[Tex/LaTex] the difference between \let and \edef


When you write


the results are often identical. What is the difference between these two commands? When do they behave the same? When do they not?

(The question isn't for me; inspired by this question comparing \let and \def, I'm asking this question so it can be used as a reference to point people towards in the future.)

Best Answer

\edef expands the argument, whereas \let doesn't. Here is an example to illustrate the difference:

\bar -> a
\bar -> b


\bar -> a
\bar -> a

There are also other differences, say, the arguments and so on. But how to expansion may be the most important(?).

This is an interesting question. May I expand the question further more?

What is the difference between \let and \expandafter\def\expandafter\foo\expandafter ? Do they always behave the same?


{\tt \string\bar = \meaning\bar}\par
{\tt \string\BAR = \meaning\bar}

Sneaky inline answer to this rhetorical question, since I wrote the original question :)

If you are doing this on macros that take no arguments, then the difference between them is negligible. OTOH, you cannot use the \expandafter\def... construct if you're trying to copy the definition of a macro that takes arguments.

In fact, this brings to light one of the aspects that I was hoping people would discuss here. \let creates a literal copy of a macro at the instant that it is executed, whereas \edef will take the contents of the macro and expand it recursively to create a new macro entirely. When you are only using the macros as places to store data (such as \def\name{Will}) then the differences are largely inconsequential, but when the macros are being used as ‘functions’ that take arguments or have contents that have various expansion restrictions applied to it (with \protected, \noexpand, and so on) then the differences can be very important indeed.