Idea for Ada 200x: Arguments that are procedures

[vsnyder@gyre.jpl.nasa.gov (Van Snyder)]

Procedures that are arguments are restricted in Ada because of the
possibility of copying the pointer to somewhere that has a longer
lifetime than the up-level environment of the procedure.

If an additional attribute were added to formal arguments to indicate
that taking a copy of a procedure pointer is prohibited, restrictions
on procedures that could be arguments could be reduced.

Since "limited" already puts restrictions on assignment, how about
using that word for the purpose of declaring a formal argument for
which it's prohibited to take a copy.  It could only be used to
access a procedure, or passed as an actual argument to a formal
argument that also had the "limited" attribute.

This would let me have, for example, a library package for quadrature
or minimization or differential equations or ... that exports a type
for a procedure pointer argument, and use a procedure that's internal
to my deep-in-the-guts-of-the-system procedure as the actual argument.
I can't do this now, because the type might outlive the procedure, and
therefore a pointer to the procedure might outlive its up-level.

-- 
What fraction of Americans believe   |  Van Snyder
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