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The ?= operator [message #1790412]

Sat, 09 June 2018 22:16

Alexandros Karypidis

Messages: 104
Registered: March 2010

Senior Member

Hello,

In this grammar:

MessageSchema:
    schema=SchemaDeclaration
    types=TypeDeclarations
;

SchemaDeclaration:
    'schema' name=QName 'id' '=' id=INT 'version' '=' version=INT optionalAttrs=OptionalSchemaAttrs?;

OptionalSchemaAttrs:
    ('headerType' '=' headerType=QName)? (bigEndian ?= 'bigEndian');

TypeDeclarations:
    'types' '{' simpleTypes+=SimpleTypeDeclaration+ '}';

I want to allow for 2 optional items to the schema declaration, which I have put in rule OptionalSchemaAttrs: a header type and a switch for flipping to big endian (default is little).

The result is not exactly what I want: if I don't include header type, the presence/absence of keyword 'bigEndian' works as expected:

// this is ok:
schema org.sbelang.examples.v2.ExampleSchema 
	id = 1 version = 1 

// this is also ok:
schema org.sbelang.examples.v2.ExampleSchema 
	id = 1 version = 1 bigEndian

But if I include a header type, the keyword 'bigEndian' becomes mandatory:

// this is ok:
schema org.sbelang.examples.v2.ExampleSchema 
	id = 1 version = 1 
 	headerType = a.b.c.D
	bigEndian

types {
//...

// this is NOT ok:
schema org.sbelang.examples.v2.ExampleSchema 
	id = 1 version = 1 
 	headerType = a.b.c.D

types {
//...

The last example gives gives me the error: missing 'bigEndian' at 'types'.

According to the docs:

Quote:

The ?= sign (boolean assignment operator) expects a feature of type EBoolean and sets it to true if the right hand side was consumed, independently from the concrete value of the right hand side.

I don't understand how the presence of headerType affects the requirement for keyword 'bigEndian' to be consumed after it?

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Re: The ?= operator [message #1790413 is a reply to message #1790412]

Sun, 10 June 2018 04:56

Christian Dietrich

Messages: 12388
Registered: July 2009

Senior Member

?= only says. Assign the left hand side Boolean attribute with true if the righthand side is present.

=> you grammar is wrong

The usual pattern is

Myflag?="mykw"?

Need professional support for Xtext, Xpand, EMF?
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Re: The ?= operator [message #1790416 is a reply to message #1790413]

Sun, 10 June 2018 07:43

Alexandros Karypidis

Messages: 104
Registered: March 2010

Senior Member

Ok, I must say that even now looking at the docs, this was not apparent to me. Thanks you.

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Re: The ?= operator [message #1790420 is a reply to message #1790413]

Sun, 10 June 2018 09:41

Alexandros Karypidis

Messages: 104
Registered: March 2010

Senior Member

I've understood the issue and fixed it, but I still can't wrap my head around the fact that the right hand side must explicitly be made optional (like you pointed out, I was assuming it implicitly is).

So what is the use case for not making the right hand side optional? Can you give me an example grammar where ?= would assign false (when the right hand side is NOT optional)? Or is it practically useless, but just the way things are?

I'm just trying to make sure I understand the utility scope of this operator.

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