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 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 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