[Haskell-cafe] Wow Monads!

Sergiu Ivanov sivanov at colimite.fr
Wed Apr 19 15:56:19 UTC 2017

Thus quoth  Joachim Durchholz  at 13:58 on Wed, Apr 19 2017:
> Am 19.04.2017 um 15:13 schrieb Sergiu Ivanov:
>> However, there are a couple big "backdoors", like the IO monad.
> Well, it's so awkward that people don't *want* to use it.

Interesting point of view, I've never thought of the relative
awkwardness of IO as of an educational measure.

>> Typechecking gives zero guarantees for functions of type IO (), for
>> example.
> That doesn't match what I hear from elsewhere.
> I don't know what is the cause for that difference though.

I'm probably shouting my "zero" too loudly.

I wanted to say that, when the typechecker sees a function of type
IO (), it only knows that it _may_ have side effects, but it cannot
verify that these effects compose correctly with the effects coming from
other functions in the IO monad.

Of course, the return type IO () still gives a lot of information (e.g.,
it says the function is only useful for its side effects) and the type
system will ensure that any computation actually using this function
must be declared as (potentially) having side effects.  Yet, you have no
guarantees that such effects are composed correctly.

>> In fact, I believe having pure functions does not so much target
>> removing state as it does making the state _explicit_.
> Except State tends to make state implicit again, except for the fact
> that there *is* state (which might be actually enough, I don't have
> enough insight for any judgemental statements on the issue).

Well, a typical definition of State is parameterised in the type of the
state, so you know what it is.

Sure, a typical definition of State does not let you know whether and
how the state was modified, but if one wants that information, one can
"just" define a custom "StateLog" monad, for example, or even use
Applicative to statically "record" the effects on the state (I hear
that's one use of Applicative).

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