[Haskell-cafe] IO is not a monad

[Brian Hulley]

Brian Hulley wrote:
> 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.

Ooops! But that does not mean the equation holds because for example

    Prelude> (return 3 >>= f) `seq` 42
    42
    Prelude> (f 3) `seq` 42
    *** Exception: Prelude.undefined

In the lhs you only hit _|_ when the composite (>>=) action is actually 
being executed whereas in the rhs you hit _|_ when computing the function 
which will return the action to execute so there is difference.

Brian.
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