| Safe Haskell | None |
|---|---|
| Language | Haskell2010 |
Servant.Exception
Description
Annotate APIs with error types they might Throw. This allows for type
level guided exception handling in servers, clients and can be used by for
generating error documentation.
See https://github.com/ch1bo/servant-exceptions/blob/master/example/Main.hs for a working example.
Synopsis
- data Throws (e :: Type)
- class (Typeable e, Show e) => ToServantErr e where
- data ServantException
- toServantException :: (Exception e, ToJSON e, ToServantErr e) => e -> SomeException
- fromServantException :: Exception e => SomeException -> Maybe e
- class (Typeable e, Show e) => Exception e where
- toException :: e -> SomeException
- fromException :: SomeException -> Maybe e
- displayException :: e -> String
- mapException :: (Exception e1, Exception e2, MonadCatch m) => (e1 -> e2) -> m a -> m a
Documentation
class (Typeable e, Show e) => ToServantErr e where Source #
Type class to convert an e to servant's error types, e.g. ServerError
on the server side. Thus, servers use methods of this class, along with
Typeable and Show allow convenient definition of how error responses
should look like.
Minimal complete definition
Methods
status :: e -> Status Source #
HTTP status code to return
A human-readable message to include. Default implementation uses Show.
headers :: e -> [Header] Source #
Additional headers to include in the response. Content-type headers are created by default.
Instances
| ToServantErr ServantException Source # | |
Defined in Servant.Exception Methods status :: ServantException -> Status Source # message :: ServantException -> Text Source # headers :: ServantException -> [Header] Source # | |
data ServantException Source #
A root exception type (see Exception) to provide a common
rendering format via MimeRender for builtin content types JSON and
PlainText.
Instances
| Show ServantException Source # | |
Defined in Servant.Exception Methods showsPrec :: Int -> ServantException -> ShowS # show :: ServantException -> String # showList :: [ServantException] -> ShowS # | |
| Exception ServantException Source # | |
Defined in Servant.Exception Methods toException :: ServantException -> SomeException # | |
| ToServantErr ServantException Source # | |
Defined in Servant.Exception Methods status :: ServantException -> Status Source # message :: ServantException -> Text Source # headers :: ServantException -> [Header] Source # | |
| MimeRender JSON ServantException Source # | |
Defined in Servant.Exception Methods mimeRender :: Proxy JSON -> ServantException -> ByteString # | |
| MimeRender PlainText ServantException Source # | |
Defined in Servant.Exception Methods mimeRender :: Proxy PlainText -> ServantException -> ByteString # | |
toServantException :: (Exception e, ToJSON e, ToServantErr e) => e -> SomeException Source #
fromServantException :: Exception e => SomeException -> Maybe e Source #
class (Typeable e, Show e) => Exception e where #
Any type that you wish to throw or catch as an exception must be an
instance of the Exception class. The simplest case is a new exception
type directly below the root:
data MyException = ThisException | ThatException
deriving Show
instance Exception MyExceptionThe default method definitions in the Exception class do what we need
in this case. You can now throw and catch ThisException and
ThatException as exceptions:
*Main> throw ThisException `catch` \e -> putStrLn ("Caught " ++ show (e :: MyException))
Caught ThisException
In more complicated examples, you may wish to define a whole hierarchy of exceptions:
---------------------------------------------------------------------
-- Make the root exception type for all the exceptions in a compiler
data SomeCompilerException = forall e . Exception e => SomeCompilerException e
instance Show SomeCompilerException where
show (SomeCompilerException e) = show e
instance Exception SomeCompilerException
compilerExceptionToException :: Exception e => e -> SomeException
compilerExceptionToException = toException . SomeCompilerException
compilerExceptionFromException :: Exception e => SomeException -> Maybe e
compilerExceptionFromException x = do
SomeCompilerException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make a subhierarchy for exceptions in the frontend of the compiler
data SomeFrontendException = forall e . Exception e => SomeFrontendException e
instance Show SomeFrontendException where
show (SomeFrontendException e) = show e
instance Exception SomeFrontendException where
toException = compilerExceptionToException
fromException = compilerExceptionFromException
frontendExceptionToException :: Exception e => e -> SomeException
frontendExceptionToException = toException . SomeFrontendException
frontendExceptionFromException :: Exception e => SomeException -> Maybe e
frontendExceptionFromException x = do
SomeFrontendException a <- fromException x
cast a
---------------------------------------------------------------------
-- Make an exception type for a particular frontend compiler exception
data MismatchedParentheses = MismatchedParentheses
deriving Show
instance Exception MismatchedParentheses where
toException = frontendExceptionToException
fromException = frontendExceptionFromExceptionWe can now catch a MismatchedParentheses exception as
MismatchedParentheses, SomeFrontendException or
SomeCompilerException, but not other types, e.g. IOException:
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: MismatchedParentheses))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeFrontendException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: SomeCompilerException))
Caught MismatchedParentheses
*Main> throw MismatchedParentheses `catch` \e -> putStrLn ("Caught " ++ show (e :: IOException))
*** Exception: MismatchedParentheses
Minimal complete definition
Nothing
Methods
toException :: e -> SomeException #
fromException :: SomeException -> Maybe e #
displayException :: e -> String #
Render this exception value in a human-friendly manner.
Default implementation: show
Since: base-4.8.0.0
Instances
mapException :: (Exception e1, Exception e2, MonadCatch m) => (e1 -> e2) -> m a -> m a Source #
Catch and rethrow using mapping function f.