Lambda for Dummies…anyone, anyone? I think not

Question

In my quest to understand the very odd looking ' => ' operator, I have found a good place to start, and the author is very concise and clear:

parameters => expression

Does anyone have any tips on understanding the basics of lambdas so that it becomes easier to 'decipher' the more complex lambda statements?

For instance: if I am given something like (from an answer I received here):

filenames.SelectMany(f => 
        Assembly.LoadFrom(f).GetCustomAttributes(typeof(PluginClassAttribute), true)
        .Cast<PluginClassAttribute>()
        .Select(a => a.PluginType)
).ToList();

How can I go about breaking this down into more simple pieces?

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UPDATE: wanted to show off my first lambda expression. Don't laugh at me, but I did it without copying someone's example...and it worked the first time:

public ModuleData[] GetStartModules( )
{ return modules.FindAll(start => start.IsBatch == true).ToArray(); }

Answer 1

Lambda calculus is common in many programming languages. They're also called anonymous functions in some languages. Though different languages have different syntax for lambda, the principle is the same, and their various parts are usually identical.

Perhaps the most famous one is Javascript's anonymous functions.

lol = function() {laugh()}
# is equivalent to
function lol() {laugh()}

What's the difference? Well, sometimes you don't want to go through the trouble of creating a function just to pass it somewhere once and then never again.

window.onload = function() {laugh()}
# is way easier than
function lol() {laugh()}
window.onload = lol

You can see the wikipedia article for indept information or you can skip directly to Lambda in programming in the same article.

Answer 2

One good simple explanation aimed at developers who are firmiliar with coding but not with lambdas is this simple video on TekPub

TekPub - Concepts: #2 Lambdas

You obviously have lots of feedback here, but this is another good source and a simple explanation.

Answer 3

I know this is kinda old but i came here trying to make sense of all this lambda stuff. By the time i finished pouring over all the answers and comments, i had a better understnading of lambda and thought i should just add this simple answer(from a learner's perspective to learners):

Dont confuse a => a + 1 as meaning add 1 to a and return the result to a. (this is most likely a source of confusion for beginners. Instead see it like this: a is the input parameter into a function(unnamed function) and a + 1 is the statement(s) in the function(unnamed function constructed 'on the fly').

Hope this helps :)

Answer 4

This is just the notation of C# to write down a function value. It doesn't require giving the function a name, hence this value is sometimes called an anonymous function. Other languages have other notations, but they always contain a parameter list and a body.

The original notation invented by Alonzo Church for his Lambda calculus in the 1930ies used the greek character lambda in the expression λx.t to represent a function, hence the name.

Answer 5

Let's dissect your code sample:

filenames.SelectMany(f => 
        Assembly.LoadFrom(f).GetCustomAttributes(typeof(PluginClassAttribute), true)
        .Cast<PluginClassAttribute>()
        .Select(a => a.PluginType)
).ToList();

So, we start off with a string[] called filenames. We invoke the SelectMany extension method on the array, and then we invoke ToList on the result:

filenames.SelectMany(
   ...
).ToList();

SelectMany takes a delegate as parameter, in this case the delegate must take one parameter of the type string as input, and return an IEnumerable<T> (Where the type of T is inferred). This is where lambdas enter the stage:

filenames.SelectMany(f => 
        Assembly.LoadFrom(f).GetCustomAttributes(typeof(PluginClassAttribute), true)
).ToList()

What will happen here is that for each element in the filenames array, the delegate will be invoked. f is the input parameter, and whatever comes to the right of => is the method body that the delegate refers to. In this case, Assembly.LoadFrom will be invoked for filename in the array, passing he filename into the LoadFrom method using the f argument. On the AssemblyInstance that is returned, GetCustomAttributes(typeof(PluginClassAttribute), true) will be invoked, which returns an array of Attribute instances. So the compiler can not infer that the type of T mentioned earlier is Assembly.

On the IEnumerable<Attribute> that is returned, Cast<PluginClassAttribute>() will be invoked, returning an IEnumerable<PluginClassAttribute>.

So now we have an IEnumerable<PluginClassAttribute>, and we invoke Select on it. The Select method is similar to SelectMany, but returns a single instance of type T (which is inferred by the compiler) instead of an IEnumerable<T>. The setup is identical; for each element in the IEnumerable<PluginClassAttribute> it will invoke the defined delegate, passing the current element value into it:

.Select(a => a.PluginType)

Again, a is the input parameter, a.PluginType is the method body. So, for each PluginClassAttribute instance in the list, it will return the value of the PluginType property (I will assume this property is of the type Type).

Executive Summary
If we glue those bits and pieces together:

// process all strings in the filenames array
filenames.SelectMany(f => 
        // get all Attributes of the type PluginClassAttribute from the assembly
        // with the given file name
        Assembly.LoadFrom(f).GetCustomAttributes(typeof(PluginClassAttribute), true)
        // cast the returned instances to PluginClassAttribute
        .Cast<PluginClassAttribute>()
        // return the PluginType property from each PluginClassAttribute instance
        .Select(a => a.PluginType)
).ToList();

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Lambdas vs. Delegates
Let's finish this off by comparing lambdas to delegates. Take the following list:

List<string> strings = new List<string> { "one", "two", "three" };

Say we want to filter out those that starts with the letter "t":

var result = strings.Where(s => s.StartsWith("t"));

This is the most common approach; set it up using a lambda expression. But there are alternatives:

Func<string,bool> func = delegate(string s) { return s.StartsWith("t");};
result = strings.Where(func);

This is essentially the same thing: first we create a delegate of the type Func<string, bool> (that means that it takes a string as input parameter, and returns a bool). Then we pass that delegate as parameter to the Where method. This is what the compiler did for us behind the scenes in the first sample (strings.Where(s => s.StartsWith("t"));).

One third option is to simply pass a delegate to a non-anonymous method:

private bool StringsStartingWithT(string s)
{
    return s.StartsWith("t");
}

// somewhere else in the code:
result = strings.Where(StringsStartingWithT);

So, in the case that we are looking at here, the lambda expression is a rather compact way of defining a delegate, typically referring an anonymous method.

And if you had the energy read all the way here, well, thanks for your time :)

Answer 6

CodeProject had a nice introductory article lately: C# Delegates, Anonymous Methods, and Lambda Expressions – O My!