alt Hacker NewsI think there is a strong case that ADTs (algebraic data types) aren't so great after all. Specifically, the "tagged" unions of ADT languages like Haskell are arguably pretty clearly inferior to the "untagged" unions of TypeScript or Scala 3. Because the latter actually behave like a logical "or" rather than an artificial construct that needs to be wrapped and unwrapped.
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I at first thought it was nice to save the wrapping, but you save yourself sooo much pain, ugliness and mistakes pattern matching trying to distinguish the types once you just use tagged unions.
Most of the time when I use untagged unions, I end up adding a tag and logic to case on it anyway…
> "tagged" unions of ADT languages like Haskell are arguably pretty clearly inferior to the "untagged" unions of TypeScript
dude .. wut?? Explain to me exactly how this is true, with a real world example.
From where I stand, untagged unions are useful in an extremely narrow set of circumstances. Tagged unions, on the other hand, are incredibly useful in a wide variety of applications.
While others have addressed the programming case for tagged unions, I want to add that, to a logician, tagged unions are the natural construct corresponding to "logical or".
In intuitionistic logic (which is the most basic kind from which to view the Curry-Howard or "propositions-as-types" correspondence), a proof of "A or B" is exactly a choice of "left" or "right" disjunct together with a corresponding proof of either A or B. The "choice tag" is part of the "constructive data" telling us how to build our proof of "A or B". Translated back into the language of code, the type "A | B" would be exactly a tagged union.