[Haskell-beginners] Question about List Type Constraint and Null

[aditya siram]

> The value of `null res' for a given String obviously depends on the type at
> which res is used.
Why does it matter what the type of res is as long as it is some kind
of list? Doesn't 'reads' always gives [(a1,String)]? So why does it
matter what type 'a1' is?

Thanks for the clear explanation of the monomorphism restriction.
-deech