Cfrgtkky

I've seen the function output.check used in .bst files a number of times; I'm sure I knew what it did at some point but I've now forgotten, and I can't easily find documentation that explains it (which presumably exists, but is buried in a bunch of resources which use it but don't say much about it.

In my (merlin-generated) .bst file, it reads

FUNCTION {output.check}

{ 't :=

  duplicate$ empty$

    { pop$ "empty " t * " in " * cite$ * warning$ }

    'output.nonnull

  if$

}

but most of that is unclear to me. What does this function do?

Short version:

It checks if a required field is empty and issues a warning if so. To know which field you need to look at where the function is called. For instance, looking at the article function in plain.bst:

format.authors "author" output.check

This will check the author field and will print empty author in <entry> to the blg file.

Long version:

For instance, in the example above, the format.authors function is:

FUNCTION {format.authors}

{ author empty$

    { "" }

    { author format.names }

  if$

}

It will leave either "<Author-names>" or "" on the stack, depending if the author field was given. So, by now, our stack has as a (possibly empty) string. Now we insert a string "author" on the stack and call output.check.

output.check will do 't :=, which will put the topmost item of the stack (the literal string "author") in the variable 't. Now we only have the (possibly empty) "<Author-names>" string on the stack.

Then we have duplicate$, which will make a copy of that, and then we have empty$, which will leave a logical 1 or 0 if that string of author names is empty.

The two conditional branches are inserted on the stack and consumed when the if$ is called. The if$ will check if the empty$ returned 1 or 0 and will put the first or second branches on the stack accordingly.

If the 1 branch is taken, it means that the author field was not given, then we'll have pop$ "empty " t * " in " * cite$ * warning$ which will discard (now certainly) empty "<Author-names>" string and issue the warning. After that the execution continues normally.

If the 0 branch is taken, then 'output.nonnull if called to print a (now certainly) not empty string.