BangPatterns was not doing anything in the end, used it before pattern matching and forgot to remove it.

Doesn't the code

  return ((const shape_t *)shape)->vtable->get_area(shape);

violate the strict-aliasing rule whenever shape isn't actually a pointer to an object of type shape_t?
Wouldn't union aliasing be the correct way to do this?
(I'm quite new to C so this might sound as a dumb question, I'm still trying to understand type aliasing and how to do it right.)

This solution works for base >= 4.11.0 && < 5 (will probably still be valid for base >= 5, but that doesn't exist as of the time of writing this)

C# solution template has the wrong method name (should be UpperCamelCase intead of lowerCamelCase)

Why not use fromIntegral?

This will fail in one way or another for any enumerable which has more than int.MaxValue elements(and enumerated to that point). Other than that it seems ok :D.

EDIT: Just to clarify, depending on the context, this might either skip a lot of elements, or throw an arithmetic overflow exception

Quite pointless imo

nice recursive algorithm, and given that ints are at most 10 digits long, you won't run into a stack overflow from that recursion :D.

Same issue as haugk, cesare63 and arnitoj, you need to derive an instance of Applicative(and Functor) for your NIM type if you want a Monad instance.

Shouldn't the product of 0 numbers be 1?

Test cases assume a certain ordering of Set(HashSet) objects, which should not be done, either replace with SortedSet(TreeSet) or remove entirely.

implementing mergesort using quicksort, seems legit.

Haskell test problem?

Falsifiable (after 3 tests and 3 shrinks): 
expected: Just 0
 but got: Nothing
[0]
Positive {getPositive = 1}
-1

Why would item no' -1 be in page 0?