Write You a Haskell Stephen Diehl

, bar :: Int
} deriving (Show)
computation :: Reader MyContext (Maybe String)
computation = do
n w -> Writer w ()
e monad can be evaluated returning the collected writer context and optionally the returned value.
execWriter :: (Monoid w) => Writer w a -> w
runWriter :: (Monoid w) => Writer w a -> (a, w)
import Control.Monad.Writer
type MyWriter = Writer [Int] String
example :: MyWriter
example = do
tell [1..5]
tell [5..10]
return ”foo”
output :: (String, [Int])
output = runWriter example
ExceptT
e Exception monad allows logic to fail at any point during computation with a user-defined exception.
e exception type is the first parameter of the monad type.
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