haskell

问题 I'm kind of stuck with an assignement concerning my exams. I want to find out the types of those two functions by applying the unifying algorithm by hand: map map (\x -> x >>= (\y -> y)) Could someone point me to the right direction? The only ressource I could find until now was the wikipedia entry which is not really aiding me because of the high level of abstraction. Greetings and thank you. 回答1: Let's just do the first. map :: (a -> b) -> [a] -> [b] Now we can write it again with two

问题 I've been learning Haskell for a few weeks, and I have a question about the use of the underscore ( _ ) as a function parameter. I think my question will be better asked with a specific example. Let's say I want to define a function that extracts the element of a list based on the provided index—yes, I realize (!!) is already pre-defined. The two ways that I can define the function (I'm sure there are more) are the following: Version 1 indexedElement :: [a] -> Int -> a indexedElement xs n | n

问题 I've been learning Haskell for a few weeks, and I have a question about the use of the underscore ( _ ) as a function parameter. I think my question will be better asked with a specific example. Let's say I want to define a function that extracts the element of a list based on the provided index—yes, I realize (!!) is already pre-defined. The two ways that I can define the function (I'm sure there are more) are the following: Version 1 indexedElement :: [a] -> Int -> a indexedElement xs n | n

问题 Consider the following code in C#. public int Foo(int a) { // ... } // in some other method int? x = 0; x = Foo(x); The last line will return a compilation error cannot convert from 'int?' to 'int' which is fair enough. However, for example in Haskell there is Maybe which is a counterpart to Nullable in C#. Since Maybe is a Functor I would be able to apply Foo to x using fmap . Does C# have a similar mechanism? 回答1: We can implement such functionality ourselves: public static class FuncUtils

问题 Consider the following code in C#. public int Foo(int a) { // ... } // in some other method int? x = 0; x = Foo(x); The last line will return a compilation error cannot convert from 'int?' to 'int' which is fair enough. However, for example in Haskell there is Maybe which is a counterpart to Nullable in C#. Since Maybe is a Functor I would be able to apply Foo to x using fmap . Does C# have a similar mechanism? 回答1: We can implement such functionality ourselves: public static class FuncUtils

问题 I am writing a function called mapper2 that applies a function to two lists: mapper2 :: (a-> b -> c) -> [a] -> [b] -> [c] mapper2 f (x:xs) (y:ys) = (f x y) : (mapper2 f xs ys) mapper2 _ _ _ = [] I am able to compile the function but get an error when I apply it: *Main> mapper2 (\x -> x*2) [2,4] [4,6] <interactive>:4:1: error: • Non type-variable argument in the constraint: Num (b -> c) (Use FlexibleContexts to permit this) • When checking the inferred type it :: forall b c. (Num (b -> c), Num

问题 What I want to do is something like this: Take an arbitrary polymorphic tuple: x = (((1, ""), Nothing), ('', 6)) And reorganize with something like this type (not necessarily in the same order but the same structure.: (Int, (Char, (Maybe Int, (String, (Int, ())))) I really don't know the name for this pattern so I am unable to use google to the best of my ability. 回答1: If you only have to deal with this specific case, i.e., converting from (((Int, String), Maybe Int), (Char, Int)) to (Int,

问题 What I want to do is something like this: Take an arbitrary polymorphic tuple: x = (((1, ""), Nothing), ('', 6)) And reorganize with something like this type (not necessarily in the same order but the same structure.: (Int, (Char, (Maybe Int, (String, (Int, ())))) I really don't know the name for this pattern so I am unable to use google to the best of my ability. 回答1: If you only have to deal with this specific case, i.e., converting from (((Int, String), Maybe Int), (Char, Int)) to (Int,

问题 Is the category Hask of haskell objects an example of a locally small category? http://ncatlab.org/nlab/show/locally+small+category Maybe not.. hask as cpo http://www.cs.gunma-u.ac.jp/~hamana/Papers/cpo.pdf The haskellwiki, http://www.haskell.org/haskellwiki/Hask has very good information, showing that Hask is not Cartesian Closed. 回答1: What is Hask? If it includes all the haskell definable "functions" as morphism then definitly not data Big = Big (Big -> Big) the "hom set" of Big -> Big

问题 Is the category Hask of haskell objects an example of a locally small category? http://ncatlab.org/nlab/show/locally+small+category Maybe not.. hask as cpo http://www.cs.gunma-u.ac.jp/~hamana/Papers/cpo.pdf The haskellwiki, http://www.haskell.org/haskellwiki/Hask has very good information, showing that Hask is not Cartesian Closed. 回答1: What is Hask? If it includes all the haskell definable "functions" as morphism then definitly not data Big = Big (Big -> Big) the "hom set" of Big -> Big