Juan Miguel Vilar
El 17/5/19 a las 11:56, ALeX Kazik escribió:
Now, the problem is what can I do to have a type that contains a single Int? Is there a sort of id for types so that I can write
Identity is the solution
Thank you Alex, but this forces the use of runIdentity. [...] Sometimes they will use lists, sometimes simple times, and would like that to be as transparent as possible.
I do not like the sibling thread's suggestion of using closed type families, because I think the regulatity of always having the `f t` present is a nicer aesthetic. Using a type family to unwrap the `Identity` makes it noisier to write functions that consume your structure and are polymorphic in `f`. You can alleviate some of the syntactic noise of `runIdentity` with careful choices of type aliases or other helper functions. The type aliases (and `runFoo` functions) for monad transformers are good examples, and the `(==>)` function in dependent-sum[1] is a good additional example of the latter. To save you the click, DSum in dependent-sum is defined like this: data DSum tag f where !(tag a) :=> f a The (==>) helper is defined like this: (==>) :: Applicative f => tag a -> a -> DSum tag f tag ==> a = tag :=> f a So when f ~ Identity (or any other Applicative), the machinery becomes that much easier to use. -- Jack [1]: https://hackage.haskell.org/package/dependent-sum-0.5/docs/Data-Dependent-Su...