functor

问题 I tried to assign a lifted value to a . λ> :m Control.Applicative λ> let a = pure 1 When I evaluated a in REPL, it prints 1 . λ> a 1 Therefore, I thought there may be an implementation of show for a , and tried this: λ> show a But the GHCi throws an error: <interactive>:70:1-4: No instance for (Show (f0 a0)) arising from a use of ‘show’ The type variables ‘f0’, ‘a0’ are ambiguous Note: there are several potential instances: instance (Integral a, Show a) => Show (GHC.Real.Ratio a) -- Defined

问题 I tried to assign a lifted value to a . λ> :m Control.Applicative λ> let a = pure 1 When I evaluated a in REPL, it prints 1 . λ> a 1 Therefore, I thought there may be an implementation of show for a , and tried this: λ> show a But the GHCi throws an error: <interactive>:70:1-4: No instance for (Show (f0 a0)) arising from a use of ‘show’ The type variables ‘f0’, ‘a0’ are ambiguous Note: there are several potential instances: instance (Integral a, Show a) => Show (GHC.Real.Ratio a) -- Defined

问题 Rust does not have higher-kinded-types. For example, functor (and thus monad) cannot be written in Rust. I would like to know if there is a deep reason explaining this and why. For instance, reason that I can understand can be that there is no zero-cost abstraction making HKT possible. Or type inference is significantly more difficult. And of course, I also looking for an explaination showing me why this is a real limitation. If the anwer was already given somewhere else, could you give me

问题 Rust does not have higher-kinded-types. For example, functor (and thus monad) cannot be written in Rust. I would like to know if there is a deep reason explaining this and why. For instance, reason that I can understand can be that there is no zero-cost abstraction making HKT possible. Or type inference is significantly more difficult. And of course, I also looking for an explaination showing me why this is a real limitation. If the anwer was already given somewhere else, could you give me

问题 Rust does not have higher-kinded-types. For example, functor (and thus monad) cannot be written in Rust. I would like to know if there is a deep reason explaining this and why. For instance, reason that I can understand can be that there is no zero-cost abstraction making HKT possible. Or type inference is significantly more difficult. And of course, I also looking for an explaination showing me why this is a real limitation. If the anwer was already given somewhere else, could you give me

问题 I have an API for managing a camera configuration. There are 344 individual options to manage. When a certain value changes the API calls a callback function to notify the program. The register function takes a void RegisterCallback(Option * ptr, void (fn*)(void*)) function pointer as a callback function. I cannot use a single function for callback, because I do now where is the callback coming from . One solution is to create 344 individual callback functions: void callback0(void*); void

来源： https://stackoverflow.com/questions/63639328/how-is-arr-fst-a-natural-transformation

来源： https://stackoverflow.com/questions/63639328/how-is-arr-fst-a-natural-transformation

来源： https://stackoverflow.com/questions/63639328/how-is-arr-fst-a-natural-transformation

来源： https://stackoverflow.com/questions/54261967/what-is-representable-used-for-in-haskell