@hackage / refined-containers

This package defines ways to prove that a key exists in an associative container such as a Map, IntMap, or HashMap; so that the key can be used to index into the map without a Maybe or manually handling the "impossible" case with error or other partial functions.

To do this, the containers are tagged with a type parameter that identifies their set of keys, so that if you have another container with the same parameter, you know it has the same keys.

There is also a type of keys that have been proven to exist in such containers -- a refinement type. They are also tagged with a type parameter. If the type parameter of the key matches that of the container, indexing is guaranteed to proceed without failure.