[LANGUAGE] Add warning for unused variables

[OnnoEbbens]

My students often create a variable that they don't use. For example:

name is Fatih
print 'my name is Fatih'

It would be nice to raise a Warning for each variable that was created but not used.

This could also help a lot in level 5 with if/else. I see a lot of students doing this:

name is print 'what is your name?'
if naam is gvr print 'ok' else print 'ai'

This code always prints 'ai' because the check (naam is gvr) is always false. It can be hard for a student to discover why. Raising a warning would inform the student that they've not used the variable name in the if-statement.

[MarleenGilsing]

Good idea!

[TiBiBa]

Love this idea! I'm not really familiar with the compiler (yet), but this seems like a great issue to dive into that code. Will try to implement this!

[Felienne]

I can imagine that @fRedelaar also wants to take this into account for the debugger (f.e. show a var in red in the list if ti is not used?)

[Felienne]

And this one I think could also be fun for you @jpelay, and not too hard? I do not think @fRedelaar will still do this as she has left the team a while ago

[nycdotnet]

For what it's worth, several editors I use will slightly gray out variables that aren't used rather than marking them in red; I like this as it is a subtle hint. There are some languages such as Go where unused variables are actually a compilation error, but I don't recommend Hedy implement this as I think it's quite user hostile.

[jpelay]

And this one I think could also be fun for you @jpelay, and not too hard? I do not think @fRedelaar will still do this as she has left the team a while ago

Yes, no problem :D