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

On 29 Jun 2005, at 16:47, Malcolm Wallace wrote:

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.

Okay, but does this not force undefined to have the type Maybe a? i.e. Just x = undefined File y = undefined would cause a type error? Bob