Error message style guide, take 2

I'm about to start going through the backend's elog() calls to update
them to ereport() style, add error code numbers, polish wording, etc.
So it's time to nail down our style guide for message wording. Attached
is a revision of the draft that Peter posted on 14-March. Any further
comments?

BTW, I'd like to SGML-ify this and put it into the developer's guide
somewhere; any thoughts where exactly?

regards, tom lane

What goes where
---------------

The primary message should be short, factual, and avoid reference to
implementation details such as specific function names. "Short" means
"should fit on one line under normal conditions". Use a detail message if
needed to keep the primary message short, or if you feel a need to mention
implementation details such as the particular system call that failed.
Both primary and detail messages should be factual. Use a hint message
for suggestions about what to do to fix the problem, especially if the
suggestion might not always be applicable.

For example, instead of
IpcMemoryCreate: shmget(key=%d, size=%u, 0%o) failed: %m
(plus a long addendum that is basically a hint)
write
Primary: Could not create shared memory segment: %m
Detail: Failed syscall was shmget(key=%d, size=%u, 0%o)
Hint: the addendum

RATIONALE: keeping the primary message short helps keep it to the point,
and lets clients lay out screen space on the assumption that one line is
enough for error messages. Detail and hint messages may be relegated to a
verbose mode, or perhaps a pop-up error-details window. Also, details and
hints would normally be suppressed from the server log to save space.
Reference to implementation details is best avoided since users don't know
the details anyway.

Formatting
----------

Don't put any specific assumptions about formatting into the message
texts. Expect clients and the server log to wrap lines to fit their own
needs. In long messages, newline characters (\n) may be used to indicate
suggested paragraph breaks. Don't end a message with a newline. Don't
use tabs or other formatting characters. (In error context displays,
newlines are automatically added to separate levels of context such
as function calls.)

RATIONALE: Messages are not necessarily displayed on terminal-type
displays. In GUI displays or browsers these formatting instructions
are at best ignored.

Quotation marks
---------------

English text should use double quotes when quoting is appropriate.
Text in other languages should consistently use one kind of quotes
that is consistent with publishing customs and computer output of
other programs.

RATIONALE: The choice of double quotes over single quotes is somewhat
arbitrary, but tends to be the preferred use. Some have suggested
choosing the kind of quotes depending on the type of object according to
SQL conventions (namely, strings single quoted, identifiers double
quoted). But this is a language-internal technical issue that many users
aren't even familiar with, it won't scale to other kinds of quoted terms,
it doesn't translate to other languages, and it's pretty pointless, too.

Use of quotes
-------------

Use quotes always to delimit file names, user-supplied identifiers,
and other variables that might contain words. Do not use them to
mark up variables that will not contain words (for example, operator
names).

There are functions in the backend that will double-quote their own
output at need (for example, format_type_be()). Do not put additional
quotes around the output of such functions.

RATIONALE: Objects can have names that create ambiguity when embedded
in a message. Be consistent about denoting where a plugged-in name
starts and ends. But don't clutter messages with unnecessary or
duplicate quote marks.

Grammar and punctuation
-----------------------

The rules are different for primary error messages and for detail/hint
messages:

Primary error messages: Do not capitalize the first letter. Do not end a
message with a period. Do not even think about ending a message with an
exclamation point.

Detail and hint messages: Use complete sentences, and end each with
a period. Capitalize the starts of sentences.

RATIONALE: Avoiding punctuation makes it easier for client applications to
embed the message into a variety of grammatical contexts. Often, primary
messages are not grammatically complete sentences anyway. (And if they're
long enough to be more than one sentence, they should be split into
primary and detail parts.) However, detail and hint messages are longer
and may need to include multiple sentences. For consistency, they should
follow complete-sentence style even when there's only one sentence.

Upper case vs. lower case
-------------------------

Use lower case for message wording, including the first letter of a
primary error message. Use upper case for SQL commands and key words if
they appear in the message.

RATIONALE: It's easier to make everything look more consistent this
way, since some messages are complete sentences and some not.

Avoid passive voice
-------------------

Use the active voice. Use complete sentences when there is an acting
subject ("A could not do B"). Use telegram style without subject if
the subject would be the program itself; do not use "I" for the
program.

RATIONALE: The program is not human. Don't pretend otherwise.

Present vs past tense
---------------------

Use past tense if an attempt to do something failed, but could perhaps
succeed next time (perhaps after fixing some problem). Use present tense
if the failure is certainly permanent.

There is a nontrivial semantic difference between sentences of the
form

could not open file "%s": %m

and

cannot open file "%s"

The first one means that the attempt to open the file failed. The
message should give a reason, such as "disk full" or "file doesn't
exist". The past tense is appropriate because next time the disk
might not be full anymore or the file in question may exist.

The second form indicates the the functionality of opening the named
file does not exist at all in the program, or that it's conceptually
impossible. The present tense is appropriate because the condition
will persist indefinitely.

RATIONALE: Granted, the average user will not be able to draw great
conclusions merely from the tense of the message, but since the
language provides us with a grammar we should use it correctly.

Type of the object
------------------

When citing the name of an object, state what kind of object it is.

RATIONALE: Else no one will know what "foo.bar.baz" is.

Brackets
--------

Square brackets are only to be used (1) in command synopses to denote
optional arguments, or (2) to denote an array subscript.

RATIONALE: Anything else does not correspond to widely-known customary
usage and will confuse people.

Assembling error messages
-------------------------

When a message includes text that is generated elsewhere, embed it in
this style:

could not open file %s: %m

RATIONALE: It would be difficult to account for all possible error codes
to paste this into a single smooth sentence, so some sort of punctuation
is needed. Putting the embedded text in parentheses has also been
suggested, but it's unnatural if the embedded text is likely to be the
most important part of the message, as is often the case.

Reasons for errors
------------------

Messages should always state the reason why an error occurred.
For example:

BAD: could not open file %s
BETTER: could not open file %s (I/O failure)

If no reason is known you better fix the code. ;-)

Function names
--------------

Don't include the name of the reporting routine in the error text.
We have other mechanisms for finding that out when needed, and for
most users it's not helpful information. If the error text doesn't
make as much sense without the function name, reword it.

BAD: pg_atoi: error in "z": can't parse "z"
BETTER: invalid input syntax for integer: "z"

Avoid mentioning called function names, either; instead say what the code
was trying to do:

BAD: open() failed: %m
BETTER: could not open file %s: %m

If it really seems necessary, mention the system call in the detail
message. (In some cases, providing the actual values passed to the
system call might be appropriate information for the detail message.)

RATIONALE: Users don't know what all those functions do.

Tricky words to avoid
---------------------

unable:

"unable" is nearly the passive voice. Better use "cannot" or "could
not", as appropriate.

bad:

Error messages like "bad result" are really hard to interpret
intelligently. It's better to write why the result is "bad", e.g.,
"invalid format".

illegal:

"Illegal" stands for a violation of the law, the rest is "invalid".
Better yet, say why it's invalid.

unknown:

Try to avoid "unknown". Consider "error: unknown response". If you
don't know what the response is, how do you know it's erroneous?
"Unrecognized" is often a better choice. Also, be sure to include the
value being complained of.

BAD: unknown node type
BETTER: unrecognized node type: 42

find vs. exists:

If the program uses a nontrivial algorithm to locate a resource (e.g., a
path search) and that algorithm fails, it is fair to say that the program
couldn't "find" the resource. If, on the other hand, the expected
location of the resource is known but the program cannot access it there
then say that the resource doesn't "exist". Using "find" in this case
sounds weak and confuses the issue.

Proper spelling
---------------

Spell out words in full. For instance, avoid:

spec
stats
parens
auth
xact

RATIONALE: This will improve consistency.