[Haskell-cafe] Re: A question about "monad laws"

Dan Piponi wrote:

IOn Feb 11, 2008 9:46 AM, Miguel Mitrofanov wrote:

...

It's well known that "ListT m" monad violates this law in general (though it satisfies it for some particular monads m). For example,

I went through this example in quite a bit of detail a while ago and wrote it up here: http://sigfpe.blogspot.com/2006/11/why-isnt-listt-monad.html . I tried to show not just why the monad laws fails to hold for ListT [], but also show how it almost holds.

...and the Unimo paper[1] explains how to easily write a 'correct' ListT. BTW, Unimo is an extreme case of the monad laws holding only w.r.t. the 'right' equality, i.e. in the laws m == m' is to be understood as observe_monad m == observe_monad m' (and even this '==' is of course /not/ the Eq class method but a semantic equality.) [1] http://web.cecs.pdx.edu/~cklin/papers/unimo-143.pdf Cheers Ben