[Haskell-cafe] Re: Control.Exception.evaluate - 'correct definition' not so correct

[Iavor Diatchki]

Hello,

On Sat, May 3, 2008 at 3:56 AM, apfelmus <apfelmus at quantentunnel.de> wrote:
> Bryan Donlan wrote:
>
> >
> >           evaluate x = (return $! x) >>= return
> >
> > However, if >>= is strict on its first argument, then this definition is
> > no better than (return $! x).
> >
>
>  According to the monad law
>
>   f >>= return = f
>
>  every (>>=) ought to be strict in its first argument, so it indeed seems
> that the implementation given in the documentation is wrong.

>From the monad law we can conclude only that "(>>= return)" is strict,
not (>>=) in general.
For example, (>>=) for the reader monad is not strict in its first argument:

m >>= f = \r -> f (m r) r

So, "(undefined >> return 2) = (return 2)"

-Iavor