[Haskell-cafe] Type trickery

[Tillmann Rendel]

Hi Andrew,

Andrew Coppin wrote:
>> You could define a function:
>>
>> withContainer ∷ (∀ s. Container s → α) → α
>>
>> which creates a container, parameterizes it with an 's' that is only
>> scoped over the continuation and applies the continuation to the
>> created container.
>
> Hmm, yes. That will work, but I wonder if there's some way of doing this
> that doesn't limit the scope of the container to one single span of code...

You can write helper functions which take containers as argument by 
parameterizing these helper functions over s:

   takesTwoContainers :: Container s1 -> Container s2 -> ...
   takesTwoContainers c1 c2 = ... -- c1 and c2 can be used here

This function could be called like this:

   withContainer (\c1 ->
     withContainer (\c2 ->
       takesTwoContainers c1 c2)) -- c1 and c2 can be used here

In this example, the scope of the containers is not limited to a single 
span of code.

   Tillmann