin Campbell — Tue, 17 Mar 2009 10:13:48 GMT

I’m back again with another post in my Yet Another Project Euler Series. As a reminder, my approach to these problems is to try to find the most beautiful solution that I can using F#. While performance is an important, I’m looking for the most elegant solutions that I can find.

Project Euler problem ten is another dealing prime numbers.

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.

Find the sum of all the primes below two million.

As you can guess, there’s not too much to this problem. In fact, we’ll declare just one simple supporting function.


          let lessThan
y x = x < y 

Using the above function and our existing prime number generator, our solution is short and simple.

primes

  |> Seq.take_while (lessThan 2000000L)

  |> Seq.sum

I feel a little guilty that this post is so short, but that’s the benefit of spending two whole blog posts working out a prime number generator for problem seven.