Erlang and OTP in Action.pdf - Synrc

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the same atom for both. Do not abuse this power when you write your own functions – it
should be done in a systematic way so that it is easy for your users to remember how to call
your functions. If you create functions that differ only in arity but produce wildly different
results, you will not be thanked for it. When in doubt, opt for giving the functions clearly
different names.
At any rate, always remember that in order to exactly specify which function we are
talking about, we need to give the arity, not just the name.
2.3.3 – Built-in functions and standard library modules
Like any other programming language, Erlang comes with a standard library of useful
functions. These are spread over a large number of modules; however, some standard
library modules are more commonly used than others. In particular, the module named
erlang contains those functions that are central to the entire Erlang system, that
everything else builds on. Another useful module that we have seen already is the lists
module. The io module handles basic text input and output. The dict module provides
hash-based associative arrays (dictionaries), and the array module provides extensible
integer-indexed arrays. And so forth.
Some functions are involved with so low-level things that they are really an intrinsic part
of the language and the run-time system. These are commonly referred to as built-in
functions, or BIFs, and like the Erlang run-time system itself, they are implemented in the C
programming language. (Though some may disagree on the details, this is the most common
definition.) In particular, all the functions in the erlang module are BIFs. Some BIFs, like
our friend lists:reverse/1 from the previous section, could in principle be written directly in
Erlang (like most of the other functions in the lists module), but have been implemented
in C for efficiency reasons. In general, though, the programmer does not have to care about
how the functions are implemented—they look the same to the eye—but the term “BIF” is
used quite often in the Erlang world, so it’s useful to know what it refers to.
Finally, even the operators of the language, such as +, are really built-in functions, and
belong to the erlang module. For example, you can write erlang:'+'(1,2) for addition.
AUTOMATICALLY IMPORTED FUNCTIONS
A few functions (all found in the module erlang) are both important and very commonly
used, in Erlang programs as well as in the shell. These are automatically imported, which
means that you don’t need to write the module name explicitly. We have already seen the
function self(), which returns the process identifier (the “pid”) of the process that calls it.
This is actually a remote call to erlang:self(), but because it is one of the auto-imported
functions, you don’t need to prefix it with erlang:. Other examples are spawn(…), which
starts a new process, and length(…), which computes the length of a list.
2.3.4 – Creating modules
To make ourselves a new module that can be used, we need to:
• Write a source file
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