haskell - What types of problems helps "higher-kinded polymorphism" solve better?

Question

As I read through some sections in History of Haskell, I came across:

However, higher-kinded polymorphism has independent utility: it is entirely possible, and occasionally very useful, to declare data types parameterised over higher kinds, such as:

data ListFunctor f a = Nil | Cons a (f a)

Knowing "basic" ADTs I was a bit puzzled here, my "guess" was that the part in parens suggests a "parametric"/"dynamic" unary data constructor f? So any data constructor of kind * -> * that "can accept" type a? Is my thinking correct or am I misinterpreting the syntax? I know I'm "just guessing" but I'm hopeful to gain a "lay-programmer" intuition on this capability here, some sample scenario needing (or benefiting immensively from) this ;) mostly I can imagine (just not in what exact manner) this allowing more flexibility in those "small embedded versatile recursable config language"-ADTs that Haskell makes such a pleasure to formulate and write evals for.. close?

In GHCi, :i ListFunctor on the above gives:

type role ListFunctor representational nominal
data ListFunctor (f :: * -> *) a = Nil | Cons a (f a)

So this seems to be what's "inferred" from the crisper data declaration.

Answer 1

Yes, f can be any unary type constructor.

For instance ListFunctor [] Int or ListFunctor Maybe Char are well-kinded.

f can also be any n-ary type constructor with (n-1) arguments partially applied.

For instance ListFunctor ((->) Bool) Int or ListFunctor (Either ()) Char are well-kinded.

The basic kinding system is quite simple. If F :: * -> * -> ... -> *, then F expects type arguments. If G :: (* -> *) -> *, then G expects any thing of kind * -> * including unary type constructor and partial applications as the ones shown above. And so on.

---

A problem which is nicely solved by higher kinds is configuration options. Assume we have a record

data Opt = Opt 
   { opt1 :: Bool
   , opt2 :: String
   -- many other fields here
   }

Now, configuration settings can be found in a file and/or passed through the command line and/or in environment variables. During the parsing of all these settings sources, we need to cope with the fact that not all sources define all options. Hence, we need a more lax type to represent subsets of configuration settings:

data TempOpt = TempOpt 
   { tempOpt1 :: Maybe Bool
   , tempOpt2 :: Maybe String
   -- many other fields here
   }

-- merge all options in one single configuration, or fail
finalize :: [TempOpt] -> Maybe Opt
...

This is horrible, since it duplicates all the options! We would be tempted to remove the Opt type, and only use the weaker TempOpt, to reduce clutter. However, by doing this we will need to use some partial accessor like fromJust every time we need to access the value of an option in our program, even after the initial configuration handling part.

We can instead resort to higher kinds:

data FOpt f = FOpt 
   { opt1 :: f Bool
   , opt2 :: f String
   -- many other fields here
   }
type Opt = FOpt Identity
type TempOpt = FOpt Maybe

-- as before: merge all options in one single configuration, or fail
finalize :: [TempOpt] -> Maybe Opt
...

No more duplication. After we finalize the configuration settings, we get the static guarantee that settings are always present. We can now use the total accessor runIdentity to get them, instead of the dangerous fromJust.