plain TeX theory, \afterassignment

Question

Is there a real difference between

\newcount\acounter
\def\dosomething{\afterassignment\dosomesecret\acounter=}
\def\dosomesecret{I do something with \the\acounter}

and

\newcount\acounter
\def\dosomething#1{\acounter=#1%
    I do something with \the\acounter}

What is the plus of \afterassignment? Why should I prefer one solution over the other?

Answer 1

The main difference is that using \afterassignment you can preserve the assignment syntax. So in your counter example any number of tokens following \dosomething would be expanded until a sequence of non expandable tokens making a <number> are scanned. The second version forces a macro-argument syntax where the number has to be given as a single token or brace group. Which is preferable depends on what you are trying to do.

Another example from the latex sources

\def\protected@edef{%
   \let\@@protect\protect
   \let\protect\@unexpandable@protect
   \afterassignment\restore@protect
   \edef
}

\protected@edef takes the syntax of \edef with delimited arguments etc and restores the meaning of \protect after the \edef. So you can do

\protectected@edef\foo#1#2\@nil{.....#1...#2}

It would be rather less convenient to do that without using \afterassignment.

Another, perhaps better, example again based on usage in the latex base, the following plain TeX file

\def\removetonil#1\xx{}

\def\myset#1{\afterassignment\removetonil
             \dimen0=#1pt\relax\xx
             \immediate\write20{[\the\dimen0]}}

\myset{3}
\myset{3em}    
\myset{3cm}
\myset{\vsize}

\bye

Produces

[3.0pt]
[30.00005pt]
[85.35826pt]
[643.20255pt]

This is setting \dimen0 to a user-specified length where the argument may omit the units (defaulting to pt) or give explicit units, or be a TeX dimen register or primitive such as \vsize. By using \afterassignment the primitive assignment may or may not use tokens after #1 pt\relax will be used if the argument is a <number> but not if it is already a dimension. Because \removetonil is inserted immediately after the assignment it can clear away any unused tokens.