Classic mashup: An FSharp coding dojo

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Yesterday I attended the FSharp Dojo: Picasquez and Velasso at Skills Matter in London.

This dojo is all about manipulating images and creating mashups, like this:

Classic mashup image

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Finding overlapping rectangles

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This is a classic quick programming puzzle: write a function that, given two rectangles, will determine if the triangles overlap. I found this puzzle over on http://leetcode.com/2011/05/determine-if-two-rectangles-overlap.html, and thought it was worth talking about because the solution is wrong, and it was interesting to me because it shows how thinking about a domain can influence your implementation of behaviour.

The mistake made by the original author is with one of the assumptions / interpretations made during their reasoning process. They said “when two rectangles overlap, one rectangle’s corner point(s) must [be contained within] the other rectangle”. This appears reasonable, given the example below:

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Programming interview questions

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At my last company I was a team lead and, as such, was one of those responsible for interviewing new candidates. Although I didn’t enjoy the process much, I did become interested in the sorts of technical questions / exercises we could set to get a good judge of a candidate’s abilities.

Developer interviews typically cross a number of disciplines (often arranged in stages) that may include:

  1. a CV / résumé screening
  2. a telephone interview
  3. face-to-face questions and answers
  4. a technical exercise or written test to be done at home, or to be done in front of the interviewers

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Be careful with attribute routes in projects with both MVC and WebApi

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This is a pretty easy trap to fall into! I have been tearing my hair out recently trying to figure out why, in my pet project website, one of the controllers was not having any routes generated. I was able to inspect the generated routes during debugging by setting a breakpoint after the line

routes.MapMvcAttributeRoutes();

and inspecting the routes object. Sure enough routes for the HomeController and ChatController were there, but nothing for the BlogController. What could it be? Everything appeared to be set up correctly:

[<RoutePrefix "blog">]
type BlogController
    (
    _filterBlogPostsQuery: FilterBlogPostsQuery,
    _getBlogPostQuery: GetBlogPostQuery,
    _getAllBlogPostsQuery: GetAllBlogPostsQuery) =
 
    inherit MySiteController()
        
    [<Route "">]
    member this.Index() =
        task {
            let! posts = _getAllBlogPostsQuery()
 
            return this.View posts
        }

Eventually I figured it out after stumbling across this StackOverflow post: https://stackoverflow.com/questions/25727305/asp-mvc-5-attribute-routing-not-registering-routes/. It turns out that, when you have a project with both WebApi and MVC installed, you have two RoutePrefixAttribute classes and two RouteAttribute classes available; one in System.Web.Http and one in System.Web.Mvc. Make sure you’re referencing the right one!

This was also, perhaps, a consequence of using F#, and the fact that the Visual Studio tooling is not as feature-complete as the other languages; I didn’t get a warning from the compiler that there was a naming clash between these two Attributes, despite the fact that I had both the System.Web.Http and System.Web.Mvc namespaces opened at the top of the file. In the end, removing the System.Web.Http namespace was all that was needed to fix the problem.

Lesson learned. Still, it highlights a fragmentation issue between the Mvc and WebApi teams; one which I hope will be fixed in ASP.NET vNext (I assume this will be the case because MVC and WebApi will now share the same base controller).

SOLID, CQRS and functional dependency injection

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I was prompted to write this after reading Mark Seemann’s post (http://blog.ploeh.dk/2014/03/10/solid-the-next-step-is-functional/)

From http://kozmic.net/2012/10/23/ioc-container-solves-a-problem-you-might-not-have-but-its-a-nice-problem-to-have/:

Fol­low­ing Sin­gle Respon­si­bil­ity Prin­ci­ple will lead you down the road of hav­ing plenty of small classes.

Open Close Prin­ci­ple will lead you down the way of encap­su­lat­ing the core logic of each type in it, and then del­e­gat­ing all other work to its depen­den­cies, caus­ing most of those small classes to have mul­ti­ple dependencies.

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“The missing number” programming exercise

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This is another entry in the brain teaser / programming exercise / interview question collection. It goes like this:

Imagine a list of consecutive numbers. The list is, however, unsorted and one number is missing. Write a function that finds which number is missing with the constraint that you may only allocate O(1) memory (i.e. the amount of memory you use cannot vary with the size of the list).

The fact that the list is unsorted means that you cannot step through in a pairwise fashion and compare each element of the list to the next to see if they are consecutive. Sure, we could sort the array first but the fact that you cannot allocate memory according to the size of the array means that you cannot create a helper array in the solution and certain sorting algorithms are ruled out. An in-place sorting algorithm such as quicksort would do it, but I don’t think that sorting the array is the best way to go.

What if we sum the list? We’re talking about a sequence of integers, after all. It turns out that there is a linear formula for calculating the sum of a sequence of consecutive integers: sum(1..n) = n(n+1) / 2.

This means that you can work out what the sum of the input list should be; all we must do it actually sum the list and the difference between the two will be the missing number. Our implementation then looks like this:


let findMissing (list : int array) =
    let n = list.Length + 1
    let expectedSum = n * (n + 1) / 2
    let actualSum = Seq.sum list
    
    expectedSum - actualSum

Note the let n = list.Length + 1. We have to add one to the length of the list to get the max element in the list because one element is missing. After that it’s pretty simple!

Exceptions and error logging

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At my current work I see an awful lot of code that looks like this:

public class ServiceA : IServiceA
{
    private readonly ILogger _logger;

    public ServiceA(ILogger logger)
    {
        _logger = logger;
    }

    public void DoIt()
    {
        try
        {
            // Do something...
        }
        catch(Exception e)
        {
            _logger.LogError(e);
            throw;
        }
    }    
}

At first blush this looks reasonable: it has constructor-based dependency injection, an abstraction, error logging… What could my issue be?

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