L7R7/Append.hs

Ended up asking and was told that the constraint (Append a) is needed because append a1 a2 uses it.

So basically in this scenario, we're taking the values out of the Just and just passing them on to append. So at from there we know that a1 + a2 are of the type class Append. 😅

yes 😃 It took me a while to really understand this, as it is quite abstract. It's even more mind blowing for tuples: We can write an instance for Append (or Semigroup, which is actually the same thing) for tuples if the types in the tuple have an Append instance as well.
I think the canonical example is the "word count" example based on monoids which is described in a paper of which I forgot the name 🤔 There they are using tuples to count words, lines, sentences etc. in one go by composing monoids together

Ah that's cool 👌 I wish I could find papers readable, have always struggled with them!! 😂

Oh, for me it's the same! most of them are still too advanced for me, plus the sometimes more complex use of the english language makes it even harder for me😅 but I keep going, slowly but surely!