CODEWITHSUNDEEP
sqlrascal
Benni Katchy
Benni Katchy

Reputation: 83

Rascal Language Composition and Inheritance

I am on working on SQL grammars using Rascal MPL as a Language workbench I have noticed there are a lot of similarities between the various dialects. How can I implement an OOP-style inheritance to facilitate code reuse I will want to have a base SQL grammar that other dialects can inherit and override parts of. I know that Rascal supports composition but not inheritance, is there a workaround?

Upvotes: 2

Views: 104

Answers (2)

Jurgen Vinju
Jurgen Vinju

Reputation: 6696

Rascal grammars work like "mixins" in OO:

In one module you'd write:

module A

syntax Stat = "if" Exp "then" Stat "else" Stat;

Then in another you would write

module B

syntax Stat = "while" Exp "do" Stat;

And finally in a third module you combine the two:

module C

extend A;
extend B;

Now the syntax definitions are fused such that also at the recursive positions of Stat and Exp the additional alternatives are active.

So this is not like composition or inheritance, but like the kind of open recursion you get from mixins.

To be able to handle processing these trees, the functions in Rascal are also openly extensible, also for their recursive calls:

module A 

syntax Stat = "if" Exp "then" Stat "else" Stat;

int count(`if <Exp e> then <Stat s1> else <Stat s2>`)) = 1 + count(e) + count(s1) + count(s2)

module B

syntax Stat = "while" Exp "do" Stat;

int count(`while <Exp e> do <Stat s1>`)) = 1 + count(e) + count(s1)

module C

extend A;
extend B;

Here after the extend the recursive calls for count will deal with the new alternatives of count (i.e. the case for the while loop from the other module).

If you don't use dynamic dispatch (overloading) like this, but hide the processing inside of function bodies (visits and switches, or nested if-then-elses) then your language processor is not composable or extensible.

Upvotes: 0

Linus Wagner
Linus Wagner

Reputation: 173

Yes, rascal by design allows for modular grammars, that can be composed. The extend keyword will be the one you need, if allows you to extend an existing grammar in a different module.

Also, might be interesting to read this comparison to OO.

Upvotes: 1

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