Why latex does not compile when two newlines are inserted in math mode?

Question

When you try to compile the following code:

\begin{equation}
   \phi = \psi

\end{equation}

The compilation fails with this error message:

! Missing $ inserted.

I know how to fix this. But still, I want to understand what the heck is the compiler trying to do and why it cannot compile this seemingly correct code. I am not asking how to insert a newline in the output file.

By the way, I only tried this using the pdftex compiler. I am not sure if this happens in every compiler.

Answer 1

It is an error check built in to TeX (not latex) at its lowest level, similar to the runaway argument check that by default if you omit a closing brace to a macro \zz{.... It does not eat your entire document, the error is trapped at the end of the paragraph.

Similarly if you start math but end the paragraph before the math ends, that is an error that is flagged at the end of the paragraph,

Note that it is not latex or two newlines (which are reported as the token \par) but an occurrence of the par primitive that triggers the error, so this plain Tex document gives the same error

$$a\endgraf$$


\bye

Conversely this latex document generates no error, despite having a blank line in an equation:

\documentclass{article}

\begin{document}

\begin{equation}\let\par\relax
   \phi = \psi

\end{equation}


\end{document}