mtl- Monad transformer library

Portabilitynon-portable (multi-parameter type classes)
Safe HaskellSafe-Infered



Computation type:
Computations which may fail or throw exceptions.
Binding strategy:
Failure records information about the cause/location of the failure. Failure values bypass the bound function, other values are used as inputs to the bound function.
Useful for:
Building computations from sequences of functions that may fail or using exception handling to structure 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 a

The Error monad (also called the Exception monad).



class Error a whereSource

An exception to be thrown. An instance must redefine at least one of noMsg, strMsg.


noMsg :: aSource

Creates an exception without a message. Default implementation is strMsg "".

strMsg :: String -> aSource

Creates an exception with a message. Default implementation is noMsg.


Error String

A string can be thrown as an error.

Error IOError 

class Monad m => MonadError e m | m -> e whereSource

The 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.


throwError :: e -> m aSource

Is used within a monadic computation to begin exception processing.

catchError :: m a -> (e -> m a) -> m aSource

A handler function to handle previous errors and return to normal execution. A common idiom is:

 do { action1; action2; action3 } `catchError` handler

where the action functions can call throwError. Note that handler and the do-block must have the same return type.


MonadError IOError IO 
Error e => MonadError e (Either e) 
MonadError e m => MonadError e (ListT m) 
MonadError e m => MonadError e (ReaderT r m) 
(Monad m, Error e) => MonadError e (ErrorT e m) 
MonadError e m => MonadError e (StateT s m) 
MonadError e m => MonadError e (StateT s m) 
(Monoid w, MonadError e m) => MonadError e (WriterT w m) 
(Monoid w, MonadError e m) => MonadError e (WriterT w m) 
(Monoid w, MonadError e m) => MonadError e (RWST r w s m) 
(Monoid w, MonadError e m) => MonadError e (RWST r w s m)