A RetryPolicy is a function that takes an iteration number and possibly returns a delay in microseconds. *Nothing* implies we have reached the retry limit.

Please note that RetryPolicy is a Monoid. You can collapse multiple strategies into one using mappend or <>. The semantics of this combination are as follows:

1. If either policy returns Nothing, the combined policy returns Nothing. This can be used to inhibit after a number of retries, for example.
2. If both policies return a delay, the larger delay will be used. This is quite natural when combining multiple policies to achieve a certain effect.

Example:

One can easily define an exponential backoff policy with a limited number of retries:

> limitedBackoff = exponentialBackoff 50 <> limitRetries 5

Naturally, mempty will retry immediately (delay 0) for an unlimited number of retries, forming the identity for the Monoid.

The default under def implements a constant 50ms delay, up to 5 times:

> def = constantDelay 50000 <> limitRetries 5

For anything more complex, just define your own RetryPolicy:

> myPolicy = RetryPolicy $ \ n -> if n > 10 then Just 1000 else Just 10000

Retry combinator for actions that don't raise exceptions, but signal in their type the outcome has failed. Examples are the Maybe, Either and EitherT monads.

Let's write a function that always fails and watch this combinator retry it 5 additional times following the initial run:

>>> import Data.Maybe
>>> let f = putStrLn "Running action" >> return Nothing
>>> retrying def (const $ return . isNothing) f
Running action
Running action
Running action
Running action
Running action
Running action
Nothing

Note how the latest failing result is returned after all retries have been exhausted.

Run an action and recover from a raised exception by potentially retrying the action a number of times.

Retry ALL exceptions that may be raised. To be used with caution; this matches the exception on SomeException.

See how the action below is run once and retried 5 more times before finally failing for good:

>>> let f = putStrLn "Running action" >> error "this is an error"
>>> recoverAll def f
Running action
Running action
Running action
Running action
Running action
Running action
*** Exception: this is an error

Helper function for constructing handler functions of the form required by recovering.

Implement a constant delay with unlimited retries.

Grow delay exponentially each iteration.

Implement Fibonacci backoff.

Retry immediately, but only up to n times.

Add an upperbound to a policy such that once the given time-delay amount has been reached or exceeded, the policy will stop retrying and fail.

Set a time-upperbound for any delays that may be directed by the given policy. This function does not terminate the retrying. The policy `capDelay maxDelay (exponentialBackoff n)` will never stop retrying. It will reach a state where it retries forever with a delay of maxDelay between each one. To get termination you need to use one of the limitRetries function variants.

An infix synonym for mappend.

Since: 4.5.0.0