Documentation

MonadSnap is a type class, analogous to MonadIO for IO, that makes it easy to wrap Snap inside monad transformers.

Type of external handler passed to escapeHttp.

Used internally to implement escapeHttp.

Snap is the Monad that user web handlers run in. Snap gives you:

1. Stateful access to fetch or modify an HTTP Request.

   printRqContextPath :: Snap ()
   printRqContextPath = writeBS . rqContextPath =<< getRequest
   

2. Stateful access to fetch or modify an HTTP Response.

   printRspStatusReason :: Snap ()
   printRspStatusReason = writeBS . rspStatusReason =<< getResponse
   

3. Failure / Alternative / MonadPlus semantics: a Snap handler can choose not to handle a given request, using empty or its synonym pass, and you can try alternative handlers with the <|> operator:

   a :: Snap String
   a = pass
   
   b :: Snap String
   b = return "foo"
   
   c :: Snap String
   c = a <|> b             -- try running a, if it fails then try b
   

4. Convenience functions (writeBS, writeLBS, writeText, writeLazyText, addToOutput) for queueing output to be written to the Response, or for streaming to the response using io-streams:

   example :: (OutputStream Builder -> IO (OutputStream Builder)) -> Snap ()
   example streamProc = do
       writeBS   "I'm a strict bytestring"
       writeLBS  "I'm a lazy bytestring"
       writeText "I'm strict text"
       addToOutput streamProc
   

5. Early termination: if you call finishWith:

   a :: Snap ()
   a = do
       modifyResponse $ setResponseStatus 500 "Internal Server Error"
       writeBS "500 error"
       r <- getResponse
       finishWith r
   

   then any subsequent processing will be skipped and the supplied Response value will be returned from runSnap as-is.

6. Access to the IO monad through a MonadIO instance:

   a :: Snap ()
   a = liftIO fireTheMissiles
   

7. The ability to set or extend a timeout which will kill the handler thread after N seconds of inactivity (the default is 20 seconds):

   a :: Snap ()
   a = setTimeout 30
   

8. Throw and catch exceptions using a MonadBaseControl instance:

   import Control.Exception.Lifted (SomeException, throwIO, catch)
   
   foo :: Snap ()
   foo = bar `catch` (e::SomeException) -> baz
     where
       bar = throwIO FooException
   

9. Log a message to the error log:

   foo :: Snap ()
   foo = logError "grumble."
   

You may notice that most of the type signatures in this module contain a (MonadSnap m) => ... typeclass constraint. MonadSnap is a typeclass which, in essence, says "you can get back to the Snap monad from here". Using MonadSnap you can extend the Snap monad with additional functionality and still have access to most of the Snap functions without writing lift everywhere. Instances are already provided for most of the common monad transformers (ReaderT, WriterT, StateT, etc.).

Pass the request body stream to a consuming procedure, returning the result.

If the consuming procedure you pass in here throws an exception, Snap will attempt to clear the rest of the unread request body (using skipToEof) before rethrowing the exception. If you used terminateConnection, however, Snap will give up and immediately close the socket.

To prevent slowloris attacks, the connection will be also terminated if the input socket produces data too slowly (500 bytes per second is the default limit).

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.ByteString.Char8 as B8
ghci> import qualified Data.ByteString.Lazy as L
ghci> import Data.Char (toUpper)
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified System.IO.Streams as Streams
ghci> let r = T.put "/foo" "text/plain" "some text"
ghci> :{
ghci| let f s = do u <- Streams.map (B8.map toUpper) s
ghci|              l <- Streams.toList u
ghci|              return $ L.fromChunks l
ghci| :}
ghci> T.runHandler r (runRequestBody f >>= writeLBS)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 20:48:40 GMT

SOME TEXT

Returns the request body as a lazy bytestring. /Note that the request is not actually provided lazily!/

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.put "/foo" "text/plain" "some text"
ghci> T.runHandler r (readRequestBody 2048 >>= writeLBS)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 20:08:44 GMT

some text

Since: 0.6

Normally Snap is careful to ensure that the request body is fully consumed after your web handler runs, but before the Response body is streamed out the socket. If you want to transform the request body into some output in O(1) space, you should use this function.

Take care: in order for this to work, the HTTP client must be written with input-to-output streaming in mind.

Note that upon calling this function, response processing finishes early as if you called finishWith. Make sure you set any content types, headers, cookies, etc. before you call this function.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.ByteString.Char8 as B8
ghci> import Data.Char (toUpper)
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified System.IO.Streams as Streams
ghci> let r = T.put "/foo" "text/plain" "some text"
ghci> let f = Streams.map (B8.map toUpper)
ghci> T.runHandler r (transformRequestBody f >> readRequestBody 2048 >>= writeLBS)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 20:30:15 GMT

SOME TEXT

Short-circuits a Snap monad action early, storing the given Response value in its state.

IMPORTANT: Be vary careful when using this with things like a DB library's withTransaction function or any other kind of setup/teardown block, as it can prevent the cleanup from being called and result in resource leaks.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import Control.Applicative
ghci> let r = T.get "/" M.empty
ghci> T.runHandler r ((ifTop $ writeBS "TOP") <|> finishWith emptyResponse)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 16:58:57 GMT

TOP
ghci> let r' = T.get "/foo/bar" M.empty
ghci> T.runHandler r' ((ifTop $ writeBS "TOP") <|> finishWith emptyResponse)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 17:50:50 GMT

Capture the flow of control in case a handler calls finishWith.

WARNING: in the event of a call to transformRequestBody it is possible to violate HTTP protocol safety when using this function. If you call catchFinishWith it is suggested that you do not modify the body of the Response which was passed to the finishWith call.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.ByteString.Char8 as B8
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import Control.Applicative
ghci> let r = T.get "/foo/bar" M.empty
ghci> let h = (ifTop $ writeBS "TOP") <|> finishWith emptyResponse
ghci> T.runHandler r (catchFinishWith h >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 18:35:42 GMT

Left HTTP/1.1 200 OK

Fails out of a Snap monad action. This is used to indicate that you choose not to handle the given request within the given handler.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r pass
HTTP/1.1 404 Not Found
server: Snap/test
date: Thu, 07 Aug 2014 13:35:42 GMT

<!DOCTYPE html>
<html>
<head>
<title>Not found</title>
</head>
<body>
<code>No handler accepted "/foo/bar"/code
</body></html>

Runs a Snap monad action only if the request's HTTP method matches the given method.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (method GET $ writeBS "OK")
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 13:38:48 GMT

OK
ghci> T.runHandler r (method POST $ writeBS "OK")
HTTP/1.1 404 Not Found
...

Runs a Snap monad action only if the request's HTTP method matches one of the given methods.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (methods [GET, POST] $ writeBS "OK")
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 13:38:48 GMT

OK
ghci> T.runHandler r (methods [POST] $ writeBS "OK")
HTTP/1.1 404 Not Found
...

Runs a Snap monad action only when the rqPathInfo of the request starts with the given path. For example,

dir "foo" handler

Will fail if rqPathInfo is not "/foo" or "/foo/...", and will add "foo/" to the handler's local rqContextPath.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (dir "foo" $ writeBS "OK")
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 14:52:24 GMT

OK
ghci> T.runHandler r (dir "baz" $ writeBS "OK")
HTTP/1.1 404 Not Found
...

Runs a Snap monad action only for requests where rqPathInfo is exactly equal to the given string. If the path matches, locally sets rqContextPath to the old value of rqPathInfo, sets rqPathInfo="", and runs the given handler.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> T.runHandler (T.get "/foo" M.empty) (path "foo" $ writeBS "bar")
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 14:15:42 GMT

bar
ghci> T.runHandler (T.get "/foo" M.empty) (path "bar" $ writeBS "baz")
HTTP/1.1 404 Not Found
...

Runs a Snap monad action only when the first path component is successfully parsed as the argument to the supplied handler function.

Note that the path segment is url-decoded prior to being passed to fromBS; this is new as of snap-core 0.10.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/11/foo/bar" M.empty
ghci> let f = (\i -> if i == 11 then writeBS "11" else writeBS "???")
ghci> T.runHandler r (pathArg f)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 14:27:10 GMT

11
ghci> let r' = T.get "/foo/11/bar" M.empty
ghci> T.runHandler r' (pathArg f)
HTTP/1.1 404 Not Found
...

Runs a Snap monad action only when rqPathInfo is empty.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/" M.empty
ghci> T.runHandler r (ifTop $ writeBS OK)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 14:56:39 GMT

OK
ghci> let r' = T.get "/foo" M.empty
ghci> T.runHandler r' (ifTop $ writeBS "OK")
HTTP/1.1 404 Not Found
...

Local Snap version of get.

Local Snap monad version of modify.

Grabs the Request object out of the Snap monad.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeBS . rqURI =<< getRequest)
HTTP/1.1 200 OK
server: Snap/test
date: Sat, 02 Aug 2014 07:51:54 GMT

/foo/bar

Grabs the Response object out of the Snap monad.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeBS . rspStatusReason =<< getResponse)
HTTP/1.1 200 OK
server: Snap/test
date: Sat, 02 Aug 2014 15:06:00 GMT

OK

Grabs something out of the Request object, using the given projection function. See gets.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeBS =<< getsRequest rqURI)
HTTP/1.1 200 OK
server: Snap/test
date: Sat, 02 Aug 2014 07:51:54 GMT

/foo/bar

Grabs something out of the Response object, using the given projection function. See gets.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeBS =<< getsResponse rspStatusReason)
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 13:35:45 GMT

OK

Puts a new Request object into the Snap monad. Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> :{
ghci| let hndlr = do rq <- T.buildRequest (T.get "/bar/foo" M.empty)
ghci|                putRequest rq
ghci|                uri' <- getsRequest rqURI
ghci|                writeBS uri'
ghci| :}
ghci> T.runHandler (T.get "/foo/bar" M.empty) hndlr
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 15:13:46 GMT

/bar/foo

Puts a new Response object into the Snap monad.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let rsp = setResponseCode 404 emptyResponse
ghci> let req = T.get "/foo/bar" M.empty
ghci> T.runHandler req (putResponse rsp)
HTTP/1.1 404 Not Found
server: Snap/test
date: Wed, 06 Aug 2014 13:59:58 GMT

Modifies the Request object stored in a Snap monad. Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> r' <- T.buildRequest $ T.get "/bar/foo" M.empty
ghci> T.runHandler r (modifyRequest (const r') >> getsRequest rqURI >>= writeBS)
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 15:24:25 GMT

/bar/foo

Modifes the Response object stored in a Snap monad. Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (modifyResponse $ setResponseCode 404)
HTTP/1.1 404 Not Found
server: Snap/test
date: Wed, 06 Aug 2014 15:27:11 GMT

Performs a redirect by setting the Location header to the given target URL/path and the status code to 302 in the Response object stored in a Snap monad. Note that the target URL is not validated in any way. Consider using redirect' instead, which allows you to choose the correct status code.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (redirect "http://snapframework.com")
HTTP/1.1 302 Found
content-length: 0
location: http://snapframework.com
server: Snap/test
date: Thu, 07 Aug 2014 08:52:11 GMT
Content-Length: 0

Performs a redirect by setting the Location header to the given target URL/path and the status code (should be one of 301, 302, 303 or 307) in the Response object stored in a Snap monad. Note that the target URL is not validated in any way.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (redirect' "http://snapframework.com" 301)
HTTP/1.1 307 Temporary Redirect
content-length: 0
location: http://snapframework.com
server: Snap/test
date: Thu, 07 Aug 2014 08:55:51 GMT
Content-Length: 0

Log an error message in the Snap monad.

Example:

ghci> import qualified Data.ByteString.Char8 as B8
ghci> runSnap (logError "fatal error!") (error . B8.unpack) undefined undefined
*** Exception: fatal error!

Run the given stream procedure, adding its output to the Response stored in the Snap monad state.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Builder as B
ghci> import qualified System.IO.Streams as Streams
ghci> let r = T.get "/foo/bar" M.empty
ghci> :{
ghci| let f str = do {
ghci|   Streams.write (Just $ B.byteString "Hello, streams world") str;
ghci|   return str }
ghci| :}
ghci> T.runHandler r (addToOutput f)
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 17:55:47 GMT

Hello, streams world

Adds the given Builder to the body of the Response stored in the | Snap monad state.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Builder as B
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeBuilder $ B.byteString "Hello, world")
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 17:33:33 GMT

Hello, world

Adds the given strict ByteString to the body of the Response stored in the Snap monad state.

Warning: This function is intentionally non-strict. If any pure exceptions are raised by the expression creating the ByteString, the exception won't actually be raised within the Snap handler.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeBS "Hello, bytestring world")
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 17:34:27 GMT

Hello, bytestring world

Adds the given lazy ByteString to the body of the Response stored in the Snap monad state.

Warning: This function is intentionally non-strict. If any pure exceptions are raised by the expression creating the ByteString, the exception won't actually be raised within the Snap handler.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeLBS "Hello, lazy bytestring world")
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 17:35:15 GMT

Hello, lazy bytestring world

Adds the given strict Text to the body of the Response stored in the Snap monad state.

Warning: This function is intentionally non-strict. If any pure exceptions are raised by the expression creating the ByteString, the exception won't actually be raised within the Snap handler.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeText "Hello, text world")
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 17:36:38 GMT

Hello, text world

Adds the given lazy Text to the body of the Response stored in the Snap monad state.

Warning: This function is intentionally non-strict. If any pure exceptions are raised by the expression creating the ByteString, the exception won't actually be raised within the Snap handler.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (writeLazyText "Hello, lazy text world")
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 17:37:41 GMT

Hello, lazy text world

Sets the output to be the contents of the specified file.

Calling sendFile will overwrite any output queued to be sent in the Response. If the response body is not modified after the call to sendFile, Snap will use the efficient sendfile() system call on platforms that support it.

If the response body is modified (using modifyResponseBody), the file will be read using mmap().

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> writeFile "/tmp/snap-file" "Hello, sendFile world"
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (sendFile "/tmp/snap-file")
HTTP/1.1 200 OK
content-length: 21
server: Snap/test
date: Wed, 06 Aug 2014 17:45:10 GMT
Content-Length: 21

Hello, sendFile world

Sets the output to be the contents of the specified file, within the given (start,end) range.

Calling sendFilePartial will overwrite any output queued to be sent in the Response. If the response body is not modified after the call to sendFilePartial, Snap will use the efficient sendfile() system call on platforms that support it.

If the response body is modified (using modifyResponseBody), the file will be read using mmap().

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> writeFile "/tmp/snap-file" "Hello, sendFilePartial world"
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (sendFilePartial "/tmp/snap-file" (7, 28))
HTTP/1.1 200 OK
content-length: 21
server: Snap/test
date: Wed, 06 Aug 2014 17:47:20 GMT
Content-Length: 21

sendFilePartial world

Runs a Snap action with a locally-modified Request state object. The Request object in the Snap monad state after the call to localRequest will be unchanged. Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> r' <- T.buildRequest $ T.get "/bar/foo" M.empty
ghci> let printRqURI = getsRequest rqURI >>= writeBS >> writeBS "\n"
ghci> T.runHandler r (printRqURI >> localRequest (const r') printRqURI)
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 15:34:12 GMT

/foo/bar
/bar/foo

Fetches the Request from state and hands it to the given action. Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import Control.Monad.IO.Class
ghci> let r = T.get "/foo/bar" M.empty
ghci> let h = withRequest (\rq -> liftIO (T.requestToString rq) >>= writeBS)
ghci> T.runHandler r h
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 15:44:24 GMT

GET /foo/bar HTTP/1.1
host: localhost

Fetches the Response from state and hands it to the given action. Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (withResponse $ writeBS . rspStatusReason)
HTTP/1.1 200 OK
server: Snap/test
date: Wed, 06 Aug 2014 15:48:45 GMT

OK

Modifies the Request in the state to set the rqRemoteAddr field to the value in the X-Forwarded-For header. If the header is not present, this action has no effect.

This action should be used only when working behind a reverse http proxy that sets the X-Forwarded-For header. This is the only way to ensure the value in the X-Forwarded-For header can be trusted.

This is provided as a filter so actions that require the remote address can get it in a uniform manner. It has specifically limited functionality to ensure that its transformation can be trusted, when used correctly.

Modifies the Request in the state to set the rqRemoteAddr field to the value from the header specified. If the header specified is not present, this action has no effect.

This action should be used only when working behind a reverse http proxy that sets the header being looked at. This is the only way to ensure the value in the header can be trusted.

This is provided as a filter so actions that require the remote address can get it in a uniform manner. It has specifically limited functionality to ensure that its transformation can be trusted, when used correctly.

This function brackets a Snap action in resource acquisition and release. This is provided because MonadCatchIO's bracket function doesn't work properly in the case of a short-circuit return from the action being bracketed.

In order to prevent confusion regarding the effects of the aquisition and release actions on the Snap state, this function doesn't accept Snap actions for the acquire or release actions.

This function will run the release action in all cases where the acquire action succeeded. This includes the following behaviors from the bracketed Snap action.

1. Normal completion
2. Short-circuit completion, either from calling fail or finishWith
3. An exception being thrown.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let br = bracketSnap (putStrLn "before") (const $ putStrLn "after")
ghci> T.runHandler (T.get "/" M.empty) (br $ const $ writeBS "OK")
before
after
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 18:41:50 GMT

OK

This exception is thrown if the handler you supply to runSnap fails.

Terminate the HTTP session with the given exception.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Control.Exception as E
ghci> let r = T.get "/foo/bar" M.empty
ghci> T.runHandler r (terminateConnection $ E.AssertionFailed "Assertion failed!")
*** Exception: <terminated: Assertion failed!>

Terminate the HTTP session and hand control to some external handler, escaping all further HTTP traffic.

The external handler takes three arguments: a function to modify the thread's timeout, and a read and a write ends to the socket.

Runs a Snap monad action.

This function is mostly intended for library writers; instead of invoking runSnap directly, use httpServe or runHandler (for testing).

Post-process a finalized HTTP response:

• fixup content-length header
• properly handle 204/304 responses
• if request was HEAD, remove response body

Note that we do NOT deal with transfer-encoding: chunked or "connection: close" here.

See rqParam. Looks up a value for the given named parameter in the Request. If more than one value was entered for the given parameter name, getParam gloms the values together with intercalate " ".

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let r = T.get "/foo/bar" $ M.fromList [("foo", ["bar"])]
ghci> T.runHandler r (getParam "foo" >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Mon, 11 Aug 2014 12:57:20 GMT

Just "bar"

See rqPostParam. Looks up a value for the given named parameter in the POST form parameters mapping in Request. If more than one value was entered for the given parameter name, getPostParam gloms the values together with: intercalate " ".

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let r = T.postUrlEncoded "/foo/bar" $ M.fromList [("foo", ["bar"])]
ghci> T.runHandler r (getPostParam "foo" >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Mon, 11 Aug 2014 13:01:04 GMT

Just "bar"

See rqQueryParam. Looks up a value for the given named parameter in the query string parameters mapping in Request. If more than one value was entered for the given parameter name, getQueryParam gloms the values together with intercalate " ".

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let r = T.postUrlEncoded "/foo/bar" M.empty >> T.setQueryStringRaw "foo=bar&foo=baz"
ghci> T.runHandler r (getQueryParam "foo" >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Mon, 11 Aug 2014 13:06:50 GMT

Just "bar baz"

See rqParams. Convenience function to return Params from the Request inside of a MonadSnap instance.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let r = T.get "/foo/bar" $ M.fromList [("foo", ["bar"])]
ghci> T.runHandler r (getParams >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Mon, 11 Aug 2014 13:02:54 GMT

fromList [("foo",["bar"])]

See rqParams. Convenience function to return Params from the Request inside of a MonadSnap instance.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let r = T.postUrlEncoded "/foo/bar" $ M.fromList [("foo", ["bar"])]
ghci> T.runHandler r (getPostParams >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Mon, 11 Aug 2014 13:04:34 GMT

fromList [("foo",["bar"])]

See rqParams. Convenience function to return Params from the Request inside of a MonadSnap instance.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let r = T.postUrlEncoded "/foo/bar" M.empty >> T.setQueryStringRaw "foo=bar&foo=baz"
ghci> T.runHandler r (getQueryParams >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Mon, 11 Aug 2014 13:10:17 GMT

fromList [("foo",["bar","baz"])]

Gets the HTTP Cookie with the specified name.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> import qualified Data.ByteString.Char8 as B8
ghci> let cookie = Cookie "name" "value" Nothing Nothing Nothing False False
ghci> let r = T.get "/foo/bar" M.empty >> T.addCookies [cookie]
ghci> T.runHandler r (getCookie "name" >>= writeBS . B8.pack . show)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 12:16:58 GMT

Just (Cookie {cookieName = "name", cookieValue = "value", ...})

Gets the HTTP Cookie with the specified name and decodes it. If the decoding fails, the handler calls pass.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let cookie = Cookie "name" "value" Nothing Nothing Nothing False False
ghci> let r = T.get "/foo/bar" M.empty >> T.addCookies [cookie]
ghci> T.runHandler r (readCookie "name" >>= writeBS)
HTTP/1.1 200 OK
server: Snap/test
date: Thu, 07 Aug 2014 12:20:09 GMT

value

Expire given Cookie in client's browser.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Data.Map as M
ghci> import qualified Snap.Test as T
ghci> let r = T.get "/foo/bar" M.empty
ghci> let cookie = Cookie "name" "" Nothing (Just "/subsite") Nothing True False
ghci> T.runHandler r (expireCookie cookie)
HTTP/1.1 200 OK
set-cookie: name=; path=/subsite; expires=Sat, 24 Dec 1994 06:28:16 GMT; Secure
server: Snap/test

date: Thu, 07 Aug 2014 12:21:27 GMT
ghci> let cookie = Cookie "name" "value" Nothing Nothing Nothing False False
ghci> let r2 = T.get "/foo/bar" M.empty >> T.addCookies [cookie]
ghci> T.runHandler r (getCookie "name" >>= maybe (return ()) expireCookie)
HTTP/1.1 200 OK
set-cookie: name=; expires=Sat, 24 Dec 1994 06:28:16 GMT
server: Snap/test

Causes the handler thread to be killed n seconds from now.

Causes the handler thread to be killed at least n seconds from now.

Modifies the amount of time remaining before the request times out.

Returns an IO action which you can use to modify the timeout value.

Represents an HTTP response.

Contains all of the information about an incoming HTTP request.

A type alias for the HTTP parameters mapping. Each parameter key maps to a list of ByteString values; if a parameter is specified multiple times (e.g.: "GET /foo?param=bar1&param=bar2"), looking up "param" in the mapping will give you ["bar1", "bar2"].

A datatype representing an HTTP cookie.

Represents a (major, minor) version of the HTTP protocol.

Enumerates the HTTP method values (see http://tools.ietf.org/html/rfc2068.html#section-5.1.1).

A typeclass for datatypes which contain HTTP headers.

Adds a header key-value-pair to the HasHeaders datatype. If a header with the same name already exists, the new value is appended to the headers list.

Example:

ghci> import qualified Snap.Types.Headers as H
ghci> addHeader "Host" "localhost" H.empty
H {unH = [("host","localhost")]}
ghci> addHeader "Host" "127.0.0.1" it
H {unH = [("host","localhost,127.0.0.1")]}

Sets a header key-value-pair in a HasHeaders datatype. If a header with the same name already exists, it is overwritten with the new value.

Example:

ghci> import qualified Snap.Types.Headers as H
ghci> setHeader "Host" "localhost" H.empty
H {unH = [("host","localhost")]}
ghci> setHeader "Host" "127.0.0.1" it
H {unH = [("host","127.0.0.1")]}

Gets a header value out of a HasHeaders datatype.

Example:

ghci> import qualified Snap.Types.Headers as H
ghci> getHeader "Host" $ setHeader "Host" "localhost" H.empty
Just "localhost"

Lists all the headers out of a HasHeaders datatype. If many headers came in with the same name, they will be catenated together.

Example:

ghci> import qualified Snap.Types.Headers as H
ghci> listHeaders $ setHeader "Host" "localhost" H.empty
[("host","localhost")]

Clears a header value from a HasHeaders datatype.

Example:

ghci> import qualified Snap.Types.Headers as H
ghci> deleteHeader "Host" $ setHeader "Host" "localhost" H.empty
H {unH = []}

Equate the special case constructors with their corresponding Method name variant.

Looks up the value(s) for the given named parameter. Parameters initially come from the request's query string and any decoded POST body (if the request's Content-Type is application/x-www-form-urlencoded). Parameter values can be modified within handlers using "rqModifyParams".

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Snap.Test as T
ghci> import qualified Data.Map as M
ghci> :{
ghci| rq <- T.buildRequest $ do
ghci|         T.postUrlEncoded "/foo/bar" $ M.fromList [("baz", ["qux"])]
ghci|         T.setQueryStringRaw "baz=quux"
ghci| :}
ghci> rqParam "baz" rq
Just ["qux","quux"]

Looks up the value(s) for the given named parameter in the POST parameters mapping.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Snap.Test as T
ghci> import qualified Data.Map as M
ghci> :{
ghci| rq <- T.buildRequest $ do
ghci|         T.postUrlEncoded "/foo/bar" $ M.fromList [("baz", ["qux"])]
ghci|         T.setQueryStringRaw "baz=quux"
ghci| :}
ghci> rqPostParam "baz" rq
Just ["qux"]

Looks up the value(s) for the given named parameter in the query parameters mapping.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Snap.Test as T
ghci> import qualified Data.Map as M
ghci> :{
ghci| rq <- T.buildRequest $ do
ghci|         T.postUrlEncoded "/foo/bar" $ M.fromList [("baz", ["qux"])]
ghci|         T.setQueryStringRaw "baz=quux"
ghci| :}
ghci> rqQueryParam "baz" rq
Just ["quux"]

Modifies the parameters mapping (which is a Map ByteString ByteString) in a Request using the given function.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Snap.Test as T
ghci> import qualified Data.Map as M
ghci> :{
ghci| rq <- T.buildRequest $ do
ghci|         T.postUrlEncoded "/foo/bar" $ M.fromList [("baz", ["qux"])]
ghci|         T.setQueryStringRaw "baz=quux"
ghci| :}
ghci> rqParams rq
fromList [("baz",["qux","quux"])]
ghci> rqParams $ rqModifyParams (M.delete "baz") rq
fromList []

Writes a key-value pair to the parameters mapping within the given request.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified Snap.Test as T
ghci> import qualified Data.Map as M
ghci> :{
ghci| rq <- T.buildRequest $ do
ghci|         T.postUrlEncoded "/foo/bar" $ M.fromList [("baz", ["qux"])]
ghci|         T.setQueryStringRaw "baz=quux"
ghci| :}
ghci> rqParams rq
fromList [("baz",["qux","quux"])]
ghci> rqParams $ rqSetParam "baz" ["corge"] rq
fromList [("baz", ["corge"])]

An empty Response.

Example:

ghci> emptyResponse
HTTP/1.1 200 OK

Sets an HTTP response body to the given stream procedure.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified System.IO.Streams as Streams
ghci> import qualified Data.ByteString.Builder as Builder
ghci> :{
ghci| let r = setResponseBody
ghci|         (out -> do
ghci|             Streams.write (Just $ Builder.byteString "Hello, world!") out
ghci|             return out)
ghci|         emptyResponse
ghci| :}
ghci> r
HTTP/1.1 200 OK

Hello, world!

Sets the HTTP response status. Note: normally you would use setResponseCode unless you needed a custom response explanation.

Example:

ghci> :set -XOverloadedStrings
ghci> setResponseStatus 500 "Internal Server Error" emptyResponse
HTTP/1.1 500 Internal Server Error

Sets the HTTP response code.

Example:

ghci> setResponseCode 404 emptyResponse
HTTP/1.1 404 Not Found

Modifies a response body.

Example:

ghci> :set -XOverloadedStrings
ghci> import qualified System.IO.Streams as Streams
ghci> import qualified Data.ByteString.Builder as Builder
ghci> :{
ghci| let r = setResponseBody
ghci|         (out -> do
ghci|             Streams.write (Just $ Builder.byteString "Hello, world!") out
ghci|             return out)
ghci|         emptyResponse
ghci| :}
ghci> r
HTTP/1.1 200 OK

Hello, world!
ghci> :{
ghci| let r' = modifyResponseBody
ghci|          (f out -> do
ghci|              out' <- f out
ghci|              Streams.write (Just $ Builder.byteString "\nBye, world!") out'
ghci|              return out') r
ghci| :}
ghci> r'
HTTP/1.1 200 OK

Hello, world!
Bye, world!

Sets the Content-Type in the Response headers.

Example:

ghci> :set -XOverloadedStrings
ghci> setContentType "text/html" emptyResponse
HTTP/1.1 200 OK
content-type: text/html

Adds an HTTP Cookie to Response headers.

Example:

ghci> :set -XOverloadedStrings
ghci> let cookie = Cookie "name" "value" Nothing Nothing Nothing False False
ghci> getResponseCookie "name" $ addResponseCookie cookie emptyResponse
Just (Cookie {cookieName = "name", cookieValue = "value", ...})

Gets an HTTP Cookie with the given name from Response headers.

Example:

ghci> :set -XOverloadedStrings
ghci> getResponseCookie "cookie-name" emptyResponse
Nothing

Returns a list of Cookies present in Response

Example:

ghci> getResponseCookies emptyResponse
[]

Deletes an HTTP Cookie from the Response headers. Please note this does not necessarily erase the cookie from the client browser.

Example:

ghci> :set -XOverloadedStrings
ghci> let cookie = Cookie "name" "value" Nothing Nothing Nothing False False
ghci> let rsp    = addResponseCookie cookie emptyResponse
ghci> getResponseCookie "name" rsp
Just (Cookie {cookieName = "name", cookieValue = "value", ...})
ghci> getResponseCookie "name" $ deleteResponseCookie "name" rsp
Nothing

Modifies an HTTP Cookie with given name in Response headers. Nothing will happen if a matching Cookie can not be found in Response.

Example:

ghci> :set -XOverloadedStrings
ghci> import Data.Monoid
ghci> let cookie = Cookie "name" "value" Nothing Nothing Nothing False False
ghci> let rsp    = addResponseCookie cookie emptyResponse
ghci> getResponseCookie "name" rsp
Just (Cookie {cookieName = "name", cookieValue = "value", ...})
ghci> let f ck@(Cookie { cookieName = name }) = ck { cookieName = name <> "'"}
ghci> let rsp' = modifyResponseCookie "name" f rsp
ghci> getResponseCookie "name'" rsp'
Just (Cookie {cookieName = "name'", ...})
ghci> getResponseCookie "name" rsp'
Just (Cookie {cookieName = "name", ...})

A note here: if you want to set the Content-Length for the response, Snap forces you to do it with this function rather than by setting it in the headers; the Content-Length in the headers will be ignored.

The reason for this is that Snap needs to look up the value of Content-Length for each request, and looking the string value up in the headers and parsing the number out of the text will be too expensive.

If you don't set a content length in your response, HTTP keep-alive will be disabled for HTTP/1.0 clients, forcing a Connection: close. For HTTP/1.1 clients, Snap will switch to the chunked transfer encoding if Content-Length is not specified.

Example:

ghci> setContentLength 400 emptyResponse
HTTP/1.1 200 OK
Content-Length: 400

Removes any Content-Length set in the Response.

Example:

ghci> clearContentLength $ setContentLength 400 emptyResponse
HTTP/1.1 200 OK

Convert a CTime into common log entry format.

Convert a CTime into an HTTP timestamp.

Example:

ghci> formatHttpTime . fromIntegral $ 10
"Thu, 01 Jan 1970 00:00:10 GMT"

Converts an HTTP timestamp into a CTime.

Example:

ghci> :set -XOverloadedStrings
ghci> parseHttpTime "Thu, 01 Jan 1970 00:00:10 GMT"
10

Adapted from:

https://www.iana.org/assignments/http-status-codes/http-status-codes.txt