Re: Ada to 'C' parameter passing problem

[Stuart Palin <null@0.0>]

Simon Wright wrote:

> I recall Robert Dewar saying that he thought that restricting
> functions to "in" (or access) parameters was a Bad Idea -- Ada is a
> programming language, not a mathematical abstraction, and there is
> nothing in Ada to stop a function modifying global state. (Praise be,
> say I). But Robert got outvoted during the Ada 9X process.

I would think that there are [at least] a couple of
conflicting inputs to this.

1) By allowing functions to have side-effects you create
problems related to expression evaluation order - which
introduces an additional source of compiler dependencies.

2) Modifying global state is not prevented by the language
rules.  But this would be so difficult to do within the
constraints (or freedoms) of other language objectives
(principally the separate compilation of specification and
body and the hiding of internal behaviour.  Note that SPARK
addresses the issue of purity by ensuring the effect on
global state is made visible at the specification level).

Point 1 suggests it is a bad idea, point 2 shows the
language can not be entirely pure on this matter.

Would allowing function parameters to be in out/out be
encouraging a risky practice, or a recognition that the
language can not fully police the concept.

I favour the existing language rules which at least do not
'encourage' the risks that might arise through functions
with side-effects.

[RD does not usually make unconsidered statements, so I
would be interested in reading his line of arguments.  Is it
available somewhere convenient?]

--
Stuart Palin