Question

I am having problems defining recursion in a function I have wrote. I am not sure what is wrong here, it could merly be a syntax blunder.

Basically I want to define like I have done for the Not Expression that the And expression calls the method substitute again. But I am havig issues defining substititue twice for the two expressions And can take.

substitute :: Expr -> [(Variable,Expr)] -> Expr
-- ...
substitute (Not e) x = substitute e x
substitute (And e1 e2) x = substitute e1 x substitute e2 x ---- ????? 

If I get And wright then the entire function will work. 🙂 Can anyone help?

Thanks

Answer 1

This should work:

substitute (Not e) x = Not (substitute e x)
substitute (And e1 e2) x = And (substitute e1 x) (substitute e2 x)

But I’m not really sure if this is what you want. You need some other cases where substitute does some real work.