forceElems

This is a challenge to solve with Haskell. Check out the readme of the whole project first if you haven't already.

The following function "entangles" the elements of a list with its spine, so that whenever the outermost constructor gets forced, the head element also gets forced (unless the list is empty, in which case nothing extra happens):

forceElemsList :: [a] -> [a]
forceElemsList = foldr ((:) $!) []

Check out Trouble in paradise: Fibonacci if you're interested how such a thing can be useful.

Note that forceElemsList does not force the spine of its argument, it only ensures that whenever some consumer forces the spine of the list returned by forceElemsList, the elements of that list get forced as well.

Can you define a function that does the same, but for any Traversable? I.e. a function having the following type signature:

forceElems :: Traversable t => t a -> t a

Rules:

1. just like forceElemsList, forceElems does not force the spine of its argument (and so can handle infinite structures) when there are elements of type a attached to it (if there are no elements of type a, feel free to force that spine until an element pops up)
2. containers having constructors storing multiple elements of type a (the order in which those get forces does not matter) and multiple recursive occurrences are allowed
3. no need to handle weird custom Traversable instances that do not agree with DeriveTraversable (like fmap id or (pure id <*>) calls inserted manually in the middle of the definition of traverse)
4. it is expected that forceElems introduces some performance overhead, even compared to forceElemsList

And there's a hardcore mode (turned off by default, change hardcore = False to hardcore = True in test/Main.hs to turn it on):

1. you're not allowed to force any non-a field (may occur in f <*> pure x for an x that is not of type a)
2. you're not allowed to force the spine any more than the final consumer forces it. I.e. "if there are no elements of type a, feel free to force that spine until an element pops up" is no longer allowed

Replace forceElems = undefined in src/Lib.hs with an actual definition of forceElems.

There's a small test suite. I run it with stack test.