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Xtend template error:This expression is not allowed in this context, since it doesn't cause any side [message #1838127]

Wed, 17 February 2021 04:24

wings cu

Messages: 14
Registered: January 2021

Junior Member

I'm trying to avoid add a new line when generate code of rule Exp, but i get an error of "This expression is not allowed in this context, since it doesn't cause any side effects."
Why this error will happen? How can I avoid it?

private def Object compile(Exp e) {
		if(e.left !== null) {
			if (e.left instanceof Var){
				'''«(e.left as Var).compile»'''
			}
			else if (e.left instanceof Val) {
				'''«(e.left as Val).compile»'''
			}
			else {
				'''«(e.left as Exp).compile»''' //error 
				if(e.op == '[' || e.op == '(' || e.op == '.')
					'''«e.op»''' //error 
				else 
					''' «e.op» ''' //error
				
				if (e.right !== null)
					'''«(e.right as Exp).compile»'''
			}
		}
	}

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838129 is a reply to message #1838127]

Wed, 17 February 2021 05:41

Christian Dietrich

Messages: 14355
Registered: July 2009

Senior Member

'''«(e.left as Exp).compile»''' is an expression that hangs in the air and that does nothing visible to the outside world
maybe you mean something like

'''
«(e.left as Exp).compile»«IF e.op == '[' || e.op == '(' || e.op == '.'»«e.op»«ELSE»«e.op»«ENDIF» // peusdo code
'''

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838144 is a reply to message #1838127]

Wed, 17 February 2021 11:57

wings cu

Messages: 14
Registered: January 2021

Junior Member

I have changed my code as you say, but it still don't work.
I still get a "This expression is not allowed in this context, since it doesn't cause any side effects." error.
'''
«(e.left as Exp).compile»«IF e.op == '[' || e.op == '(' || e.op == '.'»«e.op»«ELSE»«e.op»«ENDIF»
''' //error

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838152 is a reply to message #1838144]

Wed, 17 February 2021 15:03

Christian Dietrich

Messages: 14355
Registered: July 2009

Senior Member

can you please post a complete runnable and self contained java main showing the problem

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838167 is a reply to message #1838144]

Thu, 18 February 2021 04:22

wings cu

Messages: 14
Registered: January 2021

Junior Member

This is my xtext file and xtend file.
I also uploaded my whole project if you need them.

Attachment: TestScriptDSL.xtext
(Size: 1.18KB, Downloaded 32 times)
Attachment: TestScriptDSLGenerator.xtend
(Size: 2.83KB, Downloaded 37 times)
Attachment: testscript.zip
(Size: 1.69MB, Downloaded 34 times)

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838172 is a reply to message #1838167]

Thu, 18 February 2021 06:41

Christian Dietrich

Messages: 14355
Registered: July 2009

Senior Member

Hi, i dont see "This expression is not allowed in this context, since it doesn't cause any side effects". in the sample code you shared.

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838175 is a reply to message #1838172]

Thu, 18 February 2021 07:36

Ed Merks

Messages: 32508
Registered: July 2009

Senior Member

Yes, I was confused by the too. One general approach I would suggest is that wherever you see an error about "no side-effect", put a 'return' keyword in front of that expression. Then at least it has a side effect, i.e., it becomes the return value of the method. If you then get "unreachable code" errors then it's pretty clear that you are just executing a statement that has no effect and that you can just delete that statement without changing meaning of the program. In other words, there is no point in using a string literal as a statement unless it is the return value of the method..

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Re: Xtend template error:This expression is not allowed in this context, since it doesn't cause any [message #1838186 is a reply to message #1838167]

Thu, 18 February 2021 10:50

wings cu

Messages: 14
Registered: January 2021

Junior Member

I reopened my project and the error disappered. I'm sorry.

But I still don't understand why this error will happen. I read the java code generated from xtend file. I find this template will generate a string and will then return. So the template code will have side effect but eclipse still give me the error. Is there any details I missed?

And I still have another problem. If I changed my code like this, the code generated by xtext will create new line that I don't need.