“Orphan” typeclass instances are instance declarations instance T A that occur in any module other than

1. the module where the class T is defined, or
2. the module where the type A is defined

The orthodox Haskeller viewpoint is that orphan instances are bad and you should never write them, because they can lead to incoherence. Incoherence is where we end up using two differing instances for the same type in our program. This can manifest in two unpleasant ways:

1. If you actually import both instances, your program will fail to compile.
2. If you do not directly import both, but rather use two modules which independently use the differing instances, you can end up with incoherent behaviour.

Both of these are pretty bad problems, in that neither of them is immediately apparent when you write the offending instance, but down the line they can cause some unsuspecting user’s code to not compile, or worse, silently misbehave.

However, these failures can only happen if is possible for an unsuspecting user to import the type and the instance separately. Moreover, in the case of compilation failure, the user must be unable to fix the source of the problem. With a little bit of care, we can use orphans quite safely so long as we can avoid the problematic cases.

If there’s one thing compiler writers spill a lot of ink over, it’s tree traversals. Tree traversals are infuriatingly simple conceptually, but can be a lot of boilerplate to actually write. This post covers a couple of tricks that I’ve found useful recently using tools from lens.

Certain people (myself included) have a habit of reflexively attacking new arguments or ideas. Amanda Askell calls this “shark curiosity”: sharks bite things partly because their only real way of interacting with the world is their mouth, so biting is their way of finding out what something is.

Taken literally this implies a rather bleak picture (you attack things because that’s the only way you know to interact with them!?). But I think that the point is rather that to the shark, biting is not necessarily an aggressive action. For curious sharks anyway!

From Annihilation by Jeff VanderMeer:

“So we have nothing.”

The surveyor ignored that, said “What do we do now?” It was clear she hated asking the question.

“Eat dinner,” I said. “Take a little stroll along the perimeter to make sure the psychologist isn’t hiding in the bushes. Think about what we’re doing tomorrow.

"”I’ll tell you one thing we’re not doing tomorrow. We’re not going back into the tunnel.”

“Tower.”

She glared at me.

Sometimes you just have to call it a day. There’s nothing more you can do (or nothing you want to do), and the best things is to wait. Either for a change in the situation so you can do more, or a change in yourself so you can see what to do.

Our circadian rhythm is something of a blessing here. Sleep allows time time to pass easily, and our minds to do their mysterious subconscious processing that defeats so many problems. And you have to do it, so “sleep on it” is much easier advice to take than “wait 8 hours and see what happens”.

Sometimes I’ve wished I never had to sleep. I wonder how many odd, contingent benefits we would lose if we no longer slept. Perhaps we would be more patient… but I suspect the opposite.

John cares about nothing but making the most amount of money that he can. He comes to believe that the best way to do this is to start a new company, but also that companies are most likely to succeed when the founder is intrinsically motivated by the work of the company. So John immerses himself in a domain and cultivates an interest. When he finally starts his company, he does it out of a genuine obsession with the idea. He is successful and makes a lot of money.

This is a prosaic example of an agent modifying themselves to the point at which they are not longer explicitly pursuing their original goals (although the idea is to achieve them nonetheless). Before immersing himself in his field, John1 primarily pursues money. Afterwards, John2 primarily pursues his startup idea, and is willing to sacrifice other ways to make easy money in order to work on it.