ghci

So I have to make a function that finds a pair with its 1st letter and returning the 2nd letter. I actually found one answer but with the map function and I couldn't get it. lookUp :: Char -> [(Char, Char)] -> Char lookUp x [] = x lookUp x ( ( st,nd ): rst) | st == x = nd | otherwise = lookUp x rst And I get this message: No instance for (Show ([(Char, Char)] -> Char)) arising from a use of `print' Possible fix: add an instance declaration for (Show ([(Char, Char In a stmt of an interactive GHCi command: print it Your code is fine, you just need to supply all the arguments at the ghci prompt,

Specs GHC 6.12.1 Mac OS X 10.6.4 x64 MacBook Pro Problem I'm having trouble using let syntax. The following code refuses to compile: module Main where main = let x = 1 y = 2 z = 3 in putStrLn $ "X = " ++ show x ++ "\nY = " ++ show y ++ "\nZ = " ++ show z I tried tabbing in y = 2 and z = 3 even more. No dice. (Undesirable) Solutions The only way I've gotten the code to compile is either Replacing hard tabs with spaces. Replacing the let clause with a where clause. Saizan on #haskell explains that the assignments in a let expression have to align, not let itself. As long as the assignments line

For practice, I'm implementing a queue data type in a module called "Queue". My data type is also called "Queue", as is its only value constructor: module Queue (Queue, enq, emptyQueue) where data Queue a = Queue { inbox :: [a], outbox :: [a] } deriving (Eq, Show) emptyQueue :: Queue a emptyQueue = Queue [] [] enq :: a -> Queue a -> Queue a enq x (Queue inb out) = Queue (x:inb) out -- other function definitions (irrelevant here)... As far as I understand, because I wrote Queue , not Queue(..) or Queue(Queue) in the export statement, I don't expect the value constructor of my data type to be

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 = "localhost", appExtra = e} f <- makeFoundation c :t f f :: App :i App data App = App {settings :: AppConfig

Is there a command in Haskell which displays (or get as a list of) all the user defined functions which have been loaded/defined in the GHCi? Thanks To see bindings you've made at the ghci prompt (e.g. with let or <- ), try :show bindings . If you've loaded some modules, you can use :show modules to get the names of loaded modules and then :browse ModuleName to list everything in scope from that module. When in ghci, use :browse or just :bro after loading the file. You may also browse unloaded modules via :browse Foo.Bar.Baz . 来源： https://stackoverflow.com/questions/10272094/haskell-display

On Mac OSX 10.9, the default c compiler bundled with Xcode is clang . I installed gcc-4.9 with homebrew . Now I have two different gcc s, one is clang , the other is gcc . The default is clang . I want to use gcc when compiling Haskell files with ghc , and I want also gcc when I launch ghci . How to do this change? Carter Tazio Schonwald Reproducing my directions I've been sharing with haskellers for the past few months via https://gist.github.com/cartazio/7131371 Type ghc --print-libdir The output will be a path like /Library/Frameworks/GHC.framework/Versions/7.6.3-x86_64/usr/lib/ghc-7.6.3 Go