[Haskell-cafe] Re: Control.Exception.evaluate - 'correct
definition' not so correct
Iavor Diatchki
iavor.diatchki at gmail.com
Sun May 4 20:30:37 EDT 2008
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
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