Modulepolysemy-1.9.2.0Haskell2010

Polysemy.Error

1 type
15 values

Packagepolysemy-1.9.2.0
Exports16
LanguageHaskell2010
LicenceBSD-3-Clause
SourceError.hs

Effect

1 declaration

datadata Error e (m :: k -> Type) (a :: k) where

This effect abstracts the throwing and catching of errors, leaving it up to the interpreter whether to use exceptions or monad transformers like ExceptT to perform the short-circuiting mechanism.

Constructors

Throw :: e -> Error e m a
Short-circuit the current program using the given error value.
Catch :: m a -> (e -> m a) -> Error e m a
Recover from an error that might have been thrown in the higher-order action given by the first argument by passing the error to the handler given by the second argument.

Actions

12 declarations

valuethrow :: Member (Error e) r => e -> Sem r a

Short-circuit the current program using the given error value.

valuecatch :: Member (Error e) r => Sem r a -> (e -> Sem r a) -> Sem r a

Recover from an error that might have been thrown in the higher-order action given by the first argument by passing the error to the handler given by the second argument.

valuefromEither :: Member (Error e) r => Either e a -> Sem r a

Upgrade an Either into an Error effect.

valuefromEitherM

:: (Member (Error e) r, Member (Embed m) r)
=> m (Either e a)
-> Sem r a

A combinator doing embed and fromEither at the same time. Useful for interoperating with IO.

valuefromException

:: (Exception e, Member (Error e) r, Member (Embed IO) r)
=> IO a
-> Sem r a

Lift an exception generated from an IO action into an Error.

valuefromExceptionVia

:: (Exception exc, Member (Error err) r, Member (Embed IO) r)
=> exc -> err
-> IO a
-> Sem r a

Like fromException, but with the ability to transform the exception before turning it into an Error.

valuefromExceptionSem

:: (Exception e, Member (Error e) r, Member (Final IO) r)
=> Sem r a
-> Sem r a

Run a Sem r action, converting any IO exception generated by it into an Error.

valuefromExceptionSemVia

:: (Exception exc, Member (Error err) r, Member (Final IO) r)
=> exc -> err
-> Sem r a
-> Sem r a

Like fromExceptionSem, but with the ability to transform the exception before turning it into an Error.

valuenote :: Member (Error e) r => e -> Maybe a -> Sem r a

Attempt to extract a Just a from a Maybe a, throwing the provided exception upon Nothing.

valuetry :: Member (Error e) r => Sem r a -> Sem r (Either e a)

Similar to catch, but returns an Either result which is (Right a) if no exception of type e was thrown, or (Left ex) if an exception of type e was thrown and its value is ex.

valuetryJust

:: Member (Error e) r
=> e -> Maybe b
-> Sem r a
-> Sem r (Either b a)

A variant of try that takes an exception predicate to select which exceptions are caught (c.f. catchJust). If the exception does not match the predicate, it is re-thrown.

valuecatchJust

:: Member (Error e) r
=> (e -> Maybe b)
Predicate to select exceptions
-> Sem r a
Computation to run
-> (b -> Sem r a)
Handler
-> Sem r a

The function catchJust is like catch, but it takes an extra argument which is an exception predicate, a function which selects which type of exceptions we're interested in.

Interpretations

3 declarations

valuerunError :: Sem (Error e ': r) a -> Sem r (Either e a)

Run an Error effect in the style of ExceptT.

valuemapError

:: Member (Error e2) r
=> e1 -> e2
-> Sem (Error e1 ': r) a
-> Sem r a

Transform one Error into another. This function can be used to aggregate multiple errors into a single type.

valueerrorToIOFinal

:: Member (Final IO) r
=> Sem (Error e ': r) a
-> Sem r (Either e a)

Run an Error effect as an IO Exception through final IO. This interpretation is significantly faster than runError.

Beware: Effects that aren't interpreted in terms of IO will have local state semantics in regards to Error effects interpreted this way. See Final.