haskell

问题 I want to overload any operator . i want to do such a simple function that for instance think about overloading of == operator .Overload == such that x==y returns x . Or x==y return x+y. It doesn't matter what . Can you show me any simple operator overloading example? I cannot find any example on the web unfortunately. For example;when i call Tree a == Tree a return 5 (it always return 5. I select it ,it is not related to any thing) or when i call 3==4 return : 7 I tried the below codes(i

问题 I have 2 lists Expressions and bindings (id = Expr), and trying to replace each expression with its equivalent from the bindings list in a new list called newE, where Expression = id .. Initially, I have only one expression: eq (div (add 2 7) (sub 5 2)) 3 I want to replace each identifier in this expression with its equivalent from the bindings list, so I tried to split this expression into a list of strings and removed brackets, to separate each identifier .. This is how I tried to implement

问题 I have 2 lists Expressions and bindings (id = Expr), and trying to replace each expression with its equivalent from the bindings list in a new list called newE, where Expression = id .. Initially, I have only one expression: eq (div (add 2 7) (sub 5 2)) 3 I want to replace each identifier in this expression with its equivalent from the bindings list, so I tried to split this expression into a list of strings and removed brackets, to separate each identifier .. This is how I tried to implement

问题 This question already has an answer here : Parse error in valid code [duplicate] (1 answer) Closed 5 years ago . when implementing the code for the "Towers of Hanoi" problem I get the following error message: hanoi.hs:4:24: parse error on input `=' Failed, modules loaded: none. Here is the code: hanoi 1 i j = [(i, j)] hanoi n i j = hanoi n' i otherT ++ [(i,j)] ++ hanoi n' otherT j where n' = n-1 otherT = 1+2+3-i-j Any Ideas? 回答1: Your editor and the compiler see the tabs differently. Avoid

问题 In the following code Haskell complains about Non-exhaustive patterns in function prime' prime :: Int -> [Int] prime x = prime' [2..x] where prime' (p:ps)= p : prime' [x | x <- ps, mod x p > 0 && prime'' x [2..div x 2]] prime'' _ [] = True prime'' n (x:xs) | mod n x == 0 = False | otherwise = prime'' n xs prime' []=[] I can't find my mistake. Could someone explain why this happens, and what it means? 回答1: Indentation. The last line defines another function called prime' . Therefore, prime

问题 I have a Yesod route handler that returns a JSON with the object { id: 1, title: "foo" content: "bar" } I would like to add a _links property with some metadata, that doesn't exist in on the Entity itself, e.g. { id: 1, title: "foo" content: "bar" _links: {self: http://localhost:3000/events/1} } How can I add the _links to the existing Entity record? Here's my handler: getEventR :: EventId -> Handler Value getEventR eid = do event <- runDB $ get404 eid render <- getUrlRender let renderedUrl =

问题 I have a Yesod route handler that returns a JSON with the object { id: 1, title: "foo" content: "bar" } I would like to add a _links property with some metadata, that doesn't exist in on the Entity itself, e.g. { id: 1, title: "foo" content: "bar" _links: {self: http://localhost:3000/events/1} } How can I add the _links to the existing Entity record? Here's my handler: getEventR :: EventId -> Handler Value getEventR eid = do event <- runDB $ get404 eid render <- getUrlRender let renderedUrl =

问题 This question already has answers here : Closed 7 years ago . Possible Duplicate: Creating unique labels in Haskell I've got a datatype Person and some input data from which I will create the Persons. I'd like to have each Person have its own ID (let's say integers [0..]). I could do this with recursion, but since I'm doing this in Haskell, I'd like to understand the monads. The State Monad is probably the best for this job, I suppose? The thing is, I don't really understand lots of things:

问题 If I have a returned type of an expression/value: :: Control.Monad.IO.Class.MonadIO m => m (Either PDFInfoError PDFInfo) How do I get the PDFInfo out of it? Perhaps more importantly, what process does one use to figure such things out. I'd like to leverage typed holes or some other process to be able to reason through these types (no pun intended) of questions on my own. Still reading through my first Haskell book, but wanting to understand how a more experienced Haskeller would solve this

问题 data Pair = P Int Int instance Show Pair where show (P n1 n2) = (show n1) ++ "\t" ++ (show n2) Result: GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help \Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( labn.hs, interpreted ) Ok, modules loaded: Main. *Main> show (P 5 6) "5\t6" OK for a pair of ints this might be an artificial problem, but my actual use case is