Database.PostgreSQL.Simple.Ok

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 pg.

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 SomeExceptions 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.

Documentation

data Ok a

Constructors

Errors [SomeException]

Ok !a

Instances

Monad Ok

Methods

(>>=) :: Ok a -> (a -> Ok b) -> Ok b

(>>) :: Ok a -> Ok b -> Ok b

return :: a -> Ok a

fail :: String -> Ok a

Functor Ok

Methods

fmap :: (a -> b) -> Ok a -> Ok b

(<$) :: a -> Ok b -> Ok a

Applicative Ok

Methods

pure :: a -> Ok a

(<*>) :: Ok (a -> b) -> Ok a -> Ok b

(*>) :: Ok a -> Ok b -> Ok b

(<*) :: Ok a -> Ok b -> Ok a

Alternative Ok

Methods

empty :: Ok a

(<|>) :: Ok a -> Ok a -> Ok a

some :: Ok a -> Ok [a]

many :: Ok a -> Ok [a]

MonadPlus Ok

Methods

mzero :: Ok a

mplus :: Ok a -> Ok a -> Ok a

Eq a => Eq (Ok a)

Two Errors cases are considered equal, regardless of what the list of exceptions looks like.

Methods

(==) :: Ok a -> Ok a -> Bool

(/=) :: Ok a -> Ok a -> Bool

Show a => Show (Ok a)

Methods

showsPrec :: Int -> Ok a -> ShowS

show :: Ok a -> String

showList :: [Ok a] -> ShowS

newtype ManyErrors

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

Constructors

ManyErrors [SomeException]

Instances

Show ManyErrors

Methods

showsPrec :: Int -> ManyErrors -> ShowS

show :: ManyErrors -> String

showList :: [ManyErrors] -> ShowS

Exception ManyErrors

Methods

toException :: ManyErrors -> SomeException

fromException :: SomeException -> Maybe ManyErrors

displayException :: ManyErrors -> String