Type Aliases: Not useful?

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I think there are two competing concerns going on here:

  1. booleans are inherently unclear
  2. type aliases are sometimes unclear

For the boolean stuff, yeah: use a custom type. I’m on board with that every time. You will almost always get a clearer, safer API. Here’s a whole talk on that: Solving the Boolean Identity Crisis by Jeremy Fairbank.

As for type aliases in general, here are my rules:

  • never use type aliases for simple types like Int or Bool. They let you express intent but, crucially, are not checked by the compiler (e.g. RoundUp and RoundToTens in your example can be exchanged and will still compile.)
  • sometimes use type aliases for types with type parameters so that Result x a or Dict comparable a become HttpResponse or Students. These have a very low chance of being accidentally swapped in, and the damage is pretty low if they are. That said, I always start with a closed custom type and open it up if necessary instead of the other way around, and has no runtime cost as the compiler will unbox them if it can.
  • always use type aliases for records. That’s it.
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