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From: randhol+abuse@pvv.org (Preben Randhol)
Newsgroups: comp.lang.ada
Subject: Re: Types vs subtypes
Date: 29 Aug 2000 06:06:45 GMT
Organization: Norwegian university of science and technology
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On Mon, 28 Aug 2000 16:20:11 -0700, Richard Riehle wrote:
>
>This is something that comes up a lot in my Ada classes.   I try to explain it
>this way:
>
>A type has
>                      1)  a name
>                      2)  a set of operations
>                      3)  a set of values
>                      4)  a wall between objects of itself and objects of other
>types with differing type names
>
>The last item, " a wall" is important in distinguishing structural equivalence
>from name equivalence in the
>Ada type system.
>
>A subtype has
>
>                     1)  a name
>                     2)  a parent type
>                     3)  the set of operations of its parent type
>                     4)  either the same set of values of its parent type or a
>constrained set of values
>                     5)  no wall between operations between itself and its
>parent type
>                     6)  no wall between itself and other subtypes derived from
>its parent or from itself
>
>A subtype is structurally equivalent to its parent type and its subtype
>siblings, but may have a smaller
>range of legal values.

Yes. I like to look at it in this (rough) way in accordance with the
syntax :

    type Height is new Integer;
    --          ^^^^^^
    -- Think of it as a new type.

    subtype Height is Integer;
    --             ^^
    -- Think of it as it _is_ Integer type and not a new one.

-- 
Preben Randhol - Ph.D student - http://www.pvv.org/~randhol/
"i too once thought that when proved wrong that i lost somehow"
                               - i was hoping, alanis morisette