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From: "Nick Roberts" <Nick.Roberts@dial.pipex.com>
Subject: Re: Fixed point multiplication ambiguity
Date: 1999/01/29
Message-ID: <78r4qh$fcu$1@plug.news.pipex.net>#1/1
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Newsgroups: comp.lang.ada
Date: 1999-01-29T00:00:00+00:00
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Tucker Taft wrote in message ...
[...]
|: ... The D*10.0 does resolve as Tucker says, giving a universal_fixed. The
0.5
|: literal is a universal_real. Thus adding them gives us the pattern
|: "+"(universal_fixed,universal_real) return unknown. However, it is the
rule
|: in RM 8.6 [29,30] preferring root types that allows this to be resolved
into
|: the pattern "+"(root_real,root_real) return root_real implied by RM 4.5.3
|: [2]. So the expression D*10.0+0.5 returns a value of type root_real,
which
|: can then be converted to an integer and assigned to Dec_Delt.
|
|Sorry, but you are wrong.  Universal_fixed is *not* implicitly convertible
|to root_real.

Tucker is, of course, quite right. My slip.

|It is implicitly convertible to any *fixed point* type,
|and to universal_real (because any real type is implicitly convertible
|both *to* and *from* universal real), but *not* to root_real (I realize
|only a died-in-the-wool language lawyer will believe this reasoning).


|: However, the expression Integer(D*10.0+0.5) causes a root_real addition
and
|: conversion to integer to be executed dynamically.
|
|No, this expression uses Duration's "+", because this is effectively
|equivalent to:
|   Integer(Duration(D*10.0)+Duration(0.5))


I am not arguing with this, but which rule (or rules) in the RM specify the
above interpretation, please? I've searched and searched (even the AI95s)!
(Then again, I reckon my brain must be coming up for it's 20,000 mile
service anyway ;-)

|The whole fixed-point multiplication model is pretty obscure,
unfortunately.


But then I would say that, in general, the RM95 does an amazingly good job
of pinning down a slippery monster. I hope I haven't added to the confusion
too much :-(

Just off for a slice of humble pie. Tuck remains champ :-)
-------------------------------------------
Nick Roberts
-------------------------------------------