Business Logic and F#

One of the reasons I like F# so much is that it allows me to think about the problem I am trying to solve, not about the language syntax and coding around language constructs.  Consider this example. 

I am putting on an art show in my neighborhood and I managed to obtain 3 paintings of cultural significance:

Starry Night

Sunday Afternoon on the Island of La Grande Jatte

Dogs Playing Poker

Each painting is in its own room and due to the volume of people that the art gallery can support, a person can only visit 1 painting.  1,000 tickets sold and all 1,000 people are going to show up.  This is a hot event.

I needed a way to forecast how many people will go into each room.  Since all 3 paintings are immensely popular, I could assume that each room will have 1/3 the number of visitors.  However, I wanted to be be a bit more precise and I know that each painting has a certain number of tags associated with them:

Assuming that people will want to go see paintings with tags that interest them, paintings that have tag overlap will split visitors, paintings with no tag overlap will see more visitors, and paintings with more tags will draw more visitors.  In Excel:

Putting this to code, I opened up the F# REPL and created my art show like so:

 1 
 2 type Painting = {id:int;name:string;tags:string}
 3 type ArtShow = {id:int;name:string;expectedAttendance:int;paintings:Painting list}
 4 
 5 let painting0 = {id=0;
 6                 name="Starry Night";
 7                 tags="Impressionism;Nature"}
 8 let painting1 = {id=1;
 9                 name="Sunday Afternoon on the Island of La Grande Jatte"; 
10                 tags="Impressionism;Nature;LeisureActivities"}
11 let painting2 = {id=2;
12                 name="Dogs Playing Poker";
13                 tags="Modernism;LeisureActivities"}
14 let paintings = [painting0;painting1;painting2]
15 
16 let artShow = {id=0;
17                 name="Art Extravaganza";
18                 expectedAttendance=1000;
19                 paintings=paintings}
20 

I then needed a way of uniquely identifying the tags.  Enter the goodness of piping and high order functions:

1 let tagSet = artShow.paintings |> Seq.map(fun p -> p.tags)
2                                     |> Seq.collect(fun t -> t.Split(';'))
3                                     |> Seq.groupBy(fun t -> t)
4                                     |> Seq.map(fun (id,t) -> id, t |> Seq.length)
5 

I then needed a way of assigning number of people to tags.  Easy enough (this could have been part of the code block above but I split it for illustrative purposes)

1 let visitorsPerTag = artShow.expectedAttendance / (tagSet |> Seq.length)
2 let tagSet' = tagSet |> Seq.map(fun (id,c) -> id, visitorsPerTag/ c )

And then a function that calculates the number of expected visitor based on that the individual painting:

1 let tagModifier(painting: Painting) =
2     let tags = painting.tags.Split(';')
3     tags |> Seq.map(fun pt -> tagSet' |> Seq.find(fun(t,c) -> pt = t))
4                                       |> Seq.sumBy(fun(t,c) -> c )

And running it against my show’s paintings gives me the expected values:

1 artShow.paintings |> Seq.map(fun p -> p, tagModifier(p))
2 

So this is why I love F#.  The REPL and the language helped me reason and solve the problem.  You can see the gist here.

After note: I sent the same challenge to some C# devs I know about how they would reason and then code the answer.  No one took me up on it.

System.AggregateException using Tweetinvi

Dear Future Jamie

If you are using TweetInvi in a new project and you get a System.AggregateException

And that exception contains a single Inner exception of System.IO.FileNotFoundException and the exception reads “cannot load System.Http.Primitives”

Install Microsoft.Net.Http in the calling project (in this case it was the unit test project).

Love

Current Jamie

PS You should really exercise more

Set For List Comparisons in F#

Dear Jamie Of The Future:

Next time you want to see if there are elements in 2 different lists, use Set

1 let tags0 = Set.ofList(["A";"B";"C"])
2 let tags1 = Set.ofList(["A";"D"])
3 let tags2 = Set.ofList(["A";"B"])
4 let tags3 = Set.ofList(["D"])
5 
6 Set.intersect tags0 tags1 
7 Set.intersect tags0 tags2 
8 Set.intersect tags0 tags3

Love, Jamie of May 2015

PS.  You really should exercise more…

Using the XML Type Provider

Dear Future Jamie:

If you want to use the XML Type Provider to read an XML document from the web and you see something like this:

You need to add a reference to System.Xml.Linq.  The easiest way is to do Add.Reference in the solution explorer and and copy/paste the path from its property window into your script:

And then you should be cooking with gas:

Love,

Jamie of May 2015

PS: You really should exercise more…