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The critical problem with structural typing is that it requires weird and arbitrary branding when dealing with unions of singletons.

whilenot-dev 5 days ago [ - ]

Branding doesn't need to be weird and arbitrary, see Pythons NewType: https://docs.python.org/3/library/typing.html#typing.NewType

Reading TFA now, Pythons NewType seems to be equal to Haskells newtype. Yes, it's a hack for the type checker to work around existing language semantics and feels unergonomic at times when Parse, Don't Validate needs to fall back to plain validation, but I wouldn't call it neither weird nor arbitrary.

o11c 5 days ago [ - ]

Python defaults uses nominal typing so isn't prone to this problem anyway.

The kind of "branding" I'm talking about is a hack only needed for structural typing systems. Consider something inspired by the C locale API, for example:

  class RealLocale:
    name: str
  const C_LOCALE = RealLocale("C");

  # Each of these can be passed to *some*, but not all, locale functions,
  # which will check for them by identity before doing the logic for `RealLocale`.
  singleton NO_LOCALE # used for both "error" and "query"
  singleton THREAD_LOCALE
  singleton GLOBAL_LOCALE
  singleton ENV_LOCALE

In a structural typing system, it is impossible to write a function that takes a union including more than one of `{NO_LOCALE, THREAD_LOCALE, GLOBAL_LOCALE, ENV_LOCALE}`, since they have no contents and thus cannot be distinguished. You have to hack around it, by some kind of casting and/or adding a member that's not actually present/useful at runtime.

And this kind of need is quite common. So I maintain that structural typing is not a serious type system proposal.

(Then again, the man proponent for structural typing is TypeScript, which even in $CURRENTYEAR still lacks even a way to specify ubiquitous needs like "integer" or "valid array index").

voidmain 5 days ago [ - ]

I think a type system that permits a type (no_locale=()) | (thread_locale=()) | (global_locale=()) | (env_locale=()) would generally be considered structural. I think newtypes and structure field names are isomorphic in functionality. Where a nominal type system would use newtype(tag,X) a structural one could use struct{tag:X} (and this holds even if X is the unit/singleton type).

b_e_n_t_o_n 6 days ago [ - ]

You mean like if you have two types which are identical but you want your type system to treat them as distinct? To me that's a data modelling issue rather than something wrong with the type system, but I understand how it can sometimes be unavoidable and you need to work around it.

I think it also makes more sense in immutable functional languages like clojure. Oddly enough I like it in Go too, despite being very different from clojure.

andyferris 6 days ago [ - ]

If I understand you correctly - in popular structurally typed languages, sure.

It seems ok in upcoming languages with polymorphic sum types (eg Roc “tags”) though?