What is the difference between \let and \def?

Question

What is the difference between the \let and \def commands in TeX/LaTeX?

Ideally please provide a simple example that will illustrate the difference between them.

Answer 1

The difference is in the time at which the 'right hand side' is evaluated.

Thus \let\foo\bar defines \foo to have the value that \bar had at the point of defintion.

On the other hand, \def\foo{\bar} defines \foo to have the value that \bar has at the point of use.

Thus, consider:

\def\bar{hello}
\let\fooi\bar
\def\fooii{\bar}
\fooi +\fooii

\def\bar{goodbye}
\fooi +\fooii

This produces

hello+hello
hello+goodbye

Answer 2

They're pretty completely different. \let copies a command to a new name, while \def creates a new command.

For example:

\def \foo {bar}

creates a new command \foo, that evaluates to bar when run.

\let \foo \bar

copies the commands from the \bar commands to the \foo command, so you can call either. Because it's a copy (and not a pointer from one to the other), redefining \foo won't change the behavior of \baz. Hence:

\def \foo {bar}
\let \baz \foo
\baz % Outputs 'bar'
\def \foo {new-definition}
\baz % Still outputs 'bar'

Answer 3

While the existing answers are all true I like to highlight one point which wasn't explicitly mentioned yet. I myself got this information recently from Joseph Wright (see his answer and our comments in Simple un-obfuscation of some LaTeX internals).

As Michael said \let\macroa\macrob "copies" the definition of \macrob to \macroa. However an IMHO important thing here is that the definition isn't actually copied, i.e. exists twice, but the command sequence \macroa now points to the same hash table entry as \macrob. This means that \let\macroa\macrob uses less memory space (very important in the early days but not anymore) and is faster then \def\macroa{\macrob} because in the second form two command sequence names have to be resolved in the hash table.

Also \let actually "copies" the definition of tokens, which do not need to be macros/command sequences. This allows the definition of command sequences representing implicit characters like \let\bgroup={.