I have a Haskell file called Types.hs that contains this definition, among others: module Types ( KnapsackItem ) where data KnapsackItem = KnapsackItem { weight :: Int, cost :: Int } deriving (Show, Eq) Then there’s my Main.hs (also shortened here for simplicity): import Types evalTotal :: [KnapsackItem] -> [KnapsackItem] -> (Int, Int, String) evalTotal… Read More Data type field name from module I defined is "not in scope"

there. Relative Haskell newbie here. I have a big bunch of data types and functions on two type variables that require that the types are Eq, Ord, and Show. I found myself writing type Foo n t = (Eq n, Eq t, Ord n, Ord t, Show n, Show t) => Bar n t –… Read More In Haskell, is there a way to define a type constructor that includes a bunch of type constraints?

trying to understand Continuation monad examples implemented in Haskell. Question: where does fn in 1st line of goto on the right – in out fn part comes from? Is it some omitted argument or some destructuring of out [same with out (fn, num) in gotoC]? Imagine you explain this for a mainstream language coder (JS,… Read More Question on Haskell syntax. Continuation monad example

I see myself using this pattern a lot: f x = do — predicate :: a -> IO Bool b <- predicate x if b then rest () else return () where rest () = do — rest of my IO operations is there a pattern other people use to remove the if then else… Read More How to best handle IO Bool logic?

I created a custom List type and I’m trying to implement the zip function for it. But I cant figure it out, it always throw an error on the last line. data List a = Empty | Cons a (List a) deriving (Eq, Ord, Show, Read) listZip :: List a -> List a -> List… Read More How to access elements of a custom List data type?

class Collection c where empty :: c key value singleton :: key -> value -> c key value insert :: Ord key => key -> value -> c key value -> c key value lookup :: Ord key => key -> c key value -> Maybe value delete :: Ord key => key -> c… Read More Don't understand this collection class syntax

I want to return empty lists if one of the parameters is Nothing, but I can’t data UtilsDiff = UtilsDiff {syntaxBlockOptions :: [String], optionsBlockOptions :: [String]} deriving Show data UtilsOrDiff = UtilsOrDiff Utils | Diff UtilsDiff deriving Show useBlockExpr :: Parser UtilsOrDiff useBlockExpr = do u1 <- syntaxExpr opts <- optionsBlockExpr _ <- absorbToStopWord "Description:"… Read More How to correctly return nested types in haskell?

I am writing a parser with the help of parsec and I have a problem data Param = Param {paramName::String, argument :: Maybe String} deriving (Show) paramExpr1 :: Parser Param paramExpr1 = do paramKeyword void $ lexeme $ char ‘-‘ paramName <- word return $ Param paramName Nothing paramExpr3 :: Parser Param paramExpr3 = do… Read More How do I get the named fields in haskell correctly?

I want to write an Haskell program that read a list of integer positive numbers from stdin, if user write a different thing like a list of negative numbers, a list of characters or a thing that isn’t a list, program need to advise user and read again from stdin until user write a correct… Read More Validate a list of positive integer numbers in Haskell

I’m new to Haskell and trying to explore how datatypes in Haskell work. For instance, I’m trying to run the following code: data Number = Int num :: Number num = 1 However with that, I get the following error: main.hs:3:7: error: * No instance for (Num Number) arising from the literal `1′ * In… Read More Rename datatype "No instance … arising from the literal"