[Haskell-cafe] IO is not a monad
gale at sefer.org
Wed Feb 7 06:59:15 EST 2007
Aaron McDaid wrote:
> Apologies for referring to this old thread...
At least for me, this is not old. It is still very
much on my mind.
> Simply, 'undefined >> undefined' is a bit more defined than simply
> 'undefined'. Just like 'undefined:undefined' is at least a non-empty
> list; which can be matched by (_:_) for example. This explains the
> differing behaviour of the two seemingly equivalent actions above.
I think you are correct. And that is the problem.
As a simple Haskell programmer, I am starting from
the fact that seq exists and works in a certain
Given that reality, I want to look at what monads
should look like and what strictness properties they
Well, even with seq around, Haskell types and
functions still form a nice category - except you
have to be careful to use strict composition
instead of regular Haskell composition when
viewing Haskell functions as morphisms in
So then what is a monad? There is
one obvious extra requirement:
Monad Strictness Law:
(>>=) must be strict in its second argument.
That law follows from the monad law:
(>>= f) .! return = f
when we take f = _|_ (and we make the
assumption that our monad is not
degenerate - return x /= _|_ for some x).
Making (>>=) strict in the second argument
would be a very minor change. That
change would affect few, if any, existing
I am becoming convinced that this should be
considered a bug in IO.
Unfortunately, I don't think that fixing this
bug will help much with other known monad
strictness problems, such as the State/StateT
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