# Haskell Error Monad Either

the Maybe MonadUse of EitherCustom Data Types for ErrorsMonadic Use of EitherExceptionsFirst Steps with ExceptionsLaziness and Exception HandlingUsing handleSelective Handling of ExceptionsI/O ExceptionsThrowing ExceptionsDynamic ExceptionsExercisesError handling in monadsA tiny parsing frameworkExercises Error handling is one haskell either monad of the most important—and overlooked—topics for programmers, regardless of the language used.

## Haskell Either Example

In Haskell, you will find two major types of error handling employed: "pure" error handling and exceptions. When haskell throw exception we speak of "pure" error handling, we are referring to algorithms that do not require anything from the IO monad. We can often implement error handling for them by simply

## Throwerror Haskell

using Haskell's expressive data type system to our advantage. Haskell also has an exception system. Due to the complexities of lazy evaluation, exceptions in Haskell can be thrown anywhere, but only caught within the IO monad. In this chapter, we'll consider both. Error Handling with Data Types Let's begin our discussion of error handling with a very simple function. either monad scala Let's say that we wish to perform division on a series of numbers. We have a constant numerator, but wish to vary the denominator. We might come up with a function like this: -- file: ch19/divby1.hs divBy :: Integral a => a -> [a] -> [a] divBy numerator = map (numerator `div`) Very simple, right? We can play around with this a bit in ghci: ghci> divBy 50 [1,2,5,8,10] [50,25,10,6,5] ghci> take 5 (divBy 100 [1..]) [100,50,33,25,20] This behaves as expected: 50 / 1 is 50, 50 / 2 is 25, and so forth. [38] This even worked with the infinite list [1..]. What happens if we sneak a 0 into our list somewhere? ghci> divBy 50 [1,2,0,8,10] [50,25,*** Exception: divide by zero Isn't that interesting? ghci started displaying the output, then stopped with an exception when it got to the zero. That's lazy evaluation at work—it calculated results as needed. As we will see later in this chapter, in the absence of an explicit exception handler, this exception will crash the program. That's obviously not desira

but removes the Error restriction that prevents a natural encoding of Apomorphisms. This module is therefore incompatible with Control.Monad.Error Synopsisdata Either a b= Left a | Right b newtype EitherT a m

## Haskell Either Monad Example

b = EitherT {runEitherT :: m (Either a b)}Documentationdata

## Haskell Eithert

Either a b The Either type represents values with two possibilities: a haskell either to maybe value of type `Either a b`

is either `Left a`

or `Right b`

. The Either type is sometimes http://book.realworldhaskell.org/read/error-handling.html used to represent a value which is either correct or an error; by convention, the Left constructor is used to hold an error value and the Right constructor is used to hold a correct value (mnemonic: "right" also means "correct"). ConstructorsLeft aRight https://hackage.haskell.org/package/category-extras-0.52.0/docs/Control-Monad-Either.html bInstancesTypeable2 EitherCoassociative Hask EitherAssociative Hask EitherSymmetric Hask EitherBraided Hask EitherBizap Either (,)Bizap (,) EitherPreCoCartesian Hask EitherQFunctor Either Hask HaskPFunctor Either Hask HaskDistributive Hask (,) EitherBifunctor Either Hask Hask HaskMonad (Either e)Functor (Either a)MonadFix (Either e)Applicative (Either e)Monoid a => HasLimit (Either a)Pointed (Either a)Cozip (Either c)Monoid a => Zip (Either a)CFunctor (Either a) Hask Hask(Eq a, Eq b) => Eq (Either a b)(Ord a, Ord b) => Ord (Either a b)(Read a, Read b) => Read (Either a b)(Show a, Show b) => Show (Either a b)Representable (EitherF a b) (Either a b)newtype EitherT a m b SourceConstructorsEitherTFieldsrunEitherT :: m (Either a b)InstancesMonad m => Monad (EitherT a m)Functor f => Functor (EitherT a f)MonadFix m => MonadFix (EitherT a m)Pointed f => Pointed (EitherT a f)Produced by Haddock version 2.8.1

Mar 2015 Bartosz Milewski View Markdown source Previous content: 9. Evaluator Next content: 11. State Monad Go up to: Basics https://www.schoolofhaskell.com/school/starting-with-haskell/basics-of-haskell/10_Error_Handling of Haskell See all content by Bartosz Milewski Sections Either May Be Better than MaybeAbstracting the Either PatternThe Either MonadType ClassesSolution to the Expression ProblemThe Monad TypeclassExercisesThe Symbolic Calculator So Far http://stackoverflow.com/questions/6290712/using-either-in-haskell Show me how you handle errors and I'll tell you what programmer you are. Error handling is fundamental to all programming. Language support for error handling varies from none whatsoever (C) haskell either to special language extensions (exceptions in C++, Java, etc.). Haskell is unique in its approach because it's expressive enough to let you build your own error handling frameworks. Haskell doesn't need built-in exception support: it implements it in libraries.We've seen one way of dealing with errors: calling the error function that terminates the program. This works fine for runtime assertions, which alert us haskell either monad to bugs in the program. But many "errors" are actually expected. We've seen one such example: Data.Map.lookup fails when called with a key that's not present in the map. The possibility of failure is encoded in the Maybe return type of lookup. It's interesting to compare this with similar functions in other languages. In C++, for instance, std::map defines multiple accessor functions varying only in their failure behavior:at throws an exceptionfind returns an empty iteratoroperator[] inserts a dummy value using a default constructor for it.The last one is the most bizarre of the three. Since the array access operator must return a reference, even if the key is not found, it has to create a dummy value. The behavior of at is potentially dangerous if the client forgets to catch the exception. Of the three, find is the safest, since the return type suggests to the client iteration rather than straight dereference; and iteration normally starts with checking for termination.In functional programming, failure is another way of saying that the computation is partial; that is, not defined for all values of arguments. In Haskell we always try

here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up Using 'Either' in Haskell up vote 6 down vote favorite I have two values, t1 and t2, of type Either String Type. The Left-value is used for error handling. These values are used in a function which returns Either String Type. What I want to do is check if both t1 and t2 are Right-values and satisfy p :: Type -> Bool. If they do, I want to return Right (the type inside t1). If both t1 and t2 are Right-values, but do not satisfy p, I want to return Left someString. If one of t1 or t2 is a Left value, I just want to pass on that value. How can I do this in an elegant way? I have a hunch that using Either as a monad is the right thing to do, but I'm not sure of how to go about it. haskell monads either share|improve this question asked Jun 9 '11 at 9:27 Viktor Dahl 1,06511531 add a comment| 4 Answers 4 active oldest votes up vote 8 down vote accepted Why monads? test p (Right t1) (Right t2) | p t1 && p t2 = Right t1 | otherwise = Left "nope" test _ (Left t1) _ = Left t1 test _ _ (Left t2) = Left t2 share|improve this answer answered Jun 9 '11 at 9:39 Landei 39.5k862154 +1: Exactly the same answer I wanted to write. –Frerich Raabe Jun 9 '11 at 9:43 add a comment| up vote 5 down vote If you do want to do it with a Monad it would look somet