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From: Peter Amey <peter.amey@praxis-cs.co.uk>
Newsgroups: comp.lang.ada
Subject: Re: In-Out Parameters for functions
Date: Mon, 02 Feb 2004 10:39:47 +0000
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[snip]
> 
> By the way, I read the paper to which you referred. What happens
> in SPARK if I have
>     package Sensor
>     --# own in State;
>     is
>         function Value return Integer;
>         --# global State;
>     end Sensor;
> and I try to write
>     delta : Integer := Sensor.Value - Sensor.Value;

You get a static semantic error: SPARK only allows variables and 
constants to be initialized by constant (i.e. fixed at compile time) 
expressions.  Functions cannot be used.  (There are other reasosn for 
this as well including avoidance of elaboration order dependencies).

> 
> Can I write a function, not a procedure, which will return
> different sensor values each time? Can I use them multiple
> times in an expression?
> 

No.  There are special rules for volatile functions.  They can only be 
used directly in simple assignment or return statements, not in general 
expressions.

Peter