Mark Needham

Thoughts on Software Development

Functional collectional parameters: Some thoughts

with 6 comments

I’ve been reading through a bit of Steve Freeman and Nat Pryce’s ‘Growing Object Oriented Software guided by tests‘ book and I found the following observation in chapter 7 quite interesting:

When starting a new area of code, we might temporarily suspend our design judgment and just write code without attempting to impose much structure.

It’s interesting that they don’t try and write perfect code the first time around which is actually something I thought experienced developers did until I came across Uncle Bob’s Clean Code book where he suggested something similar.

One thing I’ve noticed when working with collections is that if we want to do something more complicated than just doing a simple map or filter then I find myself initially trying to work through the problem in an imperative hacky way.

When pairing it sometimes also seems easier to talk through the code in an imperative way and then after we’ve got that figured out then we can work out a way to solve the problem in a more declarative way by making use of functional collection parameters.

An example of this which we came across recently was while looking to parse a file which had data like this:


The file was being processed later on and the values inserted into the database in field order. The problem was that we had removed two database fields so we needed to get rid of the 2nd and 3rd values from each line.

var stringBuilder = new StringBuilder();
using (var sr = new StreamReader("c:\\test.txt"))
    string line;
    while ((line = sr.ReadLine()) != null)
        var values = line.Split(',');
        var localBuilder = new StringBuilder();
        var count = 0;
        foreach (var value in values)
            if (!(count == 1 || count == 2))
        stringBuilder.AppendLine(localBuilder.ToString().Remove(localBuilder.ToString().Length - 1));
using(var writer = new StreamWriter("c:\\newfile.txt"))

If we wanted to refactor that to use a more declarative style then the first thing we’d look to change is the for loop populating the localBuilder.

We have a temporary ‘count’ variable which is keeping track of which column we’re up to and suggests that we should be able to use one of the higher order functions over collection which allows us to refer to the index of the item.

In this case we can use the ‘Where’ function to achieve this:

while ((line = sr.ReadLine()) != null)
    var localBuilder = line.Split(',').
                        Where((_, index) => !(index == 1 || index == 2)).
                        Aggregate(new StringBuilder(), (builder, v) => builder.Append(v).Append(","));
    stringBuilder.AppendLine(localBuilder.ToString().Remove(localBuilder.ToString().Length - 1));

I’ve been playing around with ‘Aggregate’ a little bit and it seems like it’s quite easy to overcomplicate code using that. It also seems that when using ‘Aggregate’ it makes sense if the method that we call on our seed returns itself rather than void.

I didn’t realise that ‘Append’ did that so my original code was like this:

    var localBuilder = line.Split(',').
                        Where((_, index) => !(index == 1 || index == 2)).
                        Aggregate(new StringBuilder(), (builder, v) => {
                           return builder;

I think if we end up having to call functions which return void or some other type then it would probably make sense to add on an extension method which allows us to use the object in a fluent interface style.

Of course this isn’t the best solution since we would ideally avoid the need to remove the last character to get rid of the trailling comma which could be done by creating an array of values and then using ‘String.Join’ on that.

Given that I still think the solution written using functional collection parameters is easier to follow since we’ve managed to get rid of two variable assignments which weren’t interesting as part of what we wanted to do but were details about that specific implementation.

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Written by Mark Needham

January 20th, 2010 at 10:45 pm

Posted in Coding,Hibernate

Tagged with ,

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  • One approach to thinking functionally when you have one kind of collection (in this case a text file, collection of lines of fields) and you want another kind of collection (in this case a text file, a collection of lines of fewer fields) is to start by drawing a pipeline. e.g.


    This kind of forces a transformational mindset, where starting with a loop/while/for kind of keeps you in imperative mode by habit, as you noted.

    Initially don’t worry about what goes in the steps in the middle or how many there should be. Start with one, e.g. original line of unparsed text is transformed to a list of parsed fields.

    Get that working. Then begin again, but with the list of parsed fields as your “original” – because you are one step further than before.

    How that simple drawing is turned into code depends on your language, library, and experience. But at a minimum you are forced to pass values from step to step rather than thinking about “updating in place”.

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  • All the expectations that come with thorough testing can be completely paralyzing.

    Really clear use of Aggregate(), I like it!

    Removing the last seperator like the “,” in your example is pretty common. It should be a recognizable idiom for people who look at your code later. If you stick to it.

    @Patrick: Your suggestion of drawing a picture is pretty valuable. I like it. Think first about the chain of functions, and then implement each function to build the chain.

  • Though Aggregate() is a nicely used, most people seem not to know about String.Join() That way you don’t have to mess around with commas.

  • @Thomas – yeh you’re right, someone pointed that out on a previous post I wrote about this too! I guess if we were to take that approach then getting the various items into an array and then applying String.Join would be the way to go.