[Haskell-cafe] Re: Control.Exception.evaluate -
'correct definition' not so correct
Jules Bean
jules at jellybean.co.uk
Sat May 3 07:21:40 EDT 2008
apfelmus 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.
But it is known that the monad laws only apply up to some weaker
equivalence than 'seq-equivalence'.
This has been discussed here countless times by people who understand it
better than me.
As I understand the summary the "=" sign in the monad laws mean
"represent identical actions, in terms of the effects produced and the
result returned". A kind of observational-equivalence for monad
execution, but weaker than direct equational equivalence in the presence
of seq.
(Some people view this as more of a bug in "seq" than in the monad laws)
Jules
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