$ latex \\typein[\\foo]{say quack}\\show\\foo
This is pdfTeX, Version 3.1415926-2.5-1.40.14 (TeX Live 2013)
restricted \write18 enabled.
entering extended mode
LaTeX2e <2011/06/27>
Babel <3.9f> and hyphenation patterns for 54 languages loaded.
say quack
\foo=quack
> \foo=macro:
->quack.
<*> \typein[\foo]{say quack}\show\foo
? x
No pages of output.