what is a closure?

[Terry Reedy]

"Steve Horne" <sh at ttsoftware.co.uk> wrote in message
news:8nck3tocsg8kp1u2g1du6nd3lt9gfeckr2 at 4ax.com...
> I was reading a book about developing compilers recently (Modern
> Compiler Design, by Dick Grune, Henri Bal, Ceriel Jacobs and Koen
> Langendoen), and something that was presented in it was a 'closure
> algorithm'. This took a collection of data and some related
> expressions, and repeatedly evaluated the expressions using both
> original and previously derived data until all the data it was
> possible to derive was fully derived.
>
> The thing that made it a closure algorithm seemed to be the fact that
> the expressions were applied repeatedly on an accumulating set of data
> until no further data could be derived - the closure being the
> complete collection of data.

This sounds like 'transitive closure', often denoted by '*', which is
different from function closure.