haskell

问题 In this question the OP asks what the type of the expression return 5 is and the answer has already been given in that question: it is a generic type, as can be verified by typing :t return 5 in the Haskell interpreter: return 5 :: (Num a, Monad m) => m a The specific implementation of return is determined by the context in which it appears: type inference will restrict m to a specific monad such as Maybe , [] , IO , and so on. I can also force the interpreter to pick a specific monad by

问题 In this question the OP asks what the type of the expression return 5 is and the answer has already been given in that question: it is a generic type, as can be verified by typing :t return 5 in the Haskell interpreter: return 5 :: (Num a, Monad m) => m a The specific implementation of return is determined by the context in which it appears: type inference will restrict m to a specific monad such as Maybe , [] , IO , and so on. I can also force the interpreter to pick a specific monad by

问题 using the run from Network.Wai.Handler.Warp function to server rest api run :: Port -> Application -> IO () but while doing post request, getting an error CORS header ‘Access-Control-Allow-Origin’ . any idea how to overcome this in servant/haskell 回答1: You could use wai-cors middleware to add CORS headers. At the end you'll have something like app = simpleCors $ serve api serverImpl where simpleCors is a Middleware from wai-cors serve turns servant handlers into wai Application api :: Proxy

问题 using the run from Network.Wai.Handler.Warp function to server rest api run :: Port -> Application -> IO () but while doing post request, getting an error CORS header ‘Access-Control-Allow-Origin’ . any idea how to overcome this in servant/haskell 回答1: You could use wai-cors middleware to add CORS headers. At the end you'll have something like app = simpleCors $ serve api serverImpl where simpleCors is a Middleware from wai-cors serve turns servant handlers into wai Application api :: Proxy

问题 using the run from Network.Wai.Handler.Warp function to server rest api run :: Port -> Application -> IO () but while doing post request, getting an error CORS header ‘Access-Control-Allow-Origin’ . any idea how to overcome this in servant/haskell 回答1: You could use wai-cors middleware to add CORS headers. At the end you'll have something like app = simpleCors $ serve api serverImpl where simpleCors is a Middleware from wai-cors serve turns servant handlers into wai Application api :: Proxy

问题 using the run from Network.Wai.Handler.Warp function to server rest api run :: Port -> Application -> IO () but while doing post request, getting an error CORS header ‘Access-Control-Allow-Origin’ . any idea how to overcome this in servant/haskell 回答1: You could use wai-cors middleware to add CORS headers. At the end you'll have something like app = simpleCors $ serve api serverImpl where simpleCors is a Middleware from wai-cors serve turns servant handlers into wai Application api :: Proxy

问题 I remember seeing a presentation in which SPJ said that lazy evaluation forced them to keep Haskell pure (or something along that line). I often see many Haskellers saying the same. So, I would like to understand how lazy evaluation strategy forced them to keep Haskell pure as opposed to a strict evaluation stragegy ? 回答1: I think the answer by Jubobs already sums it up nicely (with good references). But, in my own words, what I think SPJ and friends are referring to is this: Having to go

问题 I was reading a lecture note on Haskell when I came across this paragraph: This “not caring” is what the “parametric” in parametric polymorphism means. All Haskell functions must be parametric in their type parameters; the functions must not care or make decisions based on the choices for these parameters. A function can't do one thing when a is Int and a different thing when a is Bool. Haskell simply provides no facility for writing such an operation. This property of a langauge is called

问题 I was reading a lecture note on Haskell when I came across this paragraph: This “not caring” is what the “parametric” in parametric polymorphism means. All Haskell functions must be parametric in their type parameters; the functions must not care or make decisions based on the choices for these parameters. A function can't do one thing when a is Int and a different thing when a is Bool. Haskell simply provides no facility for writing such an operation. This property of a langauge is called

问题 I'm just beginning this nice hashkell beginners tutorial: http://learnyouahaskell.com on this page on lists he explains that lists are compared in compared in lexicographical order, he gives this example: ghci> [3,2,1] > [2,10,100] True From some googling it seems to me that lexicographical ordering means in alphabetical or sequential number ordering(?), but I still can't make sense of this evaluating to True. I'm missing something obvious here, can anybody help? 回答1: This evaluates to True