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Add description of error style.
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Bruce Momjian
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To: Peter Eisentraut <peter_e@gmx.net>
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cc: pgsql-hackers@postgresql.org
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Subject: [HACKERS] Error message style guide, take 2
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Date: Wed, 14 May 2003 16:58:18 -0400
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From: Tom Lane <tgl@sss.pgh.pa.us>
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I'm about to start going through the backend's elog() calls to update
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them to ereport() style, add error code numbers, polish wording, etc.
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So it's time to nail down our style guide for message wording. Attached
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is a revision of the draft that Peter posted on 14-March. Any further
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comments?
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BTW, I'd like to SGML-ify this and put it into the developer's guide
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somewhere; any thoughts where exactly?
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regards, tom lane
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What goes where
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---------------
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The primary message should be short, factual, and avoid reference to
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implementation details such as specific function names. "Short" means
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"should fit on one line under normal conditions". Use a detail message if
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needed to keep the primary message short, or if you feel a need to mention
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implementation details such as the particular system call that failed.
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Both primary and detail messages should be factual. Use a hint message
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for suggestions about what to do to fix the problem, especially if the
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suggestion might not always be applicable.
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For example, instead of
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IpcMemoryCreate: shmget(key=%d, size=%u, 0%o) failed: %m
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(plus a long addendum that is basically a hint)
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write
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Primary: Could not create shared memory segment: %m
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Detail: Failed syscall was shmget(key=%d, size=%u, 0%o)
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Hint: the addendum
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RATIONALE: keeping the primary message short helps keep it to the point,
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and lets clients lay out screen space on the assumption that one line is
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enough for error messages. Detail and hint messages may be relegated to a
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verbose mode, or perhaps a pop-up error-details window. Also, details and
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hints would normally be suppressed from the server log to save space.
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Reference to implementation details is best avoided since users don't know
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the details anyway.
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Formatting
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----------
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Don't put any specific assumptions about formatting into the message
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texts. Expect clients and the server log to wrap lines to fit their own
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needs. In long messages, newline characters (\n) may be used to indicate
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suggested paragraph breaks. Don't end a message with a newline. Don't
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use tabs or other formatting characters. (In error context displays,
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newlines are automatically added to separate levels of context such
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as function calls.)
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RATIONALE: Messages are not necessarily displayed on terminal-type
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displays. In GUI displays or browsers these formatting instructions
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are at best ignored.
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Quotation marks
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---------------
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English text should use double quotes when quoting is appropriate.
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Text in other languages should consistently use one kind of quotes
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that is consistent with publishing customs and computer output of
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other programs.
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RATIONALE: The choice of double quotes over single quotes is somewhat
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arbitrary, but tends to be the preferred use. Some have suggested
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choosing the kind of quotes depending on the type of object according to
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SQL conventions (namely, strings single quoted, identifiers double
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quoted). But this is a language-internal technical issue that many users
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aren't even familiar with, it won't scale to other kinds of quoted terms,
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it doesn't translate to other languages, and it's pretty pointless, too.
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Use of quotes
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-------------
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Use quotes always to delimit file names, user-supplied identifiers,
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and other variables that might contain words. Do not use them to
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mark up variables that will not contain words (for example, operator
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names).
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There are functions in the backend that will double-quote their own
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output at need (for example, format_type_be()). Do not put additional
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quotes around the output of such functions.
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RATIONALE: Objects can have names that create ambiguity when embedded
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in a message. Be consistent about denoting where a plugged-in name
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starts and ends. But don't clutter messages with unnecessary or
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duplicate quote marks.
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Grammar and punctuation
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-----------------------
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The rules are different for primary error messages and for detail/hint
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messages:
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Primary error messages: Do not capitalize the first letter. Do not end a
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message with a period. Do not even think about ending a message with an
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exclamation point.
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Detail and hint messages: Use complete sentences, and end each with
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a period. Capitalize the starts of sentences.
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RATIONALE: Avoiding punctuation makes it easier for client applications to
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embed the message into a variety of grammatical contexts. Often, primary
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messages are not grammatically complete sentences anyway. (And if they're
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long enough to be more than one sentence, they should be split into
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primary and detail parts.) However, detail and hint messages are longer
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and may need to include multiple sentences. For consistency, they should
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follow complete-sentence style even when there's only one sentence.
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Upper case vs. lower case
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-------------------------
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Use lower case for message wording, including the first letter of a
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primary error message. Use upper case for SQL commands and key words if
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they appear in the message.
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RATIONALE: It's easier to make everything look more consistent this
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way, since some messages are complete sentences and some not.
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Avoid passive voice
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-------------------
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Use the active voice. Use complete sentences when there is an acting
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subject ("A could not do B"). Use telegram style without subject if
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the subject would be the program itself; do not use "I" for the
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program.
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RATIONALE: The program is not human. Don't pretend otherwise.
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Present vs past tense
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---------------------
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Use past tense if an attempt to do something failed, but could perhaps
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succeed next time (perhaps after fixing some problem). Use present tense
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if the failure is certainly permanent.
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There is a nontrivial semantic difference between sentences of the
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form
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could not open file "%s": %m
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and
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cannot open file "%s"
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The first one means that the attempt to open the file failed. The
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message should give a reason, such as "disk full" or "file doesn't
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exist". The past tense is appropriate because next time the disk
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might not be full anymore or the file in question may exist.
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The second form indicates the the functionality of opening the named
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file does not exist at all in the program, or that it's conceptually
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impossible. The present tense is appropriate because the condition
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will persist indefinitely.
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RATIONALE: Granted, the average user will not be able to draw great
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conclusions merely from the tense of the message, but since the
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language provides us with a grammar we should use it correctly.
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Type of the object
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------------------
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When citing the name of an object, state what kind of object it is.
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RATIONALE: Else no one will know what "foo.bar.baz" is.
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Brackets
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--------
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Square brackets are only to be used (1) in command synopses to denote
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optional arguments, or (2) to denote an array subscript.
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RATIONALE: Anything else does not correspond to widely-known customary
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usage and will confuse people.
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Assembling error messages
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-------------------------
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When a message includes text that is generated elsewhere, embed it in
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this style:
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could not open file %s: %m
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RATIONALE: It would be difficult to account for all possible error codes
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to paste this into a single smooth sentence, so some sort of punctuation
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is needed. Putting the embedded text in parentheses has also been
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suggested, but it's unnatural if the embedded text is likely to be the
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most important part of the message, as is often the case.
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Reasons for errors
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------------------
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Messages should always state the reason why an error occurred.
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For example:
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BAD: could not open file %s
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BETTER: could not open file %s (I/O failure)
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If no reason is known you better fix the code. ;-)
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Function names
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--------------
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Don't include the name of the reporting routine in the error text.
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We have other mechanisms for finding that out when needed, and for
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most users it's not helpful information. If the error text doesn't
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make as much sense without the function name, reword it.
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BAD: pg_atoi: error in "z": can't parse "z"
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BETTER: invalid input syntax for integer: "z"
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Avoid mentioning called function names, either; instead say what the code
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was trying to do:
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BAD: open() failed: %m
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BETTER: could not open file %s: %m
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If it really seems necessary, mention the system call in the detail
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message. (In some cases, providing the actual values passed to the
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system call might be appropriate information for the detail message.)
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RATIONALE: Users don't know what all those functions do.
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Tricky words to avoid
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---------------------
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unable:
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"unable" is nearly the passive voice. Better use "cannot" or "could
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not", as appropriate.
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bad:
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Error messages like "bad result" are really hard to interpret
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intelligently. It's better to write why the result is "bad", e.g.,
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"invalid format".
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illegal:
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"Illegal" stands for a violation of the law, the rest is "invalid".
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Better yet, say why it's invalid.
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unknown:
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Try to avoid "unknown". Consider "error: unknown response". If you
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don't know what the response is, how do you know it's erroneous?
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"Unrecognized" is often a better choice. Also, be sure to include the
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value being complained of.
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BAD: unknown node type
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BETTER: unrecognized node type: 42
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find vs. exists:
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If the program uses a nontrivial algorithm to locate a resource (e.g., a
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path search) and that algorithm fails, it is fair to say that the program
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couldn't "find" the resource. If, on the other hand, the expected
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location of the resource is known but the program cannot access it there
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then say that the resource doesn't "exist". Using "find" in this case
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sounds weak and confuses the issue.
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Proper spelling
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---------------
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Spell out words in full. For instance, avoid:
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spec
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stats
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parens
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auth
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xact
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RATIONALE: This will improve consistency.
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---------------------------(end of broadcast)---------------------------
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TIP 3: if posting/reading through Usenet, please send an appropriate
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subscribe-nomail command to majordomo@postgresql.org so that your
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message can get through to the mailing list cleanly
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