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Types are definitely very useful for complex refactoring. However, I disagree with the author that it wasn’t possible before types. Combining a healthy amount o

Types are definitely very useful for complex refactoring. However, I disagree with the author that it wasn’t possible before types. Combining a healthy amount of unit tests along with some integration tests makes complex refactoring totally possible even without types.

runeks 4y ago
Tests can almost by definition give stronger guarantees than a type system, since the former is Turing complete while the latter is (almost always) not. This means that, for any type, it’s possible to replicate the check done by the compiler at compile-time in a unit test but not vice versa. However, the ergonomics of this is worse than types, I’d argue, since a type system offers a much simpler way to assert basic stuff at compile-time (e.g. the presence of certain properties of an object). Also, type annotations are inline instead of being defined in a separate source file, which aids in code readability.
- dragonwriter 4y ago
  > Tests can almost by definition give stronger guarantees than a type system, False. > This means that, for any type, it’s possible to replicate the check done by the compiler at compile-time in a unit test but not vice versa. False. I mean, Turing completeness does mean you could replicate any type guarantee in code, but that code would generally not be a unit test; guarantees provided by type systems are universally quantified, to confirm that with testing would require testing every combination of values in the function’s domain (as determined by its parameter types.) The code would instead be a static typechecker that consumed the untyped source code plus a representation of the type assertions to be checked and where they attach to the untyped source code, and then did exactly the same things that any other static typechecker would do. That is... standard optional static typecheckers for dynamic languages. (Though those usually also support inline type assertions, perhaps via comments in languages without specific syntactic support, e.g., Python 2.x) You can approximate universally quantified guarantees in a testing regime with property-based testing, but that's at best probabilistic rather than actual universal quantification.
sbergot 4y ago
In my experience unit tests tend to slow down refactoring.

runeks 4y ago

Tests can almost by definition give stronger guarantees than a type system, since the former is Turing complete while the latter is (almost always) not. This means that, for any type, it’s possible to replicate the check done by the compiler at compile-time in a unit test but not vice versa. However, the ergonomics of this is worse than types, I’d argue, since a type system offers a much simpler way to assert basic stuff at compile-time (e.g. the presence of certain properties of an object). Also, type annotations are inline instead of being defined in a separate source file, which aids in code readability.

dragonwriter 4y ago
> Tests can almost by definition give stronger guarantees than a type system, False. > This means that, for any type, it’s possible to replicate the check done by the compiler at compile-time in a unit test but not vice versa. False. I mean, Turing completeness does mean you could replicate any type guarantee in code, but that code would generally not be a unit test; guarantees provided by type systems are universally quantified, to confirm that with testing would require testing every combination of values in the function’s domain (as determined by its parameter types.) The code would instead be a static typechecker that consumed the untyped source code plus a representation of the type assertions to be checked and where they attach to the untyped source code, and then did exactly the same things that any other static typechecker would do. That is... standard optional static typecheckers for dynamic languages. (Though those usually also support inline type assertions, perhaps via comments in languages without specific syntactic support, e.g., Python 2.x) You can approximate universally quantified guarantees in a testing regime with property-based testing, but that's at best probabilistic rather than actual universal quantification.

sbergot 4y ago

In my experience unit tests tend to slow down refactoring.