monads

问题 IO , just like Maybe , is just an instance of Monad . On the other hand we have all data constructors for Maybe ( Just and Nothing ), but no constructors for IO . Reader and Writer do not export constructors too, they have functions, which return instance of this type ( reader and writer ) and more importantly runReader and runWriter , which unwrap computation result from Monad. Is there a way to unwrap IO Monad? I would like to have pure function which do some impure IO computations under

问题 IO , just like Maybe , is just an instance of Monad . On the other hand we have all data constructors for Maybe ( Just and Nothing ), but no constructors for IO . Reader and Writer do not export constructors too, they have functions, which return instance of this type ( reader and writer ) and more importantly runReader and runWriter , which unwrap computation result from Monad. Is there a way to unwrap IO Monad? I would like to have pure function which do some impure IO computations under

问题 IO , just like Maybe , is just an instance of Monad . On the other hand we have all data constructors for Maybe ( Just and Nothing ), but no constructors for IO . Reader and Writer do not export constructors too, they have functions, which return instance of this type ( reader and writer ) and more importantly runReader and runWriter , which unwrap computation result from Monad. Is there a way to unwrap IO Monad? I would like to have pure function which do some impure IO computations under

问题 Original question LYAH, in For a Few Monads More shows this function, solveRPN :: String -> Maybe Double solveRPN st = do [result] <- foldM foldingFunction [] (words st) return result which uses pattern-matching in conjunction with do expression to esure that the monad coming out of foldM wraps a singleton list . In order to truly understand the nature of the do expression as well as of Monad , I have been rewriting most example from that book using >>= and >> instead of the do expression, a

问题 I know how to implement the single linked list monad transformer but couldn't get its array counterpart running. The problem is that there is a grouping effect which renders the transformer only valid for commutative base monads. Here is an example where for the sake of simplicity both the transformer and the base monad are arrays and there is no transformer type wrapper: // ARRAY const arrMap = f => xs => xs.map((x, i) => f(x, i)); const arrAp = tf => xs => arrFold(acc => f => arrAppend(acc)

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 Are list comprehensions simply a language feature? What's the easiest way to fake a list comprehension using pure Haskell? Do you have to use a do block/ >>= to do this or could you use some other method for hacking a list comprehension together? Clarification: By "fake" a list comprehension I mean create a function that takes the same input and produces the same input, i.e. a form for the return values, lists to crunch together, and a predicate or multiple predicates. 回答1: Section 3.11 in

问题 Are list comprehensions simply a language feature? What's the easiest way to fake a list comprehension using pure Haskell? Do you have to use a do block/ >>= to do this or could you use some other method for hacking a list comprehension together? Clarification: By "fake" a list comprehension I mean create a function that takes the same input and produces the same input, i.e. a form for the return values, lists to crunch together, and a predicate or multiple predicates. 回答1: Section 3.11 in

问题 Are list comprehensions simply a language feature? What's the easiest way to fake a list comprehension using pure Haskell? Do you have to use a do block/ >>= to do this or could you use some other method for hacking a list comprehension together? Clarification: By "fake" a list comprehension I mean create a function that takes the same input and produces the same input, i.e. a form for the return values, lists to crunch together, and a predicate or multiple predicates. 回答1: Section 3.11 in

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