[Haskell-cafe] What is MonadPlus good for?

Jacques Carette carette at mcmaster.ca
Mon Feb 14 10:07:41 EST 2005

Josef Svenningsson <josef.svenningsson at gmail.com> wrote:
> You claimed that monad transformers break the
> mzero-is-right-identity-for-bind law because they can be applied to
> IO. I say, it's not the monad transformers fault. They cannot possibly
> be expected to repair the law if they are given a faulty monad.

Doesn't that argue for allowing proven and unproven Monads in Haskell?

Using the Curry-Howard correspondance, wouldn't it make sense to provide (proof) terms in a Monad's signature that are 
witnesses for the monad laws?

One could then have signature-only Monads (ie a rather weak requirement) and signature+laws Monads (much stronger).


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