[Haskell-cafe] IO is not a monad

[Brian Hulley]

Brian Hulley wrote:
> Yitzchak Gale wrote:
>> I wrote:
>>>> Prelude> let f = undefined :: Int -> IO Int
>>>> Prelude> f `seq` 42
>>>> *** Exception: Prelude.undefined
>>>> Prelude> ((>>= f) . return) `seq` 42
>>>> 42
>> The monad laws say that (>>= f) . return must be
>> identical to f.
>
> I thought it was:
>
>    return x >>= f = f x
>
> so here the lhs is saturated, so will hit _|_ when the action is
> executed just as the rhs will.
> I think the problem you're encountering is just that the above law
> doesn't imply:
>
>    (>>= f) . return = f
>
> in Haskell
ok so far...

> because the lhs is of the form \x -> _|_

No I got this wrong. Evaluating the lhs to WHNF doesn't hit the _|_. 
(Incidentally the version using .! evaluates to exactly the same thing since 
(>>= f) `seq` ((>>= f) . return) = ((>>= f) . return) since (\x -> x >>= f) 
is already in WHNF afaiu)

Brian.
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