Distinguishing type names from other identifiers

["Nick Roberts" <Nick.Roberts@dial.pipex.com>]

The technique I generally use for inventing a type identifier distinct from
identifiers I might use for objects and parameters of that type is to make
the type identifier one step more specific (and thus, usually, longer).

The justification for using a longer identifier for the type, rather than
for the objects and parameters, is that type identifiers are generally used
less often in practical source text.

For example:

   type Hour_Of_Day is range 0..23;
   type Minute_Of_Hour is range 0..59;
   type Seconds_Past_Minute is delta 0.001 range 0.000 .. 59.999;

   function To_Seconds_Past_Midnight
                   (Hour:    Hour_Of_Day;
                    Minute:  Minute_Of_Day;
                    Seconds: Seconds_Past_Minute) return Day_Duration;

This example illustrates (I hope ;-) that the use of a plural or singular
form for an identifier is not naturally to do with whether it denotes a
type, an object, a parameter, a subprogram, or whatever, but rather with
whatever concept it describes.  The choice of Seconds_Past_Minute is
perhaps a little clumsy.  I think this is the reason for many scientists'
habit of using the singular form in formal language (as well as other
archaic forms) where in common language the plural would be used; e.g. "...
if Train A be travelling at twelve metre per second ...".

On the subject of the standard Calendar package, I would probably have
chosen the identifier Day_Of_Month rather than Day_Number, and
Month_Of_Year rather than Month_Number.  However, either way, the
identifiers Day and Month can be conveniently used for identifiers of
objects and parameters of these types.

Adding _Type can be a useful way to distinguish a type, as a last resort I
would suggest.  I've always felt the 'size_t' introduced by ANSI C to be a
relatively happy compromise between size and explicitness (NPI again!).  So
I reckon adding _T is not such a sin.  But is it really worth saving the
three -- count them, THREE -- extra letters?

Come what may, I do quite strongly feel that a good programmer will strive
to achieve a maximum of clarity, and a minimum of ambiguity, when choosing
identifiers, and will not allow worries about typing sully this ideal.  It
is surely a fact that, in a few rare cases, choosing short (even terse)
notation will have advantages which outweigh other considerations.  But
these cases are RARE, so say I.

Finally, I think the declaration of the fixed-point type
Seconds_Past_Minute in the example raises another interesting question. 
Should I have used 59.999 for the upper bound, or 60.000?  Is it a bad
omission that Ada does not provide for both open and closed bounds?

-- 

Nick Roberts
Croydon, UK

Proprietor, ThoughtWing Software; Independent Software Development
Consultant
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