Re: [Hat] Qualified variable in pattern: T.mkRoot

Thomas Davie writes:

Okay, I see that, now I'm slightly intrigued -- if, as before, we have

Just foo = undefined

Doesn't that have 2 effects: foo = undefined undefined :: Maybe a

and thus cause a type error (i.e. undefined has become too specific)?

No, 'foo' does not literally gain the value of 'undefined'. In fact, foo gets no value at all, because the computation diverges before the pattern can be matched. /Semantically/ divergence is equivalent to 'undefined', but syntactically, 'foo' and 'undefined' are separate bindings, and their types are therefore not constrained to be equal. Regards, Malcolm