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From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.lang.ada
Subject: Re: Initialization
Message-ID: <7540@fy.sei.cmu.edu>
Date: 14 Jun 90 17:38:46 GMT
References: <1957@sparko.gwu.edu>
Reply-To: firth@sei.cmu.edu (Robert Firth)
Organization: Software Engineering Institute, Pittsburgh, PA
List-Id: <comp.lang.ada>

In article <1957@sparko.gwu.edu> mfeldman@seas.gwu.edu (Michael Feldman) writes:
>I have a rationale-type questions regarding an apparent irregularity
>in Ada, namely in the area of default initializations.

There is no good reason for the irregularity.  Since a record
component can be of any type, the implementation of

	record
	  Component : Thing := Initial_Value;
	end record;

is no more and no less difficult than the implementation of

	type Thing is Type_Definition := Initial_Value;

would be.  There are no serious parsing difficulties with the
above syntax, and no additional semantic issues.

The reason for the irregularity is historical.  In the original
Green language, the syntax of a record type definition read

	record_type_definition ::= RECORD component_list END RECORD

	component_list ::= {object_declaration} [variant_part] | NULL ;

Because the syntax of object_definition was reused, the ability to
initialise the component got carried over automatically.  When this
was noticed, it was decided that this was a useful thing to have.

I recall asking Jean why he didn't make the type model more regular
by allowing all types to have initialisation expressions, and his
reply was that you could always achieve this effect by nesting the
type you wanted in a record, and he didn't want to add extra syntax.
Please do not ask me to defend this position.