As already mentioned, features typical for functional languages are now available in many non-functional languages like C 3.
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Lambda functions are one of the additions to C in the 3. The development of generics for C and. NET 2. Type inference, which simplifies declaration of variables in C 3. The succinctness of the F programming style is possible only thanks to the use of type inference, but thanks to the combination with generics we can get the usual safety of statically typed languages.
Parallel execution means that two pieces of code execute at the same time for example on a multi-core processor.
Let me demonstrate what I mean using an example. A typical implementation of something like this in C would look like this:. As you can see, we created a variable res and modified it two times when reading the input value from the console. The only occurrence of this symbol in a second example is when initializing a variable value, which has a different meaning then assignment operator — instead of changing a value of an existing variable it creates a new variable with a specified initial value.
You can probably imagine how to write some extremely simple programs without using traditional variables and assignment operator, but once you start thinking about slightly more complicated problems, things become difficult. Thus we need to do a more fundamental change in the code and use a technique called recursion instead of using loops:. As you probably already know, recursion means that a function SumNumbers in our case calls itself in our case this is when we calculate a value for sumRest variable.
The state of the computation, which was originally stored in a mutable variable, is now expressed using recursion. As you may guess from this introduction, the use of values instead of variables and keeping program state using recursion is an important feature whenever you want to write any interesting F program. How can we separate the functionality that will vary during every use from a recursive part of the code?
Let me demonstrate the idea on a previous example. This function should take two integers as an argument and should return an integer as a result. The use of functions as an argument is called using functions as values, because we use a function in a place where other values are used such as method argument or return value.
Functional data types are usually immutable as well, which means that a value of the type cannot change. The only way we can change a value is by creating a copy with a modified value. An example that you already know is a string type, which is immutable in. As you can see in this example, working with immutable data types is quite simple. Instead of modifying the value, we just declare a variable and store the new value returned by the operation in this variable and leave the original value unchanged. Of course, this approach can be used for other data types as well.
This previous example can be of course written more elegantly, without declaring all the unnecessary temporary variables. We could just call the Add operation straight away on the returned temporary list:. Instead of copying the whole content of l1 , we can simply create an object for example l2 that contains the new value 0 and a reference to the original list l2.
The use of immutable data types means that a code cannot perform any side-effect. By side-effect I mean for example modifying a value of global variable. A function written in this way is much closer to the mathematical notion of a function and is usually called pure function by functional programmers. Functions like this are very interesting, because they have similar properties as mathematical functions. The only thing that the function can do is to return a value, which means that the order of evaluation is not important and evaluating a function two times leads always to the same result.
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This means that the following two examples perform exactly the same thing:. In the second version, we optimize the code to run the ProcessList function only once:. Minimizing the use of side-effects is very useful if you want to have clearer way for thinking about the code, but it is also extremely valuable when developing applications that run on multi-core systems.
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As already mentioned, F is a. NET language, so we can access. NET libraries, but there is also a lot of functionality in the F libraries, that is suited especially for functional programming in F. F can be downloaded from the F Web site [ 2 ] and it contains integration with Visual Studio you can even use the free Shell version [ 3 ]. It can be used either from console fsi.
This command sends the selected text to the F interactive window compiles it and executes it, so we can immediately see the results. The following listing shows the content of the F interactive window after entering and executing a line of code that calls the printfn function, which is a function defined in the standard F library:. Next, you can see a line of code with the printfn function followed by a two semicolons - two semicolons are used to terminate the input, so that you can enter multiple lines of text when writing the code directly in F interactive.
Finally, the last line shows the result of the calculation. In F , you can treat every block of code as a calculation with some result. The first command entered to the F interactive is a declaration of the function to compute the factorial.
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The function is declared using let rec keyword. These keywords are followed by the function name and by the arguments of the function here we have just one argument called num and by the equals sign. The equals sign separates the function header from the body. In our example, the body is just one match expression. In our example, we have just two patterns. The first pattern assigns the value to a variable called n and tests whether n is less than or equal to one. This is written as an additional condition using when clause. This code demonstrates one more essential F feature, which is type inference.
If you look at the result printed on the next line, you can see that we declared a function that takes int as an argument and returns int as a result.
What the F compiler did is that it analyzed the code. The type inference mechanism in F is quite advanced and as you can see it attempts to deduce as much as it can automatically. It creates a simple form with a label on it. You can see the source code in the following listing with a screenshot of the running application below:. This example demonstrates several F features that are essential for accessing. NET components. First, you can see the use of open keyword, which makes types from a. The next block demonstrates creating the controls.
This is done by creating standard. NET objects using the usual new keyword for creating objects.
When creating an object instance in F , it is also possible to specify some properties of the object directly during initialization. Finally, we call two. NET methods to add label to the form and show the form. Run instead of frm. Using F interactive has one interesting advantage — we can develop the program in an iterative style, meaning that we can write some code, immediately see the result and then add some features and see how it affects the running program. The code first creates a standard. NET random number generator object, which is then used in the handleClick function for generating a new color of the background.
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