This module is used to mitigate several pitfalls with the capnproto format, which could potentially lead to denial of service vulnerabilities.

In particular, while they are illegal according to the spec, it is possible to encode objects which have many pointers pointing the same place, or even cycles. A naive traversal therefore could involve quite a lot of computation for a message that is very small on the wire.

Accordingly, most implementations of the format keep track of how many bytes of a message have been accessed, and start signaling errors after a certain value (the "traversal limit") has been reached. The Haskell implementation is no exception; this module implements that logic. We provide a monad transformer and mtl-style type class to track the limit; reading from the message happens inside of this monad.