3 ms·

> Python shows us that types and protocols/interfaces are two very different thing. In Python, asking 'isinstance()' is usually a bad idea Interesting perspect

> Python shows us that types and protocols/interfaces are two very different thing. In Python, asking 'isinstance()' is usually a bad idea Interesting perspective. This would seem to be pretty well described by parametric polymorphism with constraints (Haskell-style polymorphism with typeclasses). In that context, too, we are often well served staying as generic as possible, and asking at runtime about the type of something is a bit of a code smell (and only possible where you have a Typeable constraint available).

clusmore 10y ago
Yes. Haskell's type deduction is exactly what I was thinking of as I read the article. You write a function using what you need and it deducts the requirements based on usage, giving you the most general type. I think this is actually very close to compile-time duck-typing. Actually I think if you took the class declarations out of Haskell and removed the instance lines (but left the subsequent implementations), and had the compiler deduce typeclasses for you, you'd get something that felt very much like compile-time duck-typing.
- int_19h 10y ago
  If you want an even closer example, it would be OCaml with its structurally typed object system with inferred types. For example: # let f obj x y = (obj#m x)#n y;; val f : < m : 'a -> < n : 'b -> 'c; .. >; .. > -> 'a -> 'b -> 'c = <fun> Here I defined a function taking 3 arguments, called method m on obj, passing x as argument, then called method n on the result of that, passing y as argument. Note the inferred type signature for this: the first argument is of type <m : 'a -> <n : 'b -> 'c; ..>; ..> - i.e. any object that has a method named m (and possibly some other unspecified members - that's what ".." means), with said method m having a signature that allows us to pass some 'a, and returning another object of type <n : 'b -> 'c; ..> - i.e. something with a method named n that accepts 'b and returns 'c. 'a and 'b' are then used as types of x and y, respectively, and 'c is the the result of f. So effectively, this is full-fledged duck typing in the vein of Python objects, but captured on type system level.
  - clusmore 10y ago
    >Note the inferred type signature for this Wow, that's really powerful. Thanks for showing me this!
dbcurtis 10y ago
Agreed. Haskell and Python have type systems that work. Stuff in between, not so much. Or: Maybe haskelltyping

clusmore 10y ago

Yes. Haskell's type deduction is exactly what I was thinking of as I read the article. You write a function using what you need and it deducts the requirements based on usage, giving you the most general type. I think this is actually very close to compile-time duck-typing. Actually I think if you took the class declarations out of Haskell and removed the instance lines (but left the subsequent implementations), and had the compiler deduce typeclasses for you, you'd get something that felt very much like compile-time duck-typing.

int_19h 10y ago
If you want an even closer example, it would be OCaml with its structurally typed object system with inferred types. For example: # let f obj x y = (obj#m x)#n y;; val f : < m : 'a -> < n : 'b -> 'c; .. >; .. > -> 'a -> 'b -> 'c = <fun> Here I defined a function taking 3 arguments, called method m on obj, passing x as argument, then called method n on the result of that, passing y as argument. Note the inferred type signature for this: the first argument is of type <m : 'a -> <n : 'b -> 'c; ..>; ..> - i.e. any object that has a method named m (and possibly some other unspecified members - that's what ".." means), with said method m having a signature that allows us to pass some 'a, and returning another object of type <n : 'b -> 'c; ..> - i.e. something with a method named n that accepts 'b and returns 'c. 'a and 'b' are then used as types of x and y, respectively, and 'c is the the result of f. So effectively, this is full-fledged duck typing in the vein of Python objects, but captured on type system level.
- clusmore 10y ago
  >Note the inferred type signature for this Wow, that's really powerful. Thanks for showing me this!

dbcurtis 10y ago

Agreed. Haskell and Python have type systems that work. Stuff in between, not so much. Or: Maybe haskelltyping