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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="ERROR-STYLE-GUIDE"
>47.3. Error Message Style Guide</A
></H1
><P
>    This style guide is offered in the hope of maintaining a consistent,
    user-friendly style throughout all the messages generated by
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
>.
   </P
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97647"
>47.3.1. What Goes Where</A
></H2
><P
>    The primary message should be short, factual, and avoid reference to
    implementation details such as specific function names.
    <SPAN
CLASS="QUOTE"
>"Short"</SPAN
> means <SPAN
CLASS="QUOTE"
>"should fit on one line under normal
    conditions"</SPAN
>.  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.
   </P
><P
>    For example, instead of:
</P><PRE
CLASS="PROGRAMLISTING"
>IpcMemoryCreate: shmget(key=%d, size=%u, 0%o) failed: %m
(plus a long addendum that is basically a hint)</PRE
><P>
    write:
</P><PRE
CLASS="PROGRAMLISTING"
>Primary:    could not create shared memory segment: %m
Detail:     Failed syscall was shmget(key=%d, size=%u, 0%o).
Hint:       the addendum</PRE
><P>
   </P
><P
>    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 can 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.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97656"
>47.3.2. Formatting</A
></H2
><P
>    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) can 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.)
   </P
><P
>    Rationale: Messages are not necessarily displayed on terminal-type
    displays.  In GUI displays or browsers these formatting instructions are
    at best ignored.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97660"
>47.3.3. Quotation Marks</A
></H2
><P
>    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.
   </P
><P
>    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.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97664"
>47.3.4. Use of Quotes</A
></H2
><P
>    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).
   </P
><P
>    There are functions in the backend that will double-quote their own output
    at need (for example, <CODE
CLASS="FUNCTION"
>format_type_be()</CODE
>).  Do not put
    additional quotes around the output of such functions.
   </P
><P
>    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.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97670"
>47.3.5. Grammar and Punctuation</A
></H2
><P
>    The rules are different for primary error messages and for detail/hint
    messages:
   </P
><P
>    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.
   </P
><P
>    Detail and hint messages: Use complete sentences, and end each with
    a period.  Capitalize the first word of sentences.  Put two spaces after
    the period if another sentence follows (for English text; might be
    inappropriate in other languages).
   </P
><P
>    Error context strings: Do not capitalize the first letter and do
    not end the string with a period.  Context strings should normally
    not be complete sentences.
   </P
><P
>    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 might need to include multiple sentences.  For consistency, they should
    follow complete-sentence style even when there's only one sentence.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97677"
>47.3.6. Upper Case vs. Lower Case</A
></H2
><P
>    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.
   </P
><P
>    Rationale: It's easier to make everything look more consistent this
    way, since some messages are complete sentences and some not.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97681"
>47.3.7. Avoid Passive Voice</A
></H2
><P
>    Use the active voice.  Use complete sentences when there is an acting
    subject (<SPAN
CLASS="QUOTE"
>"A could not do B"</SPAN
>).  Use telegram style without
    subject if the subject would be the program itself; do not use
    <SPAN
CLASS="QUOTE"
>"I"</SPAN
> for the program.
   </P
><P
>    Rationale: The program is not human.  Don't pretend otherwise.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97687"
>47.3.8. Present vs. Past Tense</A
></H2
><P
>    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.
   </P
><P
>    There is a nontrivial semantic difference between sentences of the form:
</P><PRE
CLASS="PROGRAMLISTING"
>could not open file "%s": %m</PRE
><P>
and:
</P><PRE
CLASS="PROGRAMLISTING"
>cannot open file "%s"</PRE
><P>
    The first one means that the attempt to open the file failed.  The
    message should give a reason, such as <SPAN
CLASS="QUOTE"
>"disk full"</SPAN
> or
    <SPAN
CLASS="QUOTE"
>"file doesn't exist"</SPAN
>.  The past tense is appropriate because
    next time the disk might not be full anymore or the file in question might
    exist.
   </P
><P
>    The second form indicates that 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.
   </P
><P
>    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.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97697"
>47.3.9. Type of the Object</A
></H2
><P
>    When citing the name of an object, state what kind of object it is.
   </P
><P
>    Rationale: Otherwise no one will know what <SPAN
CLASS="QUOTE"
>"foo.bar.baz"</SPAN
>
    refers to.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97702"
>47.3.10. Brackets</A
></H2
><P
>    Square brackets are only to be used (1) in command synopses to denote
    optional arguments, or (2) to denote an array subscript.
   </P
><P
>    Rationale: Anything else does not correspond to widely-known customary
    usage and will confuse people.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97706"
>47.3.11. Assembling Error Messages</A
></H2
><P
>   When a message includes text that is generated elsewhere, embed it in
   this style:
</P><PRE
CLASS="PROGRAMLISTING"
>could not open file %s: %m</PRE
><P>
   </P
><P
>    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.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97711"
>47.3.12. Reasons for Errors</A
></H2
><P
>    Messages should always state the reason why an error occurred.
    For example:
</P><PRE
CLASS="PROGRAMLISTING"
>BAD:    could not open file %s
BETTER: could not open file %s (I/O failure)</PRE
><P>
    If no reason is known you better fix the code.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97715"
>47.3.13. Function Names</A
></H2
><P
>    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.
</P><PRE
CLASS="PROGRAMLISTING"
>BAD:    pg_atoi: error in "z": cannot parse "z"
BETTER: invalid input syntax for integer: "z"</PRE
><P>
   </P
><P
>    Avoid mentioning called function names, either; instead say what the code
    was trying to do:
</P><PRE
CLASS="PROGRAMLISTING"
>BAD:    open() failed: %m
BETTER: could not open file %s: %m</PRE
><P>
    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.)
   </P
><P
>    Rationale: Users don't know what all those functions do.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97722"
>47.3.14. Tricky Words to Avoid</A
></H2
><DIV
CLASS="FORMALPARA"
><P
><B
>Unable. </B
>    <SPAN
CLASS="QUOTE"
>"Unable"</SPAN
> is nearly the passive voice.  Better use
    <SPAN
CLASS="QUOTE"
>"cannot"</SPAN
> or <SPAN
CLASS="QUOTE"
>"could not"</SPAN
>, as appropriate.
   </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Bad. </B
>    Error messages like <SPAN
CLASS="QUOTE"
>"bad result"</SPAN
> are really hard to interpret
    intelligently.  It's better to write why the result is <SPAN
CLASS="QUOTE"
>"bad"</SPAN
>,
    e.g., <SPAN
CLASS="QUOTE"
>"invalid format"</SPAN
>.
   </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Illegal. </B
>    <SPAN
CLASS="QUOTE"
>"Illegal"</SPAN
> stands for a violation of the law, the rest is
    <SPAN
CLASS="QUOTE"
>"invalid"</SPAN
>. Better yet, say why it's invalid.
   </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Unknown. </B
>    Try to avoid <SPAN
CLASS="QUOTE"
>"unknown"</SPAN
>.  Consider <SPAN
CLASS="QUOTE"
>"error: unknown
    response"</SPAN
>.  If you don't know what the response is, how do you know
    it's erroneous? <SPAN
CLASS="QUOTE"
>"Unrecognized"</SPAN
> is often a better choice.
    Also, be sure to include the value being complained of.
</P><PRE
CLASS="PROGRAMLISTING"
>BAD:    unknown node type
BETTER: unrecognized node type: 42</PRE
><P>
   </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Find vs. Exists. </B
>    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 <SPAN
CLASS="QUOTE"
>"find"</SPAN
> 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 <SPAN
CLASS="QUOTE"
>"exist"</SPAN
>.  Using
    <SPAN
CLASS="QUOTE"
>"find"</SPAN
> in this case sounds weak and confuses the issue.
   </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>May vs. Can vs. Might. </B
>    <SPAN
CLASS="QUOTE"
>"May"</SPAN
> suggests permission (e.g., "You may borrow my rake."),
    and has little use in documentation or error messages.
    <SPAN
CLASS="QUOTE"
>"Can"</SPAN
> suggests ability (e.g., "I can lift that log."),
    and <SPAN
CLASS="QUOTE"
>"might"</SPAN
> suggests possibility (e.g., "It might rain
    today.").  Using the proper word clarifies meaning and assists
    translation.
   </P
></DIV
><DIV
CLASS="FORMALPARA"
><P
><B
>Contractions. </B
>    Avoid contractions, like <SPAN
CLASS="QUOTE"
>"can't"</SPAN
>;  use
    <SPAN
CLASS="QUOTE"
>"cannot"</SPAN
> instead.
   </P
></DIV
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97765"
>47.3.15. Proper Spelling</A
></H2
><P
>    Spell out words in full.  For instance, avoid:
  <P
></P
></P><UL
><LI
><P
>     spec
    </P
></LI
><LI
><P
>     stats
    </P
></LI
><LI
><P
>     parens
    </P
></LI
><LI
><P
>     auth
    </P
></LI
><LI
><P
>     xact
    </P
></LI
></UL
><P>
   </P
><P
>    Rationale: This will improve consistency.
   </P
></DIV
><DIV
CLASS="SIMPLESECT"
><H2
CLASS="SIMPLESECT"
><A
NAME="AEN97780"
>47.3.16. Localization</A
></H2
><P
>    Keep in mind that error message texts need to be translated into other
    languages.  Follow the guidelines in <A
HREF="nls-programmer.html#NLS-GUIDELINES"
>Section 48.2.2</A
>
    to avoid making life difficult for translators.
   </P
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