What does “canonical” mean?

Question

Sometimes I read a sentence containing the word canonical, but I cannot find appropriate meaning of the word.

For example, in this link:

Returns a canonical representation for the string object.

It's hard for me to find an appropriate definition from Google:

ca·non·i·cal Adjective /kəˈnänikəl/

1. According to or ordered by canon law
   • the canonical rites of the Roman Church
2. Included in the list of sacred books officially accepted as genuine
   • the canonical Gospels of the New Testament
3. Accepted as being accurate and authoritative
   • the canonical method of comparative linguistics
4. (of an artist or work) Belonging to the literary or artistic canon
   • canonical writers like Jane Austen
5. According to recognized rules or scientific laws
   • canonical nucleotide sequences
6. Of or relating to a general rule or standard formula
7. Of or relating to a cathedral chapter or a member of it

Answer 1

When this word is used by programmers, it is a synonym for "authoritative," "standard," or "official." That's meaning number 3 in the definition you quote.

It implies that there is one best, most official, most standard way to represent the string object and this is it.

For example sometimes things can be represented using a relative link or an absolute link. Relative might be something like ../foo.txt meaning "The file foo.txt in the folder above this one" while absolute is something like \\www.example.com\fileserver\share\foo.txt which can only refer to one file on the Internet. One might say that the latter name is canonical.

Answer 2

In computing, the "canonical" form of something usually means something like "a standardised base form, to which any given variant resolves when you apply any relevant conversions". For example, you could have various paths "$NEILSDIR$\Directory", "$USERPATH$\Neil\Directory", but the canonical form of these might be "C:\Users\Neil\Directory".

Another type of conversion that the word commonly applies to is character encoding.

Unfortunately, I don't think the general dictionary entry you quote gives you so much of a clue in this case as to the technical use.

Answer 3

Of the dictionary meanings given, three of them are very similar:

3. Accepted as being accurate and authoritative
5. According to recognized rules or scientific laws
6. Of or relating to a general rule or standard formula

The first two of these imply reference to an authority or recognized rule, which is usually not the case for a programming usage like canonical representation. The last (definition 6) is the most appropriate: according to a general rule.

For example, strings might be considered equivalent whether in upper case or lower case. For a particular program, the designer might have a rule that all strings are to be stored in upper case. That means that upper case is the canonical representation.

Answer 4

This is an interesting word within and out of programming context. A nice mix of explanations from both perspectives can be found in good old jargon file - see canonical.

The usual or standard state or manner of something. This word has a somewhat more technical meaning in mathematics. Two formulas such as 9 + x and x + 9 are said to be equivalent because they mean the same thing, but the second one is in canonical form because it is written in the usual way, with the highest power of x first. Usually there are fixed rules you can use to decide whether something is in canonical form. The jargon meaning, a relaxation of the technical meaning, acquired its present loading in computer-science culture largely through its prominence in Alonzo Church's work in computation theory and mathematical logic (see Knights of the Lambda Calculus). Compare vanilla.

Answer 5

Regarding that particular usage, describing the function of Java's intern function is always a challenge, and the writer was no doubt casting about for some descriptive term to use.

All Java "Strings" are "canonical" in that there is only one form that they can be in, with only the values of the characters contained within distinguishing them.

However, at most one String with a given character value may be designated as the "interned" String, for reasons that have nothing to do with English. So "canonical" is a stretch, but it's the only word the writer could think of that sorta fit.

Answer 6

From http://www.webopedia.com/TERM/C/canonical.html

(adj.) Authoritative or standard; conforming to an accepted rule or procedure. When referring to programming, canonical means conforming to well-established patterns or rules. The term is typically used to describe whether or not a programming interface follows the already established standard.

In Java string literals are stored in the source code depending on your encoding e.g. Latin-1, internaly UTF-8 is used. The canonical representation means that Unicode "\u000a" (6 bytes) will be transformed to the UTF-8 representation of a linefeed (1 byte).

I'm not sure whether canonical could be applied to variable expansion, for directories I would give another example: C:\users\admin\..\neil would be normalized to C:\users\neil

Answer 7

Canonical.

As already established, the meaning of certain words vary (interpretation within the scope of an event) when applied to a certain environment. Fortunately, "Canonical" has been inquired only in the technical aspect. Thus the clarification keeping in mind this highly advanced era of 1s and 0s.

"Most relevant and accurate based on a set of predefined rules within a system"

This is a system/environment depenent explanation. The canonical output of System A may or may not mirror the result of System B; it will however if the set of rules under which the result is evaluated are identical.