[Haskell-cafe] What is MonadPlus good for?

ajb at spamcop.net ajb at spamcop.net
Sun Feb 13 19:08:26 EST 2005

G'day all.

Quoting Josef Svenningsson <josef.svenningsson at gmail.com>:

> I think it's unfair to the monad transformers to simply say that they
> don't obey the law. The interesting thing is whether they *preserve*
> the law. A monad transformer T preserves a law if given a monad M
> which obeys the law holds then the monad T M obeys the law.

The law in question is that mzero is a right-zero for bind.  How can an
underlying monad be said to "obey" this law if it doesn't support mzero?

Andrew Bromage

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