3 ms·

If a method can handle n different types at runtime, then there are 2n - 1 possible union types that can be passed to the method: A, B, C becomes A, B, C, (A |

by ezrast 8y ago

If a method can handle n different types at runtime, then there are 2n - 1 possible union types that can be passed to the method: A, B, C becomes A, B, C, (A | B), (A | C), (B | C), (A | B | C), and that's without taking into account generics and nested type hierarchies. Not all of these unions will necessarily come up organically in code, but the class hierarchy of AST nodes used within the compiler was large enough that it was causing issues in real life.

weberc2 8y ago
Correct me if I'm wrong, but you don't need to generate every permutation to determine whether or not a given union type can be passed into a method, right? If the parameter is of type `A | B` and the function takes `A | B | C`, you don't need to generate the whole space to determine that `A | B` satisfies `A | B | C`. Also, for v1.0 of the language, it doesn't seem like it would be a big deal to put the onus on developers to recast their `A | B` to a `A | B | C`.
- ezrast 8y ago
  It's not just about validating types. If B and C are implemented sufficiently differently, the compiler output for a function that takes (A|B) will be different than for the same function that takes (A|C), and both of those outputs can end up coexisting in the final executable. Unfortunately I don't know enough about the compiler internals to go into further detail. As for manual recasting, sure, it's possible, but it's not a great tradeoff. Either you do type inference correctly and have developers crashing the compiler unless they recast their variables in seemingly arbitrary ways, or you cheat on inference and have developers getting "undefined method" errors on classes they didn't think they were using until they learn the rules. Ultimately it was decided that the latter was less common and easier to deal with in practice (just add a PlacentalMammal subclass to your class hierarchy that includes Cat and Dog but not Echidna). It's not a decision anyone is thrilled with but I think it's reasonable.
  - weberc2 8y ago
    I guess "larger binaries at v1 but correct" and "slightly less ergonomic at v1 but correct" both seem like pretty good tradeoffs to my mind, but at this point we seem to be pretty deep into speculation.

by ezrast 8y ago

weberc2 8y ago

Correct me if I'm wrong, but you don't need to generate every permutation to determine whether or not a given union type can be passed into a method, right? If the parameter is of type `A | B` and the function takes `A | B | C`, you don't need to generate the whole space to determine that `A | B` satisfies `A | B | C`. Also, for v1.0 of the language, it doesn't seem like it would be a big deal to put the onus on developers to recast their `A | B` to a `A | B | C`.

ezrast 8y ago
It's not just about validating types. If B and C are implemented sufficiently differently, the compiler output for a function that takes (A|B) will be different than for the same function that takes (A|C), and both of those outputs can end up coexisting in the final executable. Unfortunately I don't know enough about the compiler internals to go into further detail. As for manual recasting, sure, it's possible, but it's not a great tradeoff. Either you do type inference correctly and have developers crashing the compiler unless they recast their variables in seemingly arbitrary ways, or you cheat on inference and have developers getting "undefined method" errors on classes they didn't think they were using until they learn the rules. Ultimately it was decided that the latter was less common and easier to deal with in practice (just add a PlacentalMammal subclass to your class hierarchy that includes Cat and Dog but not Echidna). It's not a decision anyone is thrilled with but I think it's reasonable.
- weberc2 8y ago
  I guess "larger binaries at v1 but correct" and "slightly less ergonomic at v1 but correct" both seem like pretty good tradeoffs to my mind, but at this point we seem to be pretty deep into speculation.