ghci

问题 How to, for example, insert a new User into a database using Yesod application's models? Or is there a better way? I am dealing with scaffolded application. Now I created App instance and dont know how to perform requests using it. :i Extra data Extra = Extra {extraCopyright :: Data.Text.Internal.Text, extraAnalytics :: Maybe Data.Text.Internal.Text} -- Defined in `Settings let e = Extra "asdf" Nothing let c = AppConfig {appEnv = Development, appPort = 3000, appRoot = "/", appHost =

问题 How to, for example, insert a new User into a database using Yesod application's models? Or is there a better way? I am dealing with scaffolded application. Now I created App instance and dont know how to perform requests using it. :i Extra data Extra = Extra {extraCopyright :: Data.Text.Internal.Text, extraAnalytics :: Maybe Data.Text.Internal.Text} -- Defined in `Settings let e = Extra "asdf" Nothing let c = AppConfig {appEnv = Development, appPort = 3000, appRoot = "/", appHost =

问题 How to I define the equivalent of this function (taken from learnyouahaskell) inside GHCi? import Data.List numUniques :: (Eq a) => [a] -> Int numUniques = length . nub Without the type declaration, GHCi accepts the function definition, but it ends up with an unhelpful type: Prelude Data.List> import Data.List Prelude Data.List> let numUniques' = length . nub Prelude Data.List> :t numUniques' numUniques' :: [()] -> Int The resulting function only accepts a list of units as a parameter. Is

问题 How to I define the equivalent of this function (taken from learnyouahaskell) inside GHCi? import Data.List numUniques :: (Eq a) => [a] -> Int numUniques = length . nub Without the type declaration, GHCi accepts the function definition, but it ends up with an unhelpful type: Prelude Data.List> import Data.List Prelude Data.List> let numUniques' = length . nub Prelude Data.List> :t numUniques' numUniques' :: [()] -> Int The resulting function only accepts a list of units as a parameter. Is

问题 How can I clear a terminal screen after my user has selected an option from my application's menu? 回答1: :! run the shell command :! cls under windows :! clear under linux and OS X 回答2: This is what you may be looking for: ansi-terminal: Simple ANSI terminal support, with Windows compatibility You can find it in Hackage and install using cabal install ansi-terminal . It specifically has functions for clearing the screen, displaying colors, moving the cursor, etc. Using it to clear the screen

问题 While doing some exercises in GHCi I typed and got the following> ghci> (1 "one") <interactive>:187:1: No instance for (Num ([Char] -> a0)) arising from a use of ‘it’ In a stmt of an interactive GHCi command: print it which is an error, howeve if I ask GHCi for the type of the expression it does not give any error: ghci> :type (1 "one") (1 "one") :: Num ([Char] -> t) => t What is the meaning of (1 "one") ? Why does this expression gives an error, but GHCi tells it is well typed? What is the

问题 If I put the following 2 lines into foobar.hs f 1 = 1 f x = f (x-1) then $ ghci > :load foobar.hs > f 5 1 but if I do $ ghci > let f 1 = 1 > let f x = f (x-1) > f 5 ^CInterrupted. then it does not return. Why? 回答1: The latter binding overrides the former. Use this in ghci: Prelude> :{ Prelude| let f 1 = 1 Prelude| f x = f (x-1) Prelude| :} Prelude> f 5 1 Or, without the layout: Prelude> let f 1 = 1; f x = f (x-1) Prelude> f 5 1 回答2: You have to enter it all in on one line, or using :{ and :}

问题 From a Haskell tutorial: We can write functions on integers by cases. -- Compute the sum of the integers from 1 to n. sumtorial :: Integer -> Integer sumtorial 0 = 0 sumtorial n = n + sumtorial (n-1) However, here's what happens when I try it: $ ghci GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help Prelude> foo 0 = print 999 Prelude> foo n = print n Prelude> foo 0 0 What am I missing? 回答1: To use your definition in GHCi exactly as you wrote it (i.e. with multiple equations in

问题 How can I see substitutions of variable names to their values in haskell? e.g. c2i zero ==> c2i (Chr (\x y -> y)) ==> (\x y -> y) (+ 1) 0 ==> 0 As explained in this answer. 来源： https://stackoverflow.com/questions/36650044/show-substitutions-in-ghci

问题 I am an absolute beginner. Going through LYAH using emacs. My current Set up: Ubuntu 12.04 LTS (Use Experience - beginner) GNU Emacs 23 (Use Experience - beginner) able to work in haskell major mode Finding difficult to follow instructions (to bring haskell libraries) at Point 2 described here. Also need guidance to enable Scion IDE. Problem: .hs code data Maybe a = Nothing | Just a While running the code, I get following error: Please ignore typo error, Originally posted: *Main> just "Haha"