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

Christopher Done chrisdone at googlemail.com
Fri Oct 1 09:36:33 EDT 2010

On 1 October 2010 15:27, Henning Thielemann
<lemming at henning-thielemann.de> wrote:
> Given the following code, that is accepted by GHC:
>> data Exist = forall a. Exist a
>> exist :: Exist
>> exist = Exist undefined
> What type has the 'undefined' ?

I think its type is `a'.
> So far I assumed that at runtime all objects have a concrete type. This
> seems not to be true.

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.

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