内部Haskell Monads

Question

I have the following code that compiles and runs fine. I tried to make it more compact by replacing case newEmployee of with case scanEmployee p of , but it didn't work. There's probably an easy way to remove newEmployee (and newTeam ) from the code right?

module Main( main ) where
import Control.Monad.State

data Employee  = EmployeeSW  Int Int | EmployeeHW Int String deriving ( Show )
data Employee' = EmployeeSW'     Int | EmployeeHW'    String deriving ( Show )

scanTeam :: [Employee] -> State (Int,Int) (Either String [Employee'])
scanTeam [    ] = return (Right [])
scanTeam (p:ps) = do
    newEmployee <- scanEmployee p
    case newEmployee of
        Left errorMsg -> return (Left errorMsg)
        Right e -> do
            newTeam <- scanTeam ps
            case newTeam of
                Right n -> return (Right (e:n))
                Left errorMsg -> return (Left errorMsg)

scanEmployee :: Employee -> State (Int,Int) (Either String Employee')
-- actual code for scanEmployee omitted ...

Answer 1

You could use LambdaCase and be explicit with >>= instead of using do blocks. The result is not much shorter:

scanEmployee p >>= \case
    Left errorMsg -> return (Left errorMsg)
    Right e       -> do ...

Answer 2

You can simplify your code a bit with mapM and sequence :

mapM scanEmployee :: [Employee] -> State (Int, Int) [Either String Employee')

sequence :: [ Either String a ] -> Either String [ a ]

(Note that these type signatures are simplifications and the actual types are more general. Specifically mapM and sequence work for any monad (not just Either String ) and any traversable (not just ([]) ))

And write a simple solution:

scanTeam = fmap sequence . mapM scanEmployee