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From The Free On-line Dictionary of Computing (05 January 2017) [foldoc]:
canonical
(Historically, "according to religious law")
1. A standard way of writing a formula. 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.
Things in canonical form are easier to compare.
2. The usual or standard state or manner of
something. The term acquired this meaning 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.
This word has an interesting history. Non-technical academics
do not use the adjective "canonical" in any of the senses
defined above with any regularity; they do however use the
nouns "canon" and "canonicity" (not "canonicalness"* or
"canonicality"*). The "canon" of a given author is the
complete body of authentic works by that author (this usage is
familiar to Sherlock Holmes fans as well as to literary
scholars). "The canon" is the body of works in a given field
(e.g. works of literature, or of art, or of music) deemed
worthwhile for students to study and for scholars to
investigate.
The word "canon" derives ultimately from the Greek "kanon"
(akin to the English "cane") referring to a reed. Reeds were
used for measurement, and in Latin and later Greek the word
"canon" meant a rule or a standard. The establishment of a
canon of scriptures within Christianity was meant to define a
standard or a rule for the religion. The above non-technical
academic usages stem from this instance of a defined and
accepted body of work. Alongside this usage was the
promulgation of "canons" ("rules") for the government of the
Catholic Church. The usages relating to religious law derive
from this use of the Latin "canon". It may also be related to
arabic "qanun" (law).
Hackers invest this term with a playfulness that makes an
ironic contrast with its historical meaning. A true story:
One Bob Sjoberg, new at the MIT AI Lab, expressed some
annoyance at the incessant use of jargon. Over his loud
objections, GLS and RMS made a point of using as much of
it as possible in his presence, and eventually it began to
sink in. Finally, in one conversation, he used the word
"canonical" in jargon-like fashion without thinking. Steele:
"Aha! We've finally got you talking jargon too!" Stallman:
"What did he say?" Steele: "Bob just used "canonical" in the
canonical way."
Of course, canonicality depends on context, but it is
implicitly defined as the way *hackers* normally expect things
to be. Thus, a hacker may claim with a straight face that
"according to religious law" is *not* the canonical meaning of
"canonical".
(2002-02-06)