The Short Story

If some text won't all fit on the current line, we want all of that text moved to the next line instead. How can we do this?

The Medium-Length Story

Suppose we have a string of text, "banana banana." Having the first few characters of "banana [...]" on the current line and the second half of the banana string the next line is undesirable.

If the entire string will fit on the current line, then good, put the string on the current line. Otherwise put the string on the next line.

Undesirable:

apples apples apples banana

banana. cantaloupe cantaloupe

Desirable:

% a line break is inserted at the beginning

% of the bananas because they won't all fit

% on the current line.



apples apples apples 

banana banana. cantaloupe cantaloupe

How can we write LaTeX code which makes this happen?

The Long Story

Consider the following test file. It will not compile because "KEEP TOGETHER" is not yet defined. The question can be viewed as: What do we need to do the make the following file produce the desired behavior?

documentclass{minimal}

begin{document}



apples apples apples apples

apples apples apples apples

apples apples apples apples 



begin{KEEP TOGETHER}

    Banana tincidunt ante banana.

end{KEEP TOGETHER}



cantaloupe cantaloupe cantaloupe

cantaloupe cantaloupe cantaloupe

cantaloupe cantaloupe.



end{document}

Suppose that STRYNG is the text block enclosed by begin{KEEP TOGETHER} and end{KEEP TOGETHER}. We wish to have the following behavior:

SPACER = four character indent based on

         current font size and other settings.



if (STRYNG will fit on what remains

               of the current line) { 

    WILL_FIT_HERE = true;

}

if (STRYNG will fit on an almost empty

    blank line, SPACER at the beginning) {

    WILL_FIT_NEXT = true;

}    

if [(not WILL_FIT_HERE) and (WILL_FIT_NEXT)] {

    print a line break

    print SPACER on the brand new empty line.

    print all of STRYNG after the indent

    print a line-break.

else {

    print STRYNG in the output document as if STRYNG

    were not inside of a "KEEP TOGETHER" block.



    Exhibit the same behavior as if the

    lines begin{KEEP TOGETHER} and

    end{KEEP TOGETHER} were commented out.



    This probably means that STRYNG (or at least

    the beginning of string) will be printed on

    the current line.

}

Similar, but Different, Stack Exchange Questions

There is a question which embodies the same sentiments as the current question, but that question was much too vague. It was not clear what was wanted. Some people interpreted the question to mean, "how can we prevent a line break in the middle of a word?" They thought it was okay to have half of the string on on the current line and the second half on the next line. There were other interpretations as well.

The asker of prevent-line-break-in-a-span-of-text was interested in displaying URLs containing no space characters. Like me, they want a line break inserted at the very beginning of their text, instead of in the middle. However, they do not describe what behavior they want if the URL cannot all fit on one line. In the question you are reading, I do describe what behavior is desired in that instance. Also, for the other asker, maybe LaTeX was inserting line break into their urls before the right-margin was reached. Maybe they wanted characters in the URL to run all the way to the right-margin, and only then have a line break inserted then, but not any earlier than that. I am not sure what they wanted.

Unacceptable Solutions

Unacceptable: Always put a line break at the beginning

always inserting a line-break looks funny if there was plenty of space on the line above to fit the text you want to keep together.

Unacceptable: Non-breaking spaces (~)

Writing things like Cras~mattis~tincidunt~ante is not okay for a few reasons. For one, ~ must be inserted everywhere, which is a nuisance. Another is that the string to be "kept together," might not contain any space characters, won't all fit on the current line, but could fit on a new line all by itself. We want something similar to begin{command}{BLOCK OF TEXT}end{command}.

Unacceptable: nolinebreak

nolinebreak works once EXACTLY where you put it and then is not in effect after that point. Inserting nolinebreak in a dozen different places would make the source code unreadable. The issue is similar to using ~.