F#
What I’ve learnt so far
Mark Needham

What is F#?

Another language that
runs on the CLR

Interoperable with
other .NET languages

Functional...

...but with Object
Oriented features too

Statically typed with type
inference

Why did I want to learn F#?

Different mindset

Immutability

Concurrency

How did I go about learning
F#?

Functions

Can live on their own

let add a b = a + b
http://intwoplacesatonce.com/?p=9

Type inference

let add a b = a + b
val add : int ‐> int ‐> int

let add (a:string) (b:string) = a + b
val add : string ‐> string ‐> string

Lots of ways to do the same thing

let withLinks (statuses:seq<TwitterStatus>) =
Seq.filter
(fun s ‐> s.Text.Contains("http"))
statuses

let withLinks (statuses:seq<TwitterStatus>) =
statuses |>
Seq.filter (fun s ‐> s.Text.Contains("http"))

Forward operator (|>)
('a ‐> ('a ‐> 'b) ‐> 'b)

let withLinks (statuses:seq<TwitterStatus>) =
Seq.filter (fun s ‐> hasLink s) statuses
let withLinks (statuses:seq<TwitterStatus>) =
statuses |> Seq.filter (fun s ‐> hasLink s)
let withLinks statuses =
statuses |> Seq.filter (fun (s:TwitterStatus) ‐> hasLink s)
let withLinks (statuses:seq<TwitterStatus>):seq<TwitterStatus> =
statuses |> Seq.filter (fun s ‐> hasLink s)
let withLinks : seq<TwitterStatus> ‐> seq<TwitterStatus> =
Seq.filter (fun s ‐> hasLink s)

Composing functions

let statuses = seq { yield new TwitterStatus() }
let sortedStatuses = Seq.sortBy
(fun (s:TwitterStatus) ‐> s.Id)
statuses
let firstStatus = Seq.hd sortedStatuses

seq { yield new TwitterStatus() }
|> Seq.sortBy (fun (s:TwitterStatus) ‐> s.Id)
|> Seq.hd

Functions are cool but
lack structure

Refactoring to objects

type Tweets = {
TwitterStatuses:seq<TwitterStatus>
}

let myTweets = {
TwitterStatuses = TwitterService.GetStatuses 100
}

let withLinks (statuses:seq<TwitterStatus>) =
statuses |> Seq.filter (fun s ‐> s.Text.Contains("http"))
let print (statuses:seq<TwitterStatus>) =
for s in statuses do
printfn "[%s] %s" s.User.ScreenName status.Text
type Tweets with
member t.print() = print t.TwitterStatuses
member t.withLinks() =
{
TwitterStatuses = withLinks t.TwitterStatuses
}

Leads to simpler/easier to
understand code

Helped by unit testing

xUnit.NET

[<Fact>]
let should_recognise_message_with_link_as_a_link () =
let messageWithLink =
(new MessageBuilder(message = "http://www.google.com")).Build()
Assert.True(messageWithLink |> hasLink)

type MessageBuilder(?message:string, ?user:string) =
let buildMessage message user =
new TwitterStatus(
Text = message,
User = new TwitterUser(ScreenName = user)
)
member mb.Build() =
buildMessage (if message.IsSome
then message.Value
else "")
(if user.IsSome
then user.Value
else "")

[<Fact>]
let should_not_show_any_tweets_by_me () =
let messageByMe =
(new MessageBuilder(message = "",
user = "markhneedham")).Build()
let tweets = seq { yield messageByMe;
yield messageByMe;
yield messageByMe }
let tweetsExcludingMe = tweets |> excludeSelf
Assert.Equal(0, tweetsExcludingMe |> Seq.length)

Still doing all the processing at
the end

Asynchronous Workflows

“Perform a computation in a
background thread without
blocking execution on current
thread”

Couch
Twitter DB
Write to
Tweets with
Tweet
Main links
Processor
Processor
Has link?

What am I learning next?

Programming in a more
functional way

Loops -> List functions/recursive
Objects -> ADTs
Classes -> Modules/Signatures/Functors

Quotations

Where can you learn more?

Books

Hub FS (http://cs.hubfs.net)
Chris Smith’s Blog
(http://blogs.msdn.com/chrsmith/)
Wes Dyer’s Blog
(http://blogs.msdn.com/wesdyer/)
On the web
Tomas Petricek’s Blog
Matthew Podwysocki’s Blog (http://tomasp.net/blog/)
(http://weblogs.asp.net/Podwysocki/)
Robert Pickering’s Wiki
(http://www.strangelights.com/FSharp/Wiki/)

Mark Needham
mneedham@thoughtworks.com
Blog: http://www.markhneedham.com/blog
Twitter: markhneedham