[Haskell-cafe] Re: Non-existing types in existential quantification?

[Henning Thielemann]

On Sun, 3 Oct 2010, Ben Franksen wrote:

> Christopher Done wrote:
>
>> Consider the following program:
>>
>> main = putStrLn $ show $ length [undefined :: a,undefined :: b]
>>
>> A concrete type of the element in list doesn't need to be determined
>> at runtime, or any time. a unifies with b, and that unifies with x in
>> length :: [x] -> Int.
>
> A simpler example is
>
>  main = print Nothing

This seems to be a different example, because "GHCi -Wall" says that the 
type variable defaults to (). Thus 'Nothing' has monomorphic type at 
runtime. The difference is certainly that 'print' requires a Show 
instance, whereas Christopher's example does not require a type 
constraint.