Using existential types
GK at ninebynine.org
Fri Oct 10 10:40:00 EDT 2003
At 22:48 09/10/03 -0400, Derek Elkins wrote:
>As always, the wiki has some remarks about this,
Thanks for the reminder :-)
Interestingly, when I read this:
However this won't work since the elements of the list can be of SEVERAL
different types (like a sphere and a polygon and a mesh etc. etc.) but
lists need to have elements of the same type.
I was reminded of very similar issues I encountered in my own development,
which for the most part I resolved without using existential types.
Originally, I had contemplated using type classes, so that (w.r.t. the
above example) spheres and polygons would be different types that are
instances of a common class. That's how we do it with OOP, right? But it
falls foul of exactly the point made here -- you can't mix the types in
(say) a list.
It never occurred to me that existential types might be the appropriate
Instead, I replace the class instances by a single algebraic data type,
whose members are functions corresponding to OO-style class methods.
GK at NineByNine.org
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