[Haskell-cafe] Re: what is inverse of mzero and return?

[Marcin 'Qrczak' Kowalczyk]

Ashley Yakeley <ashley at semantic.org> writes:

> But only some instances (such as []) satisfy this:
>
>   (mplus a b) >>= c = mplus (a >>= c) (b >>= c)
>
> Other instances (IO, Maybe) satisfy this:
>
>   mplus (return a) b = return a
>
> I think mplus should be separated into two functions.

This would prevent using mplus in a single parser which - depending on
the underlying monad used - backtracks or not.

-- 
   __("<         Marcin Kowalczyk
   \__/       qrczak at knm.org.pl
    ^^     http://qrnik.knm.org.pl/~qrczak/