Hackage.Security.Client.Verify

Verification monad

The verification monad is similar to ResourceT in intent, in that we can register handlers to be run to release resources. Unlike ResourceT, however, we maintain _two_ handlers: a cleanup handler which is run whether or not verification succeeds, and a finalisation handler which is run only if verification succeeds.

• Cleanup handlers are registered using acquire, and are guaranteed to run just before the computation terminates (after the finalisation handler).

• The finalisation handlers are run only when verification succeeds, and can be registered with ifVerified. Finalisation can be used for instance to update the local cache (which should only happen if verification is successful).

Run an action in the Verify monad

Acquire a resource and register the corresponding cleanup handler

NOTE: Resource acquisition happens with exceptions masked. If it is important that the resource acquistion can be timed out (or receive other kinds of asynchronous exceptions), you will need to use an interruptible operation. See http://www.well-typed.com/blog/2014/08/asynchronous-exceptions/ for details.

Register an action to be run only if verification succeeds

Create a short-lived temporary file

Creates the directory where the temp file should live if it does not exist.

Lift a computation from the IO monad. This allows us to run IO computations in any monadic stack, so long as it supports these kinds of operations (i.e. IO is the base monad for the stack).

Example

import Control.Monad.Trans.State -- from the "transformers" library

printState :: Show s => StateT s IO ()
printState = do
  state <- get
  liftIO $ print state

Had we omitted liftIO, we would have ended up with this error:

• Couldn't match type ‘IO’ with ‘StateT s IO’
 Expected type: StateT s IO ()
   Actual type: IO ()

The important part here is the mismatch between StateT s IO () and IO ().

Luckily, we know of a function that takes an IO a and returns an (m a): liftIO, enabling us to run the program and see the expected results:

> evalStateT printState "hello"
"hello"

> evalStateT printState 3
3