Re: ML-like alternatives to out parameters for functions

["Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>]

"Robert I. Eachus" <rieachus@attbi.com> wrote in message
news:3E890A20.8080108@attbi.com...
> Note that this does not mean you cannot write functions in Ada that
> alter their parameters.
>
>     function Random(Gen: in Generator) return Uniformly_Distributed is
>       Local_Gen: Generator;
>       for Local_Gen'Address use Gen'Address;
>     begin...
>
> is perfectly legal Ada.

It may be legal code, but the question arises as to whether this is good
code. In most cases, parameter modes are an aid to both the compiler and the
programmer, telling them when we can assume that an actual parameter could
be altered and when its state will remain the same. In the case of the
Random function, however, the parameter mode is a lie. The function
specification indicates that the Gen parameter is not modified by a call to
Random, even though we all know that it does indeed get modified. This is an
inelegant kludge, and it stands out as a blemish on the predefined packages,
which for the most part are very well designed.