Issue 12374: MOF QVT 1.0, 8.2.2.22, Unclear specification of Unpack notation shorthand (qvt-rtf)
Source: Model Driven Solutions (Dr. Edward Willink, ed(at)willink.me.uk)
Nature: Uncategorized Issue
Severity: 
Summary: Section 8.2.2.22 provides no guidance on the meaning of:


var x:X;
var y:Y;
var z:Z;
...
var (x:X,y,z) := self.foo();


[If one of the variables in the left hand side, does exist ...]


Possible interpretations


a) x:X is always a syntactical shorthand.


Therefore the above is a shorthand for:


var x:X;
var y:Y;
var z:Z;
...
var x:X; var (x,y,z) := self.foo();


and consequently there is duplicate variable definition to be
reported as a semantic error.


b) x:X is a convenience type check assertion


Therefore the above is a shorthand for:


var x:X;
var y:Y;
var z:Z;
...
var (x,y,z) := self.foo();


with a successful validation of type consistency.


c) x:X is not available as a shorthand


Therefore the above is a syntax error.


------------------------------------


Interpretations b) and c) require semantic knowledge to resolve syntactic
sugar.


Interpretation a) is an unambiguous syntax shorthand that could be more
clearly
described by:


"The following example demonstrates a shorthand in which variables are both
declared
and assigned.


        var (x:X,y:Y,z) := self.foo();


Any variable name qualified by a type name is both a declaration and an
assignment,
with all declarations proceding the unapack assignment. 
The above example should therefore be analyzed as:


        var x:X; var y:Y; var (x,y,z) := self.foo();"


---------------------------------------------------------------------


Recommend interpretation a) with the above textual clarification.

Resolution: 
Revised Text: (1) Replace the text: 
"""If one of the variables in the left hand side does not exist, the notation is a shorthand for declaring the missing variables prior to the unpack expression. In such case, the type of the variables can be given. For instance, in the example above, if 'x' and 'y' does not exist, the expression var (x:X,y:Y,z) := self.foo(); is equivalent to: var x:X; var y:Y; var (x,y,z) := foo(); 
""" 
By the following text: 
"""The following example demonstrates a shorthand in which variables are both
declared and assigned.

       var (x:X,y:Y,z) := self.foo();

Any variable name which specifies a type name is both a declaration and an
assignment, with all declarations proceding the unpack assignment.
The above example should therefore be analyzed as:

       var x:X; var y:Y; var (x,y,z) := self.foo();"
""" 

Actions taken:
April 8, 2008: received issue
April 26, 2010: closed issue
Discussion: 

End of Annotations:=====

m: "Willink, Ed" <Ed.Willink@thalesgroup.com>
To: "'issues@omg.org'" <issues@omg.org>
Cc: 'Adolfo Sánchez-Barbudo Herrera' <adolfosbh@opencanarias.com>
Subject: MOF QVT 1.0, 8.2.2.22, Unclear specification of Unpack notation s
 horthand
Date: Tue, 8 Apr 2008 11:36:31 +0100 
X-Mailer: Internet Mail Service (5.5.2656.59)


Section 8.2.2.22 provides no guidance on the meaning of:


var x:X;
var y:Y;
var z:Z;
...
var (x:X,y,z) := self.foo();


[If one of the variables in the left hand side, does exist ...]


Possible interpretations


a) x:X is always a syntactical shorthand.


Therefore the above is a shorthand for:


var x:X;
var y:Y;
var z:Z;
...
var x:X; var (x,y,z) := self.foo();


and consequently there is duplicate variable definition to be
reported as a semantic error.


b) x:X is a convenience type check assertion


Therefore the above is a shorthand for:


var x:X;
var y:Y;
var z:Z;
...
var (x,y,z) := self.foo();


with a successful validation of type consistency.


c) x:X is not available as a shorthand


Therefore the above is a syntax error.


------------------------------------


Interpretations b) and c) require semantic knowledge to resolve syntactic
sugar.


Interpretation a) is an unambiguous syntax shorthand that could be more
clearly
described by:


"The following example demonstrates a shorthand in which variables are both
declared
and assigned.


        var (x:X,y:Y,z) := self.foo();


Any variable name qualified by a type name is both a declaration and an
assignment,
with all declarations proceding the unapack assignment. 
The above example should therefore be analyzed as:


        var x:X; var y:Y; var (x,y,z) := self.foo();"


---------------------------------------------------------------------


Recommend interpretation a) with the above textual clarification.


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