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lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay) writes:
>[Ada lets you say "type VOLT is delta 0.125"]
>[The problem is roundoff will be eighths of a volt.]
>[Conversions are bad: "volatage := 0.3" -- choose: 0.2 or 0.375]

I AMP SHOCKED by your REVOLTING example :-)

I believe that the Ada spec requires that the increments be at least
as fine as `delta', but that they can be finer if the compiler wants.
(If I'm wrong, assume that I have a language `adA' that does allow the
compiler to choose finer deltas.) Thus, if you have Ada on a machine
which does only decimal arithmetic operations (your average pocket
caluculator :-), then the implementation delta can be 0.1 units.
Given floats with sufficient range, it could even use floats.

This example points in a mostly uncharted direction for programming
languages: an assertion that something must be minimally true vs. an
assertion that something must be exactly true.

The `semantics' of Ada's delta are approximately

	type VOLT is delta 0.125;
	-- pragma require VOLT.delta <= 0.125

If you actually need a delta of 0.125, then you need to make a
stronger assertion.

	type VOLT is delta 0.125;
	-- pragma require VOLT.delta = 0.125

If I'm writing joe code that I want to run fast and some margin for
error is acceptable, then I'll probably write the above, even if all
that I want is resolution of 0.2 units.  Ideally, I should be able to
tell the compiler ``this is what I need, and here's a hint on how to
do it.''

	type VOLT is delta 0.2;
	-- pragma hint VOLT.delta = 0.125

Similarly, if you are troubled by conversions, you could hint that the
compiler should choose a finer delta

	type VOLT is delta 0.125;
	-- pragma hint VOLT.delta <= 1.0

No, no Ada compiler is required to let you do any of this.

Followups to comp.lang.misc.

	;-D on  ( Fixed pointless arithmetic )  Pardo
-- 
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