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digitalmars.D.learn - BNF Question

reply BCS <ao pathlink.com> writes:
What is the difference between an Identity Expression and a Equal Expression? 
both have a "ShiftExpression is ShiftExpression" and a "ShiftExpression !is 
ShiftExpressionand".

http://www.digitalmars.com/d/expression.html#EqualExpression
http://www.digitalmars.com/d/expression.html#IdentityExpression
Sep 02 2007
parent reply Xinok <xnknet gmail.com> writes:
An equal expression is when the data in two objects is compared.
int[] arr1 = [10, 20, 30], arr2 = [10, 20, 30];
if(arr1 == arr2){ } // True

An identity expresion is when the pointer values (and .length in arrays) 
in two objects is compared.
int[] arr1 = [10, 20, 30], arr2 = arr1;
if(arr1 is arr2){ } // True

For primitive types, there is no difference between using == and is.
int a = 15, b = 15;
if(a == b){ } // True
if(a is b){ } // True

BCS wrote:

 What is the difference between an Identity Expression and a Equal 
 Expression? both have a "ShiftExpression is ShiftExpression" and a 
 "ShiftExpression !is ShiftExpressionand".
 
 http://www.digitalmars.com/d/expression.html#EqualExpression
 http://www.digitalmars.com/d/expression.html#IdentityExpression
Sep 02 2007
parent reply BCS <ao pathlink.com> writes:
Reply to Xinok,


 An equal expression is when the data in two objects is compared.
 int[] arr1 = [10, 20, 30], arr2 = [10, 20, 30];
 if(arr1 == arr2){ } // True
 An identity expresion is when the pointer values (and .length in
 arrays)
 in two objects is compared.
 int[] arr1 = [10, 20, 30], arr2 = arr1;
 if(arr1 is arr2){ } // True
 For primitive types, there is no difference between using == and is.
 int a = 15, b = 15;
 if(a == b){ } // True
 if(a is b){ } // True
 BCS wrote:
 


However what you describe is at the semantic level and the grammar is totally 
at the syntax level. What I was looking at is that this:

a is b

can be parsed as an Identity Expression and an Equal Expression.

If the distinction is a semantic issue then this makes D context sensitive. 
If it is not a semantic issue, then it is redundant or conflicting.


 What is the difference between an Identity Expression and a Equal
 Expression? both have a "ShiftExpression is ShiftExpression" and a
 "ShiftExpression !is ShiftExpressionand".
 
 http://www.digitalmars.com/d/expression.html#EqualExpression
 http://www.digitalmars.com/d/expression.html#IdentityExpression
Sep 02 2007
parent reply Kirk McDonald <kirklin.mcdonald gmail.com> writes:
BCS wrote:

 Reply to Xinok,
 

 An equal expression is when the data in two objects is compared.
 int[] arr1 = [10, 20, 30], arr2 = [10, 20, 30];
 if(arr1 == arr2){ } // True
 An identity expresion is when the pointer values (and .length in
 arrays)
 in two objects is compared.
 int[] arr1 = [10, 20, 30], arr2 = arr1;
 if(arr1 is arr2){ } // True
 For primitive types, there is no difference between using == and is.
 int a = 15, b = 15;
 if(a == b){ } // True
 if(a is b){ } // True
 BCS wrote:


 
 However what you describe is at the semantic level and the grammar is 
 totally at the syntax level. What I was looking at is that this:
 
 a is b
 
 can be parsed as an Identity Expression and an Equal Expression.
 
 If the distinction is a semantic issue then this makes D context 
 sensitive. If it is not a semantic issue, then it is redundant or 
 conflicting.
 


The grammar is merely confusing. It might be better written as:

EqualExpression:
	ShiftExpression
	ShiftExpression == ShiftExpression
	ShiftExpression != ShiftExpression
	IdentityExpression

-- 
Kirk McDonald
http://kirkmcdonald.blogspot.com
Pyd: Connecting D and Python
http://pyd.dsource.org
Sep 02 2007
parent reply Jascha Wetzel <"[firstname]" mainia.de> writes:
This one isn't actually a problem. But you will find that there are more 
tricky ambiguities. For example:

void main()
{
   foo.bar.baz;
}

"foo.bar.baz" could be a type,
"foo.bar" could be a type and ".baz" a property,
"foo" could be an object, "bar" a member, and "baz" a member of that,
resulting in 3 different syntax trees:
BasicType
   IdentifierList
or
PrimaryExpression
   BasicType . Identifier
or
PostfixExpression
   PostfixExpression . PrimaryExpression

The differences are only semantical.

Kirk McDonald wrote:

 BCS wrote:

 Reply to Xinok,


 An equal expression is when the data in two objects is compared.
 int[] arr1 = [10, 20, 30], arr2 = [10, 20, 30];
 if(arr1 == arr2){ } // True
 An identity expresion is when the pointer values (and .length in
 arrays)
 in two objects is compared.
 int[] arr1 = [10, 20, 30], arr2 = arr1;
 if(arr1 is arr2){ } // True
 For primitive types, there is no difference between using == and is.
 int a = 15, b = 15;
 if(a == b){ } // True
 if(a is b){ } // True
 BCS wrote:


 However what you describe is at the semantic level and the grammar is 
 totally at the syntax level. What I was looking at is that this:

 a is b

 can be parsed as an Identity Expression and an Equal Expression.

 If the distinction is a semantic issue then this makes D context 
 sensitive. If it is not a semantic issue, then it is redundant or 
 conflicting.


 
 The grammar is merely confusing. It might be better written as:
 
 EqualExpression:
     ShiftExpression
     ShiftExpression == ShiftExpression
     ShiftExpression != ShiftExpression
     IdentityExpression
Sep 03 2007
parent BCS <ao pathlink.com> writes:
Reply to Jascha,


 This one isn't actually a problem. But you will find that there are
 more tricky ambiguities. For example:
 
 void main()
 {
 foo.bar.baz;
 }
 "foo.bar.baz" could be a type,
 "foo.bar" could be a type and ".baz" a property,
 "foo" could be an object, "bar" a member, and "baz" a member of that,
 resulting in 3 different syntax trees:
 BasicType
 IdentifierList
 or
 PrimaryExpression
 BasicType . Identifier
 or
 PostfixExpression
 PostfixExpression . PrimaryExpression
 The differences are only semantical.
 


Oouch!

I think I'll look at handling dot chains as "Identifier . DotChain" and figure 
out what it is at semantic time.
Sep 03 2007