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servant-exceptions
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Extensible exceptions for servant
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# servant-exceptions [](https://travis-ci.org/ch1bo/servant-exceptions) | Package | Hackage | | -------------------- | --------------------------------------------------------------------------------------------------------------------------------- | | servant-exceptions | [](https://hackage.haskell.org/package/servant-exceptions) | | servant-exceptions-server | [](https://hackage.haskell.org/package/servant-exceptions-server) | Servant servers typically run their handlers in some form of `IO`. Either directly in the builtin `Handler` monad or a custom monad transformer on top it. When APIs fail, one would typically use the `MonadError ServantError` instance via `throwError` to create an error response of type `ServantErr`. This approach has two problems: * `Handler` (basically being `ExceptT ServantErr IO`) is considered an anti-pattern by some, as it suggests to novice users that only `ServantErr` would occur, but in `IO` any exception can be raised to abort execution * `ServantErr` values need to be created at the call site of `throwError`, where the requested content type and/or headers are not available `servant-exceptions` tries to help with both by making it easy to catch specific error types with an instance of `Exception` and provide automatic encoding into the requested content-type. The API combinator `Throws e` can be used to annotate what error types `e` might be thrown by a server, for example: ```haskell type API = "api" :> Throws UsersError :> "users" :> Get '[JSON, PlainText] [User] ``` The type `UsersError` can then be used to describe expected errors and their conversion via type class instances: ```haskell data UsersError = UserNotFound | UserAlreadyExists | InternalError deriving (Show) instance Exception UsersError instance ToServantErr UsersError where status UserNotFound = status404 status UserAlreadyExists = status409 status InternalError = status500 instance ToJSON UsersError where toJSON e = object [ "type" .= show (typeOf e) , "message" .= message e ] instance MimeRender PlainText UsersError where mimeRender ct = mimeRender ct . show ``` See [example](https://github.com/ch1bo/servant-exceptions/blob/master/example/Main.hs) for a full, commented example. ## Features * Declarative conversion of errors into error responses using `ToServantErr` * Respects `Accept` headers and constructs responses accordingly using `MimeRender` * Add headers to error responses, also via `ToServantErr` * Type-driven exception handling in `ServerT` stacks * Convert "backend" errors into "api" errors using `mapException` ## Planned things This package lacks at least * `servant-client` to rethrow exceptions (using `MonadThrow` and/or `MonadError`?) * `servant-docs` support for automatic error documentation * Documentation, more examples (explain included `ServantException` helper type) ## Credit This package is inspired by `servant-checked-exceptions` (Throws combinator) and the generalized error handling in `cardano-sl`.
