haskell

问题 The default monoid for lists in the GHC Prelude is concatenation. [1,2,3] <> [4,5,6] becomes [1,2,3] ++ [4,5,6] and thus [1,2,3,4,5,6] I want to write a ZipList Monoid instance that behaves like this: [ 1 <> 4 , 2 <> 5 , 3 <> 6 ] The result is [5,7,9] assuming I am using the sum monoid. Note this behaves like zipWith (+) Potentially it would behave like this: [ Sum 1 <> Sum 4 , Sum 2 <> Sum 5 , Sum 3 <> Sum 6 ] I need to create a newtype around the ZipList newtype and the Sum newtype in order

问题 I'm just learning Haskell and IO monads. I'm wondering why wouldn't this force the program to output "hi" as well as "bye": second a b = b main = print ((second $! ((print "hi") >>= (\r -> return ()))) "bye") As far as I understand, the $! operator would force the first argument of second to be evaluated, and the >>= operator would need to run print "hi" in order to get a value off of it and pass it to \r -> return () , which would print "hi" to the screen. What's wrong with my reasoning? And

问题 I'm just learning Haskell and IO monads. I'm wondering why wouldn't this force the program to output "hi" as well as "bye": second a b = b main = print ((second $! ((print "hi") >>= (\r -> return ()))) "bye") As far as I understand, the $! operator would force the first argument of second to be evaluated, and the >>= operator would need to run print "hi" in order to get a value off of it and pass it to \r -> return () , which would print "hi" to the screen. What's wrong with my reasoning? And

问题 i've created orphaned instances for ZipList Semigroup and Monoid. However, when I run the tests from quickBatch on monoid, at the mconcat test, there is a stack overflow error. How do I resolve this error? Why is there such an error? Is it due to pure mempty , which I do not quite understand as I got this mostly from HaskellBook Chapter 17 Applicative section 17.8 ZipList Monoid? zl :: ZipList (Sum Int) zl = ZipList [1,1 :: Sum Int] instance Semigroup a => Semigroup (ZipList a) where (<>) =

问题 I understand the seq function and why it's necessary to introduce strictness for efficiency. What I don't understand is, why is this primitive called seq (and not something to do with strictness)? 回答1: TL;DR: Miranda called it seq , it was introduced when sequence was (probably) already a thing for Monads, and ($!) was known as strict for a short time. Miranda was first It is called seq because it was called seq in Miranda and previous languages, at least according to A History of Haskell:

问题 I understand the seq function and why it's necessary to introduce strictness for efficiency. What I don't understand is, why is this primitive called seq (and not something to do with strictness)? 回答1: TL;DR: Miranda called it seq , it was introduced when sequence was (probably) already a thing for Monads, and ($!) was known as strict for a short time. Miranda was first It is called seq because it was called seq in Miranda and previous languages, at least according to A History of Haskell:

问题 I understand the seq function and why it's necessary to introduce strictness for efficiency. What I don't understand is, why is this primitive called seq (and not something to do with strictness)? 回答1: TL;DR: Miranda called it seq , it was introduced when sequence was (probably) already a thing for Monads, and ($!) was known as strict for a short time. Miranda was first It is called seq because it was called seq in Miranda and previous languages, at least according to A History of Haskell:

问题 I understand the seq function and why it's necessary to introduce strictness for efficiency. What I don't understand is, why is this primitive called seq (and not something to do with strictness)? 回答1: TL;DR: Miranda called it seq , it was introduced when sequence was (probably) already a thing for Monads, and ($!) was known as strict for a short time. Miranda was first It is called seq because it was called seq in Miranda and previous languages, at least according to A History of Haskell:

问题 I understand the seq function and why it's necessary to introduce strictness for efficiency. What I don't understand is, why is this primitive called seq (and not something to do with strictness)? 回答1: TL;DR: Miranda called it seq , it was introduced when sequence was (probably) already a thing for Monads, and ($!) was known as strict for a short time. Miranda was first It is called seq because it was called seq in Miranda and previous languages, at least according to A History of Haskell:

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