respond

a Haskell library built on top of WAI for processing and routing HTTP requests and generating responses.

motivation

you might wonder why I went to the trouble of building respond when there are, at this point, plenty of libraries etc for HTTP routing in Haskell. (i could say that when I started, there weren't as many, but no one's going to believe that!) I have three justifications for developing this library.

• some HTTP APIs will be best represented using a nested routing structure. this is, in fact, the primary reason i wrote respond - while working on a different project, i found that the nested routes in the API specification for that project were not well served by other libraries. respond should serve in those situations, where groups of routes repeatedly need the same shared routing filters.
• type-safe path matching and parameter extraction is useful, and respond has it.
• it's fairly simple - the core of it is a newtype wrapper for a ReaderT, and the rest of the library contains convenience functions that interact with the monadic interface that RespondT implements. if you want to interact directly with the request, or build and send your own responses, the core interface will let you do that.

using respond

you'll probably want to look at the haddock documentation (available on Hackage) while you're reading this guide. let's get started with a brief overview of how to use this library.

a brief overview

building an app using respond should hopefully be straightforward, and familiar to anyone who has used other WAI-based web service libraries (e.g. scotty).

• first, you integrate the RespondT monad transformer into your monad stack. you can also use RespondM if you don't need a more involved stack.
• use various routing tools (defined in terms of MonadRespond) to match various aspects of the request
• produce responses from inside your routing, either by building them directly, or by using one of the tools for building responses
• convert your routing stack into a WAI app and run it in a WAI compatible server; this library provides a default warp setup you can use.

the monad transformer and monadic interface

MonadRespond is the monadic interface that most of the tools this library provides build on top of. it defines a core set of actions:

• respond is the WAI 3.0 Application continuation lifted into MonadRespond. whenever you see the type ResponseReceived, a call to respond is involved.
• getRequest gets the request that's currently being handled. if you ever find yourself using this directly, get in touch; there's probably an opportunity to add a new tool to the library or improve the existing ones!
• getHandlers gets out the FailureHandlers. these define the responding action to use when request matching or processing fails.
• withHandlers runs the inner MonadRespond action with a modified set of handlers; you can use this to add code to be run after the actual response is sent, or to change the response entirely.
• getPath gets the current PathConsumer.
• withPath runs the passed MonadRespond action with a modified PathConsumer value. this and the previous function will be explained further in the discussion of path routing.

the WAI Application type specifies that both the passed continuation and the returned value are in IO; because MonadRespond specifies a wrapping of that continuation, any instance of MonadRespond must be an instance of MonadIO.

RespondT is the monad transformer that implements the MonadRespond class (as long as it is stacked on top of a MonadIO); it's basically a newtype for a ReaderT that contains a record type containing the relevant components. there is a simple RespondM type alias defined in the Web.Respond.Run module; this alias stacks RespondT directly on IO, i.e.

type RespondM a = RespondT IO a 

running the app

there are several functions in Web.Respond.Run that you can use. the key ones build a WAI application from a RespondT route (i.e. a value of type RespondT m ResponseReceived); note that these functions (respondApp and respondAppDefault) require a function to run the rest of the monadic stack to an IO value. (the RespondM functions, of course, do not require this run function - running the RespondT already produces the necessary IO value.)

the default handlers used by the *Default functions are contained in Web.Respond.DefaultHandlers; the default warp server setup used by the serve* functions are contained in Web.Respond.DefaultServer.

request processing and routing

a number of tools are provided for (hopefully) convenient request processing and routing.

matching methods

Web.Respond.Method defines the type MethodMatcher, which is just a newtype around a Map from StandardMethod to some value. the module provides an onMethod function that takes a StandardMethod and a value and provide a MethodMatcher for just that method. functions are provided that apply onMethod to each StandardMethod. using the monoid instance of MethodMatcher, you can combine matchers to map different methods to different actions. for example:

-- | do something
act1 :: MonadRespond m => m ResponseReceived
act1 = ...

-- | do something else
act2 :: MonadRespond m => m ResponseReceived
act2 = ...

getOrPutActions ::  MonadRespond m => MethodMatcher (m ResponseReceived)
getOrPutActions = onGET act1 <> onPOST act2

matchMethod takes a MethodMatcher that will produce an appropriate responding action for each mapped HTTP method, and chooses the action that handles the current request's method.

-- continuing from before ...
route :: MonadRespond m => m ResponseReceived
route = matchMethod $ 
    onGET act1 <>
    onPOST act2

in this example, route will use act1 if the request method is GET, act2 if the method is POST, and will call handleUnsupportedMethod for any other method (including ones that are not in StandardMethod). what handleUnsupportedMethod does from there will be explained in the section on error handling below.

matching paths

PathMatcher a is a newtype for functions that take PathConsumers and produce Maybe as. the matchPath function uses these to choose the responding action based on the current path state.

PathMatcher is a functor, which lets you do things like wrap inner actions with other routing logic;

-- this is provided in the module, though with a simpler definition
-- whatever action the original matcher would perform is now only performed if
-- the method matches.
matchPathWithMethod :: MonadRespond m => StdMethod -> PathMatcher (m ResponseReceived) -> PathMatcher (m ResponseReceived)
matchPathWithMethod method matcher = (matchMethod . onMethod method) <$> matcher

PathMatcher is also an instance of Applicative and, more importantly, Alternative. the Alternative instance is what allows you to choose different actions for different request paths, for instance

-- for now, suppose this is at the top level of the API
pathExample0 :: RespondM ResponseReceived
pathExample0 = matchPath $
    -- if the request is to e.g. http://localhost:3000/, rootAction will produce the response
    pathEndOrSlash rootAction <|>
    -- however, if the request is to e.g. http://localhost:3000/one/ or http://localhost:3000/one, actionOne will get to respond
    pathLastSeg "one" actionOne

if the request is to none of those paths, then handleUnmatchedPath will get called (see further on).

path extraction

obviously, we want to do more advanced matching on paths, and often we want to get parameters out of paths. this is supported by the use of PathExtractor, which is newtype wrapper around a somewhat fearsome-looking stack of monads. you should look at the definition, but to summarize, it is meant to track PathConsumer state , and possibly produce a value.

a PathConsumer, defined in Web.Respond.Types.Path, can be thought of as a pair of the path segments that have been consumed and the path segments that have not been consumed. each field within the consumer keeps the segments in order, so it is possible to rebuild the original request path using getFullPath. pcConsumeNext produces a new consumer with the first segment in the previous unconsumed segment list appended to the consumed sequence (if there is no next segment, i.e. pcGetNext produces nothing, pcConsumeNext should produce an identical consumer).

the path extractor produced by seg "whatever" does the following when run against a PathMatcher (e.g. by using the function pathExtract)

• it pulls out the current PathConsumer state
• if there is a next segment, and it matches the string "whatever", then it produces an empty value (HList0)
• it then updates the state with the result of applying pcConsumeNext to the old state.

these extractors can then be sequenced using the (</>) combinator, which takes advantage of the Applicative instance of PathExtractor to put them together. (now, if you happen to be looking at the documentation for this function, you might be wondering what all this HList stuff is about. we'll get to that soon.)

now you can use the path function to combine extractors and inner actions to produce path matchers to use with matchPath. for example

pathExample1 :: RespondM ResponseReceived
pathExample1 = matchPath $
    path (seg "one" </> seg "two" </> endOrSlash) someAction

and someAction will only get run if the request is to e.g. http://localhost:3000/one/two/

it wouldn't make sense to call it PathExtractor if it couldn't extract values; this is where HLists come in. a simple example is the value PathExtractor; it produces a single value from a single segment if it can successfully be extracted using the PathPiece instance. when multiple value extractors are chained, they build an HList of those types (since (</>) appends each side's HList). the path function's second parameter expects a function that takes the types of the constructed HList and produces a MonadRespond action. in fact, that is all that HListElim is; a function that has a type signature that matches up with the types of an HList - the function hListUncurry uncurries such a function against a conforming HList. putting this all together, you can do something like

pathInnerAction1 :: Integer -> Text -> Text -> RespondM ResponseReceived
pathInnerAction1 num t1 t2 = ... -- does whatever

pathExample2 :: RespondM ResponseReceived
pathExample2 = matchPath $
    path (seg "id" </> value </> seg "params" </> value </> value) pathInnerAction1

and e.g. a GET against http://localhost:3000/id/42/params/one/two would lead to whatever response the action pathInnerAction1 42 "one" "two" produces.

path route nesting

an important point about the path function is that it runs the inner action with a modified PathConsumer - specifically, the consumer produced by running the PathExtractor. this is accomplished by building on the withPath function specified by the the MonadRespond class. this lets you nest routing in a sensible and hopefully pleasant way:

idAction :: Text -> Integer -> RespondM ResponseReceived
idAction t i = --whatever

pathInnerRouting1 :: Text -> RespondM ResponseReceived
pathInnerRouting1 text = matchPath $
    pathEndOrSlash (innerRootAction text) <|>
    path (seg "id" </> value </> endOrSlash) (\v -> idAction text v)

pathExample3 :: RespondM ResponseReceived
pathExample3 = matchPath $
    pathEndOrSlash outerRootAction <|>
    path (seg "loc" </> value) pathInnerRouting1

which handles requests to e.g.

• http://localhost:3000/ with outerRootAction
• http://localhost:3000/seg/loc with handleUnmatchedPath
• http://localhost:3000/seg/loc/here with pathInnerRouting1 "here", which in turn uses innerRootAction "here".
• http://localhost:3000/seg/loc/here/id/55 with pathInnerRouting1 "here", which in turn uses idAction "here" 55.

extracting request body values

respond defines the FromBody typeclass; instances of this class implement a function that either extracts a value of the instance type out of a lazy bytestring or produces an error value (see below for ReportableError).

provided instances

several newtype wrappers with FromBody instances have been provided for convenience.

• TextBody is a wrapper around a lazy Text value. it decodes the bytestring body as utf-8. it fails with a UnicodeException.
• TextBodyS is like TextBody except that it converts lazy Text into strict Text after decoding.
• Json is a wrapper around a JSON Value. it uses Aeson's eitherDecode for lazy conversion and wraps up a failure message with JsonParseError.
• JsonS is also a wrapper around a JSON Value - it works much the same way as the previous wrapper except that it uses eitherDecode' to perform immediate conversion.

getting the request body - lazy vs strict IO

in Web.Respond.Request, there are 6 functions for extracting the request body as a MonadRespond action.

• getBodyLazy uses Wai's lazyRequestBody function to lazily load the request body. this may or may not be safe for your purposes.
• getBodyStrict uses Wai's strictRequestBody function instead.
• extractBodyLazy and extractBodyStrict build on getBodyLazy and getBodyStrict, and apply the getBody method for the desired instance of FromBody.
• withRequiredBody and withRequiredBody' use extractBodyLazy and extractBodyStrict and then run the passed continuation if a value was successfully extracted. otherwise, they run handleBodyParseFailure to report the error (see section on error reporting).

authentication and authorization tools

there are several functions that run inner actions based on passed values - the main difference between the authenticate and authorize functions is that when a failure is indicated, the former call handleAuthFailed and the latter call handleAccessDenied.

generating responses

the ToResponseBody typeclass is defined to allow you to choose how a type being used a response value should be rendered based on content negotiation. this is accomplished by having the Accept header of the request be passed into the toResponseBody function. various utilities for matching on this header are provided, built on top of the http-media library.

as an example, let's say you have a type ExDocument that you want to use as a response body, and various ways of rendering it.

import qualified Data.Text as T
import Network.HTTP.Media

data ExDocument = ...

-- you have the following ways of rendering it defined appropriately ...

instance ToJSON ExDocument where
    toJSON doc = undefined 

renderDocPlaintext :: ExDocument -> T.Text
renderDocPlaintext doc = undefined 

renderDocHTML :: ExDocument -> T.Text
renderDocHTML doc = undefined

-- you can then use these rendering tools based on the Accept header
-- by defining a ToResponseBody instance.
instance ToResponseBody ExDocument where
    toResponseBody = matchToContentTypes [
        textUtf8 "text/html" renderDocHTML,
        jsonMatcher,
        textUtf8 "text/plain" renderDocPlaintext
    ]

as this example hopefully illustrates, you do not have to handle the interpretation of the Accept header value yourself - you can rely on the http-media library and the provided convenience functions to select the appropriate encoding function. let's break down what they do:

• matchToContentTypes takes a list of MediaTypeMatchers, "prepares" them, and uses the first one that matches (if any) to produce a ResponseBody. it is defined so that it can be easily used to implement toResponseBody - you give it the matcher list and it gives you the implementation.
• ResponseBody is a pair of the value to use as the content type header and the bytestring to use as the body.
• a MediaTypeMatcher is a pair of a media type value and a function that produces a bytestring for a value.
• prepareMediaTypeMatcher is a function that matchToContentTypes uses to construct a pair of media type and response body out of a value and a media type matcher.
• textUtf8 takes a media type and a Text based renderer, and produces a MediaTypeMatcher that will render the value to Text and then encode it as utf-8; it will also add the parameter specifying the encoding to the media type you pass in.
• jsonMatcher is a MediaTypeMatcher for any instance of ToJSON; unsurprisingly, it matches the media type application/json.

once you have an instance of ToResponseBody for a type, you can use the functions in Web.Respond.Response to send responses for that type. for instance, let's say your app monad has a way to fetch a document from the database ...

lookupDocument :: ExampleAppMonad m => DocId -> m ExDocument
lookupDocument id = undefined

docLookupRoute  :: (MonadRespond m, ExampleAppMonad m) -> DocId -> m ResponseReceived
docLookupRoute id = do
    doc <- lookupDocument id
    respondOk doc

now, you may be wondering what happens if the request's Accept header can't be matched; e.g. someone made a request into docLookupRoute but specified the header Accept: text/xml but ExDocument doesn't render to XML. all of the functions built on respondWith use respondUnacceptable if toResponseBody produces Nothing; this sends back a 406 Unacceptable response with an empty body and no content type.

if you know that a set of inner routes will only produce certain content types, you can handle those early on using the checkAccepts function, which takes a list of media types that the inner route can produce, and uses respondUnacceptable if the request's Accept header doesn't match any of them.

also, it is worth keeping in mind that the respond library will default to */* if the request does not set an Accept header explicitly.

ErrorReport and ReportableError

An ErrorReport is a container for information about errors that occur during request processing. it has a ReportableError instance that defaults to rendering the error report as HTML with the status code as a header, and also renders to plain text and JSON.

ReportableError is similar to ToResponseBody except that it must choose a default rendering if it can't match the Accept header. it is also given the status code that will be sent in the response so it may render the status in the body.

if you want to make other errors into ReportableErrors, you may find it convenient to define a function to convert values of your error type into an appropriate ErrorReport value and then define the ReportableError instance using reportAsErrorReport.

handling failures and errors during request processing

various routing failures are handled by using the appropriate handle* function. these functions get the appropriate function from the current FailureHandlers value and run it against the arguments.

it should be possible to modify these functions at any point during routing - installing a new function for a particular handler can allow you to modify how the inner route will respond if it fails in a way that uses the modified handler, and it should allow you to perform other actions, such as any sort of cleanup you might need.

currently, exception handling is a particularly weak part of the respond library - catchRespond only works on specific exception types, so there is no top level exception handling.