Erlang (programming language)/Tutorials/Pattern Matching

The metadata subpage is missing. You can start it via filling in this form or by following the instructions that come up after clicking on the [show] link to the right.

Do you see this on PREVIEW? then SAVE! before following a link. To find out more about the use of metadata for the management of Citizendium pages, see CZ:Metadata. You may see this box for one of two reasons. Either: A - New page has been created, or subpages template was added to an existing page B - Cluster move is in progress Click either the A or B link for further instructions. [Feel free to recommend improvements to the text or links in this template] A - For a New Cluster use the following directions Subpages format requires a metadata page. Using the following instructions will complete the process of creating this article's subpages. Click the blue "metadata template" link below to create the page. On the edit page that appears paste in the article's title across from "pagename =". You might also fill out the checklist part of the form. Ignore the rest. For background, see Using the Subpages template Don't worry--you'll get the hang of it right away. Remember to hit Save! the "metadata template". However, you can create articles without subpages. Just delete the {{subpages}} template from the top of this page and this prompt will disappear. :) Don't feel obligated to use subpages, it's more important that you write sentences, which you can always do without writing fancy code. [Feel free to recommend improvements to the text or links in this template] B - For a Cluster Move use the following directions The metadata template should be moved to the new name as the first step. Please revert this move and start by using the Move Cluster link at the top left of the talk page. The name prior to this move can be found at the following link. [Feel free to recommend improvements to the text or links in this template]

---

Erlang uses powerful pattern-matching to bind variables to values. Pattern matching can be explicit, as when the = (pattern matching operator) is used, or implicit, as when a function call is processed and a function's actual arguments are matched with its formal parameters.

Patterns

Patterns look the same as terms (they can be simple literals like atoms and numbers, compound like tuples and lists, or a mixture of both. They can also contain variables, which are alphanumeric strings that begin with a capital letter or underscore. A special "anonymous variable", _ (the underscore) is used when you don't care about the value to be matched, and won't be using it.

A pattern matches if it has the same "shape" as the term being matched, and atoms encountered are the same. For example, the following matches succeed:

• A = 1.
• 1 = 1.
• {ok, A} = {ok, 42}.
• [H|T] = [1, 2, 3].

Note that in the fourth example, the pipe (|) signifying the head and tail of the list as described in Terms.

These matches fail:

• 1 = 2.
• {ok, A} = {failure, "Don't know the question"}.
• [H|T] = [].

In the case of the pattern-matching operator, a failure generates an error and the process exits. How this can be trapped and handled is covered in Errors.

Patterns are used to select which clause of a function will be executed (this is covered in Functions; which option to select in a case expression (Expressions); and which messages to retrieve from the mailbox (Processes).

Variables

Erlang variables are single-assignment variables that do not have to be declared. They are written with a capital letter or underscore followed by an alphanumeric sequence. They are bound to values using the pattern matching mechanism. The Erlang compiler will produce an error if an unbound variable is used, and a warning if a bound variable is not used. Sometimes you might encounter a _Var variable. This variable is bound and does contain a value, however it suppresses compiler warnings regarding unused bound variables.