Error Monad Transformer

type classes)Safe HaskellSafeLanguageHaskell98Control.Monad.ErrorContentsMonads with error handlingThe ErrorT monad transformerExample 1: Custom Error Data TypeExample 2: Using ErrorT Monad TransformerDescriptionDeprecated: Use Control.Monad.Except insteadComputation type:Computations which may fail or throw exceptions.Binding strategy:Failure records information about the cause/location of the failure. Failure

Monad Transformer Category Theory

values bypass the bound function, other values are used as inputs to monad transformer tutorial the bound function.Useful for:Building computations from sequences of functions that may fail or using exception handling to structure

Haskell Monad Transformer

error handling.Zero and plus:Zero is represented by an empty error and the plus operation executes its second argument if the first fails.Example type:Either String aThe Error monad (also called scalaz monad transformer the Exception monad).Synopsisclass Monad m => MonadError e m | m -> e wherethrowError :: e -> m acatchError :: m a -> (e -> m a) -> m aclass Error a wherenoMsg :: astrMsg :: String -> anewtype ErrorT e m a :: * -> (* -> *) -> * -> * = ErrorT {runErrorT :: m (Either e a)}runErrorT free monad transformer :: ErrorT e m a -> m (Either e a)mapErrorT :: (m (Either e a) -> n (Either e' b)) -> ErrorT e m a -> ErrorT e' n bmodule Control.Monadmodule Control.Monad.Fixmodule Control.Monad.TransMonads with error handlingclass Monad m => MonadError e m | m -> e where SourceThe strategy of combining computations that can throw exceptions by bypassing bound functions from the point an exception is thrown to the point that it is handled.Is parameterized over the type of error information and the monad type constructor. It is common to use Either String as the monad type constructor for an error monad in which error descriptions take the form of strings. In that case and many other common cases the resulting monad is already defined as an instance of the MonadError class. You can also define your own error type and/or use a monad type constructor other than Either String or Either IOError. In these cases you will have to explicitly define instances of the Error and/or MonadError classes.MethodsthrowError :: e -> m a Sour

in monads and monad transformersStacking multiple monad transformersHiding our workExercisesMoving down the stackWhen explicit lifting is necessaryUnderstanding monad transformers by building oneCreating a monad transformerMore typeclass instancesReplacing the Parse type with a

State Monad Transformer

monad stackExercisesTransformer stacking order is importantPutting monads and monad transformers io monad transformer into perspectiveInterference with pure codeOverdetermined orderingRuntime overheadUnwieldy interfacesPulling it all togetherMotivation: boilerplate avoidanceMonads provide a powerful

Monad Transformers Step By Step

way to build computations with effects. Each of the standard monads is specialised to do exactly one thing. In real code, we often need to be https://hackage.haskell.org/package/mtl/docs/Control-Monad-Error.html able to use several effects at once.Recall the Parse type that we developed in Chapter 10, Code case study: parsing a binary data format, for instance. When we introduced monads, we mentioned that this type was a state monad in disguise. Our monad is more complex than the standard State monad, because it http://book.realworldhaskell.org/read/monad-transformers.html uses the Either type to allow the possibility of a parsing failure. In our case, if a parse fails early on, we want to stop parsing, not continue in some broken state. Our monad combines the effect of carrying state around with the effect of early exit.The normal State monad doesn't let us escape in this way; it only carries state. It uses the default implementation of fail: this calls error, which throws an exception that we can't catch in pure code. The State monad thus appears to allow for failure, without that capability actually being any use. (Once again, we recommend that you almost always avoid using fail!)It would be ideal if we could somehow take the standard State monad and add failure handling to it, without resorting to the wholesale construction of custom monads by hand. The standard monads in the mtl library don't allow us to combine them. Instead, the library provides

Sign in Pricing Blog Support Search GitHub This repository Watch 9 Star 264 Fork 29 kqr/gists Code Pull requests 1 Projects 0 https://github.com/kqr/gists/blob/master/articles/gentle-introduction-monad-transformers.md Pulse Graphs Permalink Branch: master Switch branches/tags Branches Tags master Nothing to show Nothing to show Find file Copy path gists/articles/gentle-introduction-monad-transformers.md cb412c4 Dec 28, 2015 limdauto Update gentle-introduction-monad-transformers.md 3 contributors https://en.wikibooks.org/wiki/Haskell/Monad_transformers Users who have contributed to this file kqr limdauto tedkornish Raw Blame History 809 lines (610 sloc) 25.3 KB A Gentle Introduction to Monad Transformers or, Values as Exceptions monad transformer Table of Contents Either Left or Right Introducing Side-Effects We Can Make Our Own Monads Implementing Instances for Common Typeclasses Using EitherIO Do You Even Lift? Signalling Errors throwE? What Is This, Java? ExceptIO Gotta Catch 'Em All Going General Appendix A: Complete Program Either Left or Right Before we break into the mysterious world of monad transformers, error monad transformer I want to start with reviewing a much simpler concept, namely the Either data type. If you aren't familiar with the Either type, you should probably not jump straight into monad transformers – they do require you to be somewhat comfortable with the most common monads. With that out of the way: Pretend we have a function that extracts the domain from an email address. Actually checking this properly is a rather complex topic which I will avoid, and instead I will assume an email address can only contain one @ symbol, and everything after it is the domain. I'm going to work with Text values rather than Strings. This means if you don't have the text library, you can either work with Strings instead, or cabal install text. If you have the Haskell platform, you have the text library. We need to import Data.Text and set the OverloadedStrings pragma. The latter lets us write string literals (such as "Hello, world!") and have them become Text values automatically. λ> :module +Data.Text λ> :set -XOverlo

example 3 A plethora of transformers 3.1 Type juggling 4 Lifting 4.1 Implementing lift 5 Implementing transformers 5.1 The State transformer 6 Acknowledgements 7 Notes Monads Prologue: IO, an applicative functor Understanding monads Maybe List do notation IO State Additive monads (MonadPlus) Monad transformers edit this chapter We have seen how monads can help handling IO actions, Maybe, lists, and state. With monads providing a common way to use such useful general-purpose tools, a natural thing we might want to do is using the capabilities of several monads at once. For instance, a function could use both I/O and Maybe exception handling. While a type like IO (Maybe a) would work just fine, it would force us to do pattern matching within IO do-blocks to extract values, something that the Maybe monad was meant to spare us from. Enter monad transformers: special types that allow us to roll two monads into a single one that shares the behavior of both. Passphrase validation[edit] Consider a real-life problem for IT staff worldwide: getting users to create strong passphrases. One approach: force the user to enter a minimum length with various irritating requirements (such as at least one capital letter, one number, one non-alphanumeric character, etc.) Here's a Haskell function to acquire a passphrase from a user: getPassphrase :: IO (Maybe String) getPassphrase = do s <- getLine if isValid s then return $ Just s else return Nothing -- The validation test could be anything we want it to be. isValid :: String -> Bool isValid s = length s >= 8 && any isAlpha s && any isNumber s && any isPunctuation s First and foremost, getPassphrase is an IO action, as it needs to get input from the user. We also use Maybe, as we intend to return Nothing in case the password does not pass the isValid. Note, however, that we aren't actually using Maybe as a monad here: the do block is in the IO monad, and we just happen to return a Maybe value into it. Monad transformers not only make it easier to write getPassphrase but also simplify all the code instances. Our passphrase acquisition