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For nonstop operations, you need multiple computers, redundant power supplies, multiple network interfaces and reliable networks, cooling systems that never fail, and cables that system administrators cannot trip over, not to mention engineers who are well practiced in their maintenance skills.
Considering that this target has been achieved at a fraction of the effort that would have been needed in a conventional programming language, it is still something to be very proud of.
How did Erlang contribute to the success of the AXD? It supports incremental development, with the absence of side effects, making it easier to add or modify single components.
Support for robustness and concurrency is built into the language and available from the start. Erlang was very popular with the programming teams that found they were building much more compact code, thus dramatically improving their productivity.
Ericsson has gone on to use Erlang on other projects across the company, including a SIP telephony stack, control software for wireless base stations, telephony gateway controllers, media gateways, broadband solutions, and in GPRS and 3G data transmission.
And these are just a few of the many we are allowed to talk about. His system consisted of three components: a storage engine, a view engine, and a query language.
The complexity of his components increased, and when he started hitting concurrency issues, he felt like he had hit a wall.
He stumbled upon Erlang, downloaded it, and quickly came to the realization that it would solve his problems. From the world Damien was coming from, Erlang initially sounded very complicated, and he believed it would be hard to learn.
But when he got down to the details, what instead struck him was the simplicity of the language. Just like Erlang, many of its features were ahead of its time.
It was never released to the public. Case Studies 11 but to get something working reliably ended up taking much less talent and time than any of the other languages he knew.
Erlang gave Damien the features he needed for CouchDB at a fraction of the effort of using conventional languages. The first benchmarks on the code, even before it was profiled, allowed in excess of 20, simultaneous connections.
Once it was released, CouchDB started getting lots of attention in the open source community. Damien made the decision to release the code under the Apache license, giving him the freedom he needed to continue development.
Today, CouchDB is one of the best known Erlang open source applications currently being used in production systems worldwide.
What happened to Damien? Indeed, this was an urban legend among Ericsson programmers long before Erlang was released as open source.
But until recently, there was very little scientific evidence to back up these claims. Quicksort Chapter 9 using list comprehensions or remote procedure call server examples Chapter 11 , both of which we cover in this book, were used to argue the case.
When comparing programming languages, however, you must benchmark whole systems in the application domain for which those languages were designed, not code snippets or simple functions.
When we first heard about this grant, our reaction was, why not speak with Ericsson and get it over with? These systems handled digital communication streams for pocket radio systems as used by emergency services.
The DM been written with fault tolerance and reliability in mind. The DCC was a pure Erlang implementation. Comparisons were made of the performance, robustness, productivity, and impact of the programming language constructs.
The interesting conclusions of this research came with the pure Erlang implementations. Although the throughput might sound surprising at first, it was a result of Erlang and its lightweight concurrency model being the right tool for the task.
The mobility application in question had lots of concurrency, short messages, and little in terms of heavy processing and number crunching.
They do, however, demonstrate an important property of Erlang-based systems, which are stable under heavy loads and recover automatically when the load drops.
Although Erlang pioneers argued their case of shorter and more compact code based on experience, this study has finally provided empirical data to support the claims.
Trinder, and D. The philosophy used to develop Erlang fits equally well with the development of Erlangbased systems.
It is not good enough to have ideas, you must also be able to implement them and know they work. Make mistakes on a small scale, not in a production project.
In line with these quotes, all successful Erlang projects should start with a prototype, and Erlang has all the support you need to get a prototype up and running quickly.
Working prototypes usually cover a subset of the functionality and allow end-to-end tests of the system.
If, for example, you were building an IM server a recurring theme throughout this book , valid functionality to test could include the ability to sign on and send messages to a remote server without worrying about issues such as redundancy, persistency, and security.
Software development in Erlang is best achieved using an agile approach, incrementally delivering systems of increasing functionality over a short cycle period.
Teams should be small in size, and, where possible, tests should be automated. Testing is aided by EUnit for unit testing, and Common Test for system testing.
Other tools include cover, providing coverage analysis, and Dialyzer, a static analysis tool that identifies software discrepancies such as type errors, dead code, and unsafe code.
Once you have achieved success on a small scale, you can start to think bigger! The website for this book and the Appendix contain links about where you can go to learn more about Erlang itself, the tools that support program development, and the Erlang community.
Erlang has assignment, but not as you know it from other imperative languages, because you can assign to each variable only once. Erlang has pattern matching, which not only determines control flow, but also binds variables and pulls apart complex data structures.
Erlang pattern matching is different in subtle ways from other functional languages. We conclude the chapter by showing how to define Erlang functions and place them into modules to create programs, but we start by surveying the basic data types in Erlang.
Integers Integers in Erlang are used to denote whole numbers. They can be positive or negative and expressed in bases other than The notion of a maximum size of integers in Erlang does not exist, and so arbitrarily large whole numbers can be used in Erlang programming.
When large integers do not fit in a word, they are internally converted to representation using an arbitrary number of words, more commonly known as bignums.
While bignums give completely accurate calculation on arbitrary-size integers, this makes their implementation less efficient than fixed-size integers.
The only limit on how large an integer can become depends on the physical constraints of the machine, namely the available memory.
More details about obtaining and running Erlang are given in the Appendix. Do not forget to terminate your expression with a period or full stop.
When you press Enter, and the line you typed in is terminated by a full stop, the shell will evaluate what you typed, and, if successful, will display the result.
Note how the various integer notations are all translated and displayed in base 10 notation. Ignore errors for the time being, as we will cover them in Chapter 3.
To recover from having made an error, just press the Enter key a few times, add a full stop, and terminate with a final press of the Enter key.
If you want to exit the shell, just type q followed by a full stop. Some examples of floats include: The E is a conventional floating-point notation stating that the decimal point has been moved 10 positions to the left: 1.
Before going on to the next section, try typing a few floats in the Erlang shell. Soft real-time aspects of telecom applications rarely rely on floats.
So historically, the implementation of efficient floating-point operations was a low priority for the Erlang virtual machine VM.
Operations on real numbers suddenly became much more efficient. This was, of course, a great boon for anyone doing real number computations e.
Mathematical Operators Operations on integers and floats include addition, subtraction, multiplication, and division.
Operations on integers alone always result in an integer, except in the case of floating-point division, where the result is a float.
Using div will result in an integer without a remainder, which has to be computed separately using the rem operator. Table lists the arithmetic operators.
Table In Table , they are listed in order of precedence. You can see here that it is possible to add an integer to a float: this is done by first coercing the integer to a float before performing the addition.
Note the results, especially when mixing floats and integers or dealing with floating-point division. The error you see is typical of errors returned by the runtime system.
We will cover this and other errors in Chapter 3. Atoms Atoms are constant literals that stand for themselves.
Atoms serve much the same purpose as values in enumeration types in other languages; for the beginner, it sometimes helps to think of them as a huge enumeration type.
The only operations on atoms are comparisons, which are implemented in a very efficient way in Erlang. The reason you use atoms in Erlang, as opposed to integers, is that they make the code clear yet efficient.
Atoms start with a lowercase letter or are delimited by single quotes. Any character code is allowed within an atom if the atom is encapsulated by single quotes.
They are, however, also commonly found in functional programming languages. Now try typing some atoms in the shell.
If at any point, the shell stops responding, you have probably opened a single quote and forgotten to close it.
Type '. Experiment with spaces, funny characters, and capital letters. Instead of a Boolean type, the atoms true and false are used together with Boolean operators.
We give more details of these comparisons later in this chapter. Erlang has a wide variety of built-in functions, usually called BIFs in the Erlang community, which can be used in your programs and in the shell.
In a tuple, when the first element is an atom, it is called a tag. This Erlang convention is used to represent different types of data, and will usually have a meaning in the program that uses it.
The use of a first position tag is to differentiate between tuples used for different purposes in the code. This greatly helps in finding the cause of errors when the wrong tuple has been mistakenly passed as an argument or returned as a result of a function call.
This is considered a best practice for Erlang. These functions are all generic in that they can be used over any kind of tuple, of any size.
Before starting to look at lists, make sure you experiment and get better acquainted with tuples and the tuple BIFs in the Erlang shell.
Lists Lists and tuples are used to store collections of elements; in both cases, the elements can be of different types, and the collections can be of any size.
Lists and tuples are very different, however, in the way that they can be processed. We begin by describing how lists are denoted in Erlang, and examine the way that strings are a special kind of list, before explaining in detail how lists can be processed.
Lists are delimited by square brackets, [ Elements in lists do not have to be of the same data type and, just like tuples, can be freely mixed.
The first element is the atom a, the second is the list [b,[c,d,e],f], and the third is the atom g. Characters and Strings Characters are represented by integers, and strings of characters are represented by lists of integers.
So, the string "Hello World" is in fact the list [72,,,,,32,87,,,,]. Every character in Erlang consumes 8 bytes in the bit emulator and 16 in the bit emulator , ensuring that characters and strings are not stored in a memory-efficient way.
Erlang does include binaries, which we discuss in Chapter 9, and these are recommended for representing long strings, particularly if they are being transported by an application rather than being analyzed in any way.
Recent releases of Erlang have improved the speed of binary processing, and it looks like this will continue. This implementation has not stopped Erlang from making inroads in string-intensive applications, however.
These systems run on small hardware clusters you can buy off eBay for a few hundred dollars. As the spread of Erlang continues into new problem domains, the current implementation will probably be more of an issue, but so far very few production systems have suffered as a result of this implementation choice.
Atoms and Strings What is the difference between atoms and strings? First, they can be processed in different ways: the only thing you can do with atoms is compare them, whereas you can process strings in a lot of different ways.
You could use a string to play the role of an atom, that is, as a constant literal. However, another difference between atoms and strings is efficiency.
If a program is to compare two strings or lists , it needs to compare the strings character by character while traversing both of them.
When comparing atoms, however, the runtime system compares an internal identifier in a single operation.
Building and Processing Lists As we said earlier, lists and tuples are processed in very different ways. A tuple can be processed only by extracting particular elements, whereas when working with lists, it is possible to break a list into a head and a tail, as long as the list is not empty.
The head refers to the first element in the list, and the tail is another list that contains all the remaining elements; this list can itself be processed further.
This is illustrated in Figure Just as a list can be split in this way, it is possible to build or construct a list from a list and an element.
The recursive definition of lists So, if you take the list [1,2,3], the head would be 1 and the tail would be [2,3].
Using the cons operator, the list can be represented as [1 [2,3]]. Breaking the tail further, you would get [1 [2 ] and [1 [2 [3 ]]].
A final valid notation for this list is of the format [1,2 [3 ]], where you can have more than one element separated by commas before appending the tail with the cons operator.
All of these lists are equivalent to the original list [1,2,3]. If the last tail term is the empty list, you have a proper or well-formed list.
When learning Erlang, the recursive definition of lists is the first hurdle that people can stumble on.
In fact, lists in Erlang do not have to be proper, meaning that the tail does not necessarily have to be a list.
Try typing [[1, 2] 3] in the shell. What is the result? Expressions such as [1 2] and [1,2 foo] are syntactically valid Erlang data structures, but are of only limited value.
Apart from this, it is one of the conventions of Erlang programming that use of nonproper lists should be avoided.
That is because it is normally impossible to determine just by inspecting the code whether their use was intentional or an error.
Writing [2 3] instead of [2 ], for example, results in a valid Erlang expression that compiles without any errors.
It will, however, generate a runtime error when the tail of the list is treated as a list and not as an atom. List Functions and Operations Lists are one of the most useful data types in Erlang, and, especially in combination with tuples, they can be used to represent all sorts of complex data structures.
In particular, lists are often used to represent collections of objects, which can be split into other collections, combined, and analyzed.
Many operations on lists are defined in the lists library module, and you can see some examples in the following shell session. These functions are not BIFs, and so are called by putting the module name in front of the function, separated by a colon : as in lists:split.
The effect of the functions should be clear from the examples. There are also three operators on lists. You already saw the [ The -- operator individually subtracts each element in the list on the righthand side from the list on the lefthand side.
So, [1,1] --  returns , whereas [1,2,3,4] -[1,4] returns [2,3]. If you evaluate [1,2] -- [1,1,3], you get the list .
This is because if elements on the list on the righthand side of the operation do not match, they are ignored.
So, when you want to add an element to the head of the list, you should always use cons [ The proplists module contains functions for working with property lists.
It is important that you understand how this works, as recursion, covered in Chapter 3, builds heavily on it. Type ".
How does the shell display them? Before the string concatenation construct was added to the language, programmers would make their strings span many lines.
When the code became unreadable, they would often break the strings into manageable chunks and concatenate them using the append function in the lists library module.
Lists 27 Term Comparison Term comparisons in Erlang take two expressions on either side of the comparison operator.
The result of the expression is one of the Boolean atoms true or false. You get around this by using the operators exactly equal to and not exactly equal to, as these operators compare not only the values on either side of the equation, but also their data types.
Table lists the comparison operators. The first elements are compared, and whichever is smaller indicates the smaller list: if they are the same, the second elements are compared, and so on.
When one list is exhausted, that is the smaller list. For the time being, do not worry about references, funs, ports, and binaries.
We will cover these data types in Chapter 9 and Chapter Using the exactly equal and not exactly equal operators will provide the compiler and type tools with more information and result in more efficient code.
As a result, the equal and not equal operators are commonly used in programs, including many of the libraries that come with the Erlang runtime system.
Start the Erlang shell and try some of the comparison operators. Though not included in the following examples, try testing with different data types and comparing the results with the various equality operators.
This is called single assignment. So, if you need to do a computation and manipulate the value of a variable, you need to store the results in a new variable.
Trying to bind it to the integer 4 fails as it is already bound. It encourages you to write shorter functions and puts in place a discipline that often results in code with fewer errors.
It also makes debugging of errors related to incorrect values easy, as tracing the source of the error to the place where the value was bound can lead to only one place.
All calls with variables in Erlang are call by value: all arguments to a function call are evaluated before the body of the function is evaluated.
The concept of call by reference does not exist, removing one way in which side effects can be caused. Global variables do not exist, making it easier to debug Erlang programs and reduce the risk of errors and bad programming practices.
Another useful feature of Erlang variables is that there is no need to declare them: you just use them. Programmers coming from a functional programming background are used to this, whereas those coming from a C or Java background will quickly learn to appreciate it.
The reason for not having to declare variables is that Erlang has a dynamic type system. Haskell, for instance, uses a type inference algorithm to deduce types of variables.
It has happened to us! Before using variables, remember: variables can be bound only once! This might be a problem in the Erlang shell, as programs are meant to run nonstop for many years and the same shell is used to interact with them.
Two operations can be used as a workaround to this problem. Using f forgets all variable bindings, whereas f Variable will unbind a specific Variable.
You can use these operations only in the shell. The Erlang Type System The reason for not having a more elaborate type system is that none of the Erlang inventors knew how to write one, so it never got done.
The advantage of a static type system is that errors can be predicted at compile time rather than at runtime, therefore allowing faults to be detected earlier and fixed at a lower cost.
A number of people have tried to build a static type system for Erlang. Unfortunately, due to design decisions taken when Erlang was invented, no project has been able to write a comprehensive type system, since with hot code loading, this is intrinsically difficult.
An excellent tool that resulted from research related to an Erlang type system by Uppsala University is TypEr, which can infer types of Erlang functions.
TypEr, taken together with the Dialyzer tool, which also came out of Uppsala University and can pick up other faults at compile time, results in a powerful mechanism for finding faults in Erlang programs.
We discuss these tools in Chapter Complex Data Structures When we refer to Erlang terms, we mean legal data structures.
Erlang terms can be simple data values, but we often use the expression to describe arbitrarily complex data structures.
In Erlang, complex data structures are created by nesting composite data types together. These data structures may contain bound variables or the simple and composite values themselves.
For C programmers, this means no more sleepless nights hunting for pointer errors or memory leakages.
Memory to store the complex data types is allocated by the runtime system when needed, and deallocated automatically by the garbage collector when the structure is no longer referenced.
Memory Management in Erlang When the first Erlang-based products were being developed in , critics said it was madness to use a language compiling for a VM with a garbage collector just like Java!
It is thanks to the design of the garbage collector, however, that the soft real-time properties of these systems are not affected.
The current implementation of the Erlang VM uses a copying, generational garbage collector. The garbage collection is done separately for each concurrent process: when no more memory is available for a particular process to store values, a garbage collection will be triggered.
A copying garbage collector works by having two separate areas heaps for storing data. When garbage collection takes place, the active memory is copied to the other heap, and the garbage left behind is overwritten in the other heap.
A garbage collection can be shallow or deep. A shallow garbage collection looks only at data in the youngest generation; all data that survives three shallow garbage collections will be moved to the old generation.
A deep garbage collection will occur only when a shallow collection fails to recycle enough memory or after a VM version dependent number of shallow collections.
The Pattern consists of data structures that can contain both bound and unbound variables, as well as literal values such as atoms, integers, or strings.
A bound variable is a variable which already has a value, and an unbound variable is one that has not yet been bound to a value.
It may not contain unbound values. What happens when a pattern match is executed? What determines whether the pattern match succeeds?
If Sum is unbound, pattern matching succeeds and Sum is bound to 3. Just to be clear, this would not bind 1 to Sum and then add 2 to it.
If Sum is already bound, pattern matching will succeed if and only if Sum is already bound to 3. You can see this in commands 3 and 4.
It looks as though this should succeed, but the variables Head and Tail are bound already, so this pattern match becomes a test of whether the expression is in fact [1,2,3,4]; you can see in command 6 that this would succeed.
If you want to extract the head and tail of the list , you need to use variables that are not yet bound, and commands 7 and 8 show that this is now successful.
The first occurrence is unbound, and results in a binding: here to the value 1. The next occurrence is bound, and will succeed only if the corresponding value is 1.
This will match with anything, and produces no bindings. Take assignment of variables as an example. If Int is unbound, it gets bound to whatever the righthand side of the equation evaluates to, in this case 1.
That is how variable assignment actually works. We are not assigning variables, but in fact pattern-matching them. The second expression will add 1 to 0 and compare it to the contents of the variable Int, currently bound to 1.
As the result is the same, the pattern matching will be successful. Attempting to compare it to the contents of Int would fail, as it is bound to 1.
Pattern matching is also used to pick the execution flow in a program. Later in this and in the following chapters, we will cover case statements, receive statements, and function clauses.
In each of these constructs, pattern matching is used to determine which of the clauses has to be evaluated.
In effect, we are testing a pattern match that either succeeds or fails. Both are tuples with three elements, so thus far, the pattern matching is successful.
Tests are now done on the individual elements of the tuple. The first A is unbound and gets bound to the atom abc.
The second A is now also bound to abc, so comparing it to the atom def will fail because the values differ. Even if both are lists, the list on the lefthand side has four elements and the one on the right has only three.
A common misconception is that D can be set to the empty list and the pattern matching succeeds. In this example, that would not be possible, as the separator between C and D is a comma and not the cons operator.
The last use of pattern matching is to extract values from compound data types. It will also extract the first element of the list stored in the third element of the tuple and bind it to the variable B.
This will succeed, with the variable Name being bound to the string "Francesco" and the variable Surname to "Cesarini". The only difference is that compiler warnings are generated if the value of the normal variable is never used.
This will cause the pattern match to fail. Using variables that start with an underscore makes the code more legible, but inserts potential bugs in the code when they are mistakenly reused in other clauses in the same function.
Since the introduction of compiler warnings for singleton variables variables that appear once in the function , programmers mechanically add an underscore, but tend to forget about the single assignment rule and about the fact that these variables are actually bound to values.
So, use them because they increase code legibility and maintainability, but use them with care, ensuring that you do not introduce bugs.
You can see from what we have said that pattern matching is a powerful mechanism, with some subtleties in its behavior that allow you to do some amazing things in one or two lines of code, combining tests, assignment, and control.
At the risk of sounding repetitive, try pattern matching in the shell. You can experiment with defining lists to be really sure you master the concept, and use pattern matching to deconstruct the lists you have built.
Make pattern-matching clauses fail and inspect the errors that are returned. When you do so, experiment with both bound and Different versions of the Erlang runtime system will format errors differently.
Pattern Matching 37 unbound variables. As pattern matching holds the key to writing compact and elegant programs, understanding it before continuing will allow you to make the most of Erlang as you progress.
In programs, of course. Erlang programs consist of functions that call each other. Functions are grouped together and defined within modules.
The name of the function is an atom. The head of a function clause consists of the name, followed by a pair of parentheses containing zero or more formal parameters.
In Erlang, the number of parameters in a function is called its arity. Before we go any further, do not try to type functions directly in the shell.
You can if you want to, but all you will get is a syntax error. Functions have to be defined in modules and compiled separately. We will cover writing, compiling, and running functions in the next section.
Example shows an Erlang function used to calculate the area of a shape. Each clause has a head specifying the expected argument patterns and a function body consisting of one or more comma-separated expressions.
These are evaluated in turn, and the return value of a function is the result of the last expression executed. Example When a function is called, its clauses are checked sequentially by pattern matching the arguments passed in the call to the patterns defined in the function heads.
If the pattern match is successful, variables are bound and the body of the clause is executed. If it is not, the next clause is selected and matched.
The second clause is chosen and the pattern match is successful, resulting in the variable Radius being bound to the integer 2.
When a clause is matched, the remaining ones are not executed. As Other is unbound, it will match any call to the function area when pattern matching fails in the first three clauses.
A common error is shadowing clauses that will never match. If we call factorial 3 , pattern matching in the first clause will fail, as 3 does not match 0.
The runtime system tries the second clause, and as N is unbound, N will successfully be bound to 3. The runtime system is unable to return any value until it has executed factorial 2 and is able to multiply its value by 3.
The call factorial 0 matches in the first clause, returning 1 as a result. Variables are local to each clause.
There is no need to allocate or deallocate them; the Erlang runtime system handles that automatically.
Modules Functions are grouped together in modules. A program will often be spread across several modules, each containing functions that are logically grouped together.
Modules consist of files with the. A module example -module demo. These functions are global, meaning they can be called from outside the module.
Make sure you use them everywhere in your code! Global calls, also called fully qualified function calls, are made by prefixing the module name to the function.
So, in Example , calling demo:double 2 would return 4. Local functions can be called only from within the module. Calling them by prefixing the call with the module name will result in a runtime error.
Functions in Erlang are uniquely identified by their name, their arity, and the module in which they are defined.
Two functions in the same module might have the same name but a different arity. If so, they are different functions and are considered unrelated.
There is no need to declare functions before they are called, as long as they are defined in the module. Compilation and the Erlang Virtual Machine To run functions exported from a module, you have to compile your code, which results in a module.
This will from now on allow you to get an Erlang shell in the right directory just by double-clicking any.
With both operating systems, you can otherwise move to the directory by using the cd Directory command in the Erlang shell. Once in the directory, you compile the code using c Module in the Erlang shell, omitting the erl suffix from the module name.
If the code contained no errors, the compilation will succeed. Large Erlang systems consist of loosely coupled Erlang modules, all compiled on a standalone basis.
Once you have compiled your code, look at the source code directory and you will find a file with the same name as the module, but with the.
This file contains the byte code that you can call from any other function. Once compiled, you need to make a fully qualified function call to run your functions.
This is because you are calling the function from outside the module. The module attribute is mandatory, and describes the module name.
Modules 41 This directive should be used only for testing purposes. Do not do like many others and forget to replace it with an export directive before your code goes into production!
The compile directive takes on other options that are useful only in special conditions. If you are curious and want to read more about them, check out the manual page for the compile module.
It allows you to import functions from other modules and call them locally. You can make up your own module attributes. User-defined attributes can have only one argument unlike some of the built-in attributes.
Module demo compiled: Date: February 25 , Time: Note that we did not have any vsn attribute in the demo module, but one appeared in the preceding example.
When vsn is not defined, the compiler sets it to the MD5 of the module. Do not worry about records, vsn, and behaviour for now, as we cover them in Chapters 7 and We covered the basics of Erlang in this chapter, and you saw some of its peculiarities: you can assign values to variables, but only once; you can pattern-match against a variable and it may turn into a test for equality with that variable.
Other features of the language, such as the module system and the basic types it contains, are more familiar. Exercises Exercise The Shell Type in the following Erlang expressions in the shell and study the results.
They will show the principles of pattern matching and single-variable assignment described in this chapter. What happens when they execute?
What values do the expressions return, and why? Exercises 43 D. Exercise Modules and Functions Copy the demo module from the example in this chapter.
Compile it and try to run it from the shell. What happens when you call demo:times 3,5? What about double 6 when omitting the module name?
Create a new module called shapes and copy the area function in it. Do not forget to include all the module and export directives.
Compile it and run the area function from the shell. When you compile it, why do you get a warning that variable Other is unused? Exercise Simple Pattern Matching Write a module boolean.
You should not use the logical constructs and, or, and not, but instead use pattern matching to achieve your goal.
Test your module from the shell. When working in functional programming languages, you replace iterative constructs such as while and for loops with recursive programming techniques.
It allows a programmer to traverse a data structure via successive calls to the same function, with the patterns of function calls mirroring the structure of the data itself.
The resulting programs are more compact and easier to understand and maintain. Functional programs are, importantly, side-effect-free, unless side effects are specifically needed for printing or for access to external storage.
You control recursion using various conditional constructs that enhance the expressive power of pattern matching; in the example of data structure traversal, different patterns correspond to different traversals: bottom-up, top-down, breadth-first, and so forth.
This chapter also introduces other features directly related to sequential programming. Through exception-handling mechanisms, programs can recover and continue execution.
The Erlang distribution comes with an extensive set of modules which contain libraries, tools, and utilities, as well as complete applications.
New modules are being added in every release, and existing libraries are often enhanced with new functionality.
Some of the libraries consist of what we in the Erlang world call built-in functions BIFs , because they are part of the Erlang runtime system.
They do things that are either not programmable in Erlang itself or would be slow to execute were they to be defined by the user.
The first form you encountered in Chapter 2: the choice of a function clause to evaluate through pattern matching over the arguments to the function.
The second is the case statement, which works in a similar way to function clause selection. The third form is the if construct, which you can view as a simplified form of the case construct.
The case Construct The case construct relies on pattern matching to choose what code to evaluate, in a strikingly similar manner to the selection of function clauses by pattern matching.
Instead of matching actual parameters against patterns for the formal parameters to the function, case evaluates an expression and matches the result against a list of patternmatching clauses, separated by semicolons.
The conditional-expression is evaluated and matched against Pattern1, Once a pattern has been matched, the selected clause expressions are evaluated in order and the result of the case construct is the result of the last expression evaluated.
In the following case construct, the list List is examined using the member function from the lists library module to determine whether the atom foo is a member of the List, in which case the atom ok is returned.
It is possible, although rare, to type case clauses directly in the Erlang shell. From now on, the constructs are getting complex enough that it makes sense for you to try what you learn by typing trial functions in a module, compiling the modules, and running their functions.
Defensive Programming Assume your program has to map to an integer an atom representing a day of the week. It is better to make your program terminate in the convert function with a clause error meaning no clause was matched , because then the error is apparent at the point where it occurred.
In the past, defensive programming like this has left us searching in 2-million-line code bases looking for the function that returned the error tuple that caused a bad match error in a completely different part of the system.
Function definitions and case expressions have a lot in common. A case expression, on the other hand, matches a single expression. So, pattern matching in function definitions can be more compact than using a case expression, but remember that a case expression can be used anywhere in a function definition and not just in the head, so each kind of pattern match has a part to play in Erlang.
A case statement with only one clause is considered bad practice in Erlang. In our example, we do it to demonstrate a point.
You should however avoid it in your code, and instead use pattern matching on its own. Variable Scope The scope of a variable is the region of the program in which that variable can be used.
The same variable name may be used in many places in the program; some uses will refer to the same variable and others to a different variable, which happens to have the same name.
In the next example, there are two separate variables with the name Y. This creates a problem when variables are bound in only some of the clauses of a case or if construct, and are later used within the same function body.
The small code example that follows demonstrates this: what would be the result of calling unsafe one and unsafe two?
You can use a variable safely only if it is bound in all the clauses of a case or if construct. This is considered bad coding practice, as it tends to make the code much harder to read and understand.
If this is Guardi, the body of the following clause is evaluated: expressioni1, expressioni2, The guard expressions are a subset of the Erlang Boolean expressions that can only contain calls to a restricted set of functions together with comparisons and arithmetic operations.
We describe exactly what the guards may contain in the next section. If none of the guards evaluates to the atom true, a runtime error is generated.
To get a catch-all clause, you can allow the last clause to have the atom true as the guard; it is not mandatory to have such a catch-all clause.
Pay special attention to the return value of these clauses, trying to bind the value to a variable and using it in later computations.
When testing, write a program with an unsafe variable defined in some clauses and not others and try to compile it. The clause will be selected only if the pattern matches and the guard expression evaluate to the atom true.
We reordered the clauses in the factorial function. In the previous version, we had to have factorial 0 as the first clause to ensure that the function terminates.
Now we select the recursive clause i. If the pattern matching together with the guards uniquely identifies what clause should be selected, their order becomes irrelevant, and that is the case here.
In the previous version of the factorial function, the function would never have returned a value, since factorial -1 calls factorial -2 , and so on.
This would eventually cause the Erlang runtime system to run out of memory and terminate. The reason for not allowing developers to implement their own guard functions, limiting them to permitted operations, is to ensure that guards are free of side effects.
The Guards 51 guards are executed for all clauses up to the successful clause, meaning that if you had an io:format call in a guard that fails, you would still see the printout even if the clause was not selected for evaluation.
The use of these BIFs is discouraged, as they are deprecated and are available only for backward compatibility reasons. To conclude this section, copy the following example and run it from the shell.
The function even will take the remainder of an integer when divided by 2; if the result is 0, it returns the atom true, and if it is 1 the integer is not even, so it returns false.
Try calling the function even with a float or an atom. What happens? If anything other than a number is passed, the function returns false: -module examples.
We will refer to built-in functions as BIFs, a practice almost universal in the Erlang community. Standard and nonstandard BIFs are listed in the manual page of the erlang module.
BIFs are usually written in C and integrated into the virtual machine VM , and can be used to manipulate, inspect, and retrieve data as well as interact with the operating system.
Originally, all built-in functions were considered to belong to the module erlang, but they have made their way to other modules for practicality and efficiency reasons.
Among the modules that contain built-in functions are ets and lists. Although most built-in functions are seen as being an integral part of Erlang, others are VM-dependent and do not necessarily exist in other VM implementations or even in specific OS ports of the existing VM.
Standard built-in functions are auto-imported, so you can call them without the module prefix. Nonstandard BIFs, however, have to be prefixed with the erlang module prefix, as in erlang:function.
Examples of nonstandard built-in functions include erlang:hash Term, Range , which returns the hash of the Term in the specified range; and erlang:display Term , which prints the term to standard output and is mainly used for debugging purposes.
This would be impossible to do efficiently within the language, even if it were possible to do at all. There are numerous type conversion functions, not only to change numerical types, but also to convert the basic types to and from a printable representation i.
The retrieval and manipulation of these values unfortunately introduces global variables into Erlang. Using the process dictionary might provide the programmer with a quick win while developing the program, but the result is code that is very hard to debug and maintain.
As most Erlang functions are side effect free, the parameters passed to the function included in the crash report usually contain enough information to solve the bug.
Introducing the process dictionary greatly complicates this task, as the state of the process dictionary is lost when the program crashes.
We will not cover these BIFs in this book, as we do not want to be seen as encouraging bad practices. If you are desperate to write ugly, hard-to-debug programs or are putting together a submission to the obfuscated Erlang competition, you can read about these BIFs in the documentation that comes with the Erlang distribution.
At least you will not be able to say you picked up this bad habit from us. Meta Programming One often refers to the ability of a function to determine what other function to call at runtime as meta programming, that is, programs that create other programs and run them.
When called, it executes the named function on the specified arguments and returns its result. They can be passed to the BIF as variables. If the number of arguments is known at compile time, you can use the following notation if there are two arguments : Mod:Fun Arg1, Arg2 instead of the more general apply Mod,Fun,[Arg1,Arg2].
We will look at other ways that functions can be created dynamically when we look at higher-order functions in Chapter 9.
Process, Port, Distribution, and System Information In the chapters dealing with concurrency, we will cover several BIFs directly related to processes, process inspection, and error handling.
The same applies to port handling and distribution. We will mention these BIFs and others in their relevant chapters throughout the book.
There is a variety of information concerning the system that we might want to know, and of course, all the access functions for this information have to be BIFs.
The information includes low-level system information, trace stacks, as well as the current time and date. The list is long, but they are all documented in the erlang module.
As a result, it can be used as a unique identifier. In this section, we describe the main functions that read from standard input and write to standard output.
These take an extra argument in the data list indicating the maximum depth for printing terms. PadC, where F is the field width of the printed argument, P is its precision, Pad is the padding character, and C is the control character.
The pretty printing mode tries to figure out what you are trying to print, and formats it accordingly. But if your list of integers happens to be valid ASCII values instead of a list of integers, you will get a string.
By joining together the solutions of several simple problems, you solve the bigger one without even realizing it! Because Erlang is a single assignment language, we have to create a new list, in which we will store the result.
If the old list is empty, the new one should also be empty. If so, we split the list to a head and a tail.
Now, the question is how do we proceed with the rest of list? We want to construct a new list where all the elements are one larger than in the old list.
But that is exactly what the bump function is supposed to do! This provides us with what we are looking for. Does the function really work?
The first one is the common technique of tackling the problem piecemeal, breaking it up into smaller problems. This resulted in a very common recursive programming pattern in Erlang.
How does this piece of magic work? We are calling the same function, reusing variables. Are they not already bound? No, they are not.
The important thing to remember is that the variables are unique to every call and considered fresh in every iteration.
For each call to a function, a frame is created on the call stack with information regarding where to return, together with the parameters to the function and its local variables.
We are now going to look at a more elaborate example, revisiting a similar type of problem and, indeed, a solution. We want to compute the average of a list of numbers.
What is the average? It is the sum of the elements divided by the length of the list. And this has solved the problem! All we need to do is to define the sum and len functions, and we are done.
To compute the sum, we do a similar case analysis to what we did for bump, breaking the problem into smaller problems. We are calling the length function len to avoid clashing with the built-in function.
The case for the empty list, of course, returns zero. It is a pattern that, with variations, is by far the most common in Erlang code.
We will now continue with an example that is a variation of this pattern. We will traverse a list, filtering out the elements that are not even.
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