Copyright	(c) 2012-2015 Leon P Smith
License	BSD3
Maintainer	leon@melding-monads.com
Stability	experimental
Safe Haskell	Safe-Inferred
Language	Haskell2010

Database.PostgreSQL.Simple.Ok

Description

The Ok type is a simple error handler, basically equivalent to Either [SomeException] . This type (without the list) was used to handle conversion errors in early versions of postgresql-simple.

One of the primary reasons why this type was introduced is that Either SomeException had not been provided an instance for Alternative , and it would have been a bad idea to provide an orphaned instance for a commonly-used type and typeclass included in base .

Extending the failure case to a list of SomeException s enables a more sensible Alternative instance definitions: <|> concatenates the list of exceptions when both cases fail, and empty is defined as 'Errors []'. Though <|> one could pick one of two exceptions, and throw away the other, and have empty provide a generic exception, this avoids cases where empty overrides a more informative exception and allows you to see all the different ways your computation has failed.

Synopsis

data Ok a
- = Errors [ SomeException ]
- | Ok !a
newtype ManyErrors = ManyErrors [ SomeException ]

Documentation

data Ok a Source #

Constructors

Errors [ SomeException ]
Ok !a

Instances

Instances details

Monad Ok Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods (>>=) :: Ok a -> (a -> Ok b) -> Ok b Source # (>>) :: Ok a -> Ok b -> Ok b Source # return :: a -> Ok a Source #
Functor Ok Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods fmap :: (a -> b) -> Ok a -> Ok b Source # (<$) :: a -> Ok b -> Ok a Source #
MonadFail Ok Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods fail :: String -> Ok a Source #
Applicative Ok Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods pure :: a -> Ok a Source # (<>) :: Ok (a -> b) -> Ok a -> Ok b Source # liftA2 :: (a -> b -> c) -> Ok a -> Ok b -> Ok c Source # (>) :: Ok a -> Ok b -> Ok b Source # (<*) :: Ok a -> Ok b -> Ok a Source #
Alternative Ok Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods empty :: Ok a Source # (<\|>) :: Ok a -> Ok a -> Ok a Source # some :: Ok a -> Ok [a] Source # many :: Ok a -> Ok [a] Source #
MonadPlus Ok Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods mzero :: Ok a Source # mplus :: Ok a -> Ok a -> Ok a Source #
Eq a => Eq ( Ok a) Source #	Two `Errors` cases are considered equal, regardless of what the list of exceptions looks like.
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods (==) :: Ok a -> Ok a -> Bool Source # (/=) :: Ok a -> Ok a -> Bool Source #
Show a => Show ( Ok a) Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods showsPrec :: Int -> Ok a -> ShowS Source # show :: Ok a -> String Source # showList :: [ Ok a] -> ShowS Source #

newtype ManyErrors Source #

a way to reify a list of exceptions into a single exception

Constructors

ManyErrors [ SomeException ]

Instances

Instances details

Show ManyErrors Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods showsPrec :: Int -> ManyErrors -> ShowS Source # show :: ManyErrors -> String Source # showList :: [ ManyErrors ] -> ShowS Source #
Exception ManyErrors Source #
Instance details Defined in Database.PostgreSQL.Simple.Ok Methods toException :: ManyErrors -> SomeException Source # fromException :: SomeException -> Maybe ManyErrors Source # displayException :: ManyErrors -> String Source #