confusing explanation of labels and names in Syntax Definitions documentation

[jurgenvinju]

See the question by ThomasH on SO: https://stackoverflow.com/questions/60411725/rascal-what-is-the-difference-between-label-and-name-in-the-syntax-definiti/60412309#60412309 about http://tutor.rascal-mpl.org/Rascal/Declarations/SyntaxDefinition/SyntaxDefinition.html

[thron7]

For the symbol position labels as you described in your SO answer, for me as a consumer of a parse tree it is interesting which value they have when a different alternative has matched.

So, to pick up on your example, slightly changed: When I have the grammar rule syntax E = E lhs "+" E rhs | E minuend "-" E subtrahend, what would be the value of minuend and subtrahend if the first (addition) rule has matched? Is there a null value in Rascal?

[jurgenvinju]

The .minuend operator for “field selection” or sometimes called projection is statically allowed on any tree of type E, but when it is dynamically applied to a tree that doesn’t have it, you will get a runtime exception. I believe NoSuchField(..).

Rascal has no generic null value at all.

To guard for the exception you can use the has operator and the ? set of operators in the context of if and such, but in general (concrete) pattern matching is a nicer way of avoiding the exception altogether.

Virtually All pattern matching is in some conditional context in rascal (function dispatch, switch, visit, if, for, catch, comprehensions), and this avoids again the need for null.

The difference with pattern matching and field projection to get at a nested value is thus that you can statically depend on the binding with pattern matching, but have to assume a dynamic exception might be thrown with projection+assignment.