🚀 go-pugleaf

    I sure wish that people would use "invalid" instead of "illegal" when
    referring to syntax validity checking or other forms of user-error
    checking on computers.  "Illegal" should be reserved for things which
    are against the law; e.g. theft of services, vandalism, fraudulent
    usage, etc.

"Illegal" more accurately describes a message with bad syntax than
"invalid".  From my Random House College Dictionary (a mediocre
dictionary):

    illegal: 1. not legal; contrary to existing statutes, regulations,
                etc.; unauthorized.

    invalid: 1. not valid; without force or foundation; indefensible.
             2. deficient in substance or cogency; weak.
             3. void or without legal force, as a contract.

    valid:   1. sound; just; well-founded:  "a valid objection."
             2. producing the desired result; effective:  "a valid remedy."
             3. having force, weight, or cogency; authoritative.
             4. Logic.  (of an argument) so constructed that if the
                premises are jointly asserted the conclusion cannot be
                denied without contradition.

"Illegal" merely implies some non-conformance with a regulation or statute
or rule.  There is no necessary requirement that there be the force of
government behind the rule.  For example, it is common usage to say that
a move in a game is illegal, i.e. doesn't conform to the rules.

"Invalid" on the other hand is mainly used to refer to arguments and
methods, especially those involving logic (thinking).  For example, you
might describe a possible compiler source transformation as invalid.

So "illegal" accurrately describes a header that does not conform to the
rules of RFC822, rules laid down by a supposedly authoritative agency,
DARPA.  "Invalid" wouldn't be nearly as good an adjective, since a message
header is not an argument or method.

It's pretty much mom and apple pie these days that error messages such
as "Illegal syntax" or "Invalid header format" are pretty useless to a
novice (as Mark points out in with his anecdote).  On the other hand,
there need be little relation between what our programs say to users and
the terminology we used to describe the implementation of the program.
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