Testable monad and mtl laws

A library for testing implementations of mtl classes.

test-monad-laws defines laws for monadic effects as QuickCheck properties.

To use this library, quickcheck-higherorder is also needed.

Supported classes:

• mtl: MonadExcept, MonadReader, MonadState, MonadWriter
• transformers: MonadTrans
• transformers-base: MonadBase
• monad-control: MonadTransControl, MonadBaseControl

This project also tests the effectiveness of these laws, by including some incorrect implementations, called mutants, and some invalid laws.

Organization

For every mtl class, for example MonadReader:

• The most important module is Test.Monad.Reader, defining laws for the class. Note that these laws are not official. But if your instance does not satisfy them, now you know.
• Mutants and bad laws are in Test.Monad.Reader.Mutants.
• For convenience, all the good laws are gathered in a single list in Test.Monad.Reader.Checkers. It can easily be consumed by the library tasty-quickcheck.

Related links and references

• validity
• checkers
• quickcheck-classes
• hedgehog-classes
• ClassLaws
• test-invariant

Papers with some relevant laws:

• Just do it: Simple Monadic Equational Reasoning. Jeremy Gibbons, Ralf Hinze.
• Proof abstraction for imperative languages. William L. Harrison.

Hackage search terms: laws, properties.