Are HLists nothing more than a convoluted way of writing tuples?

Question

I am really interested in finding out where the differences are, and more generally, to identify canonical use cases where HLists cannot be used (or rather, don\'t yield any

Answer 1

I can explain this in super simple language:

The tuple vs list naming isn't significant. HLists could be named as HTuples. The difference is that in Scala+Haskell, you can do this with a tuple (using Scala syntax):

def append2[A,B,C](in: (A,B), v: C) : (A,B,C) = (in._1, in._2, v)

to take an input tuple of exactly two elements of any type, append a third element, and return a fully typed tuple with exactly three elements. But while this is completely generic over types, it has to explicitly specify the input/output lengths.

What a Haskell style HList lets you do is make this generic over length, so you can append to any length of tuple/list and get back a fully statically typed tuple/list. This benefit also applies to homogeneously typed collections where you can append an int to a list of exactly n ints and get back a list that is statically typed to have exactly (n+1) ints without explicitly specifying n.