[Haskell-cafe] IO is a bad example for Monads [was: do]

[Lennart Augustsson]

I doubt all imperative programming will be banished from Haskell anytime
soon.  I really, really wish we had all the nice abstractions in place
already, but we just don't.

You can't write any program in Haskell without using IO, because the type of
main involves IO.
And currently I believe that almost any real program will have to involve
IO.
(BTW, the only H98 IO avoiding wrapper, interact, was included in Haskell
because I insisted on it.)
It's just from my experience.  No matter how pure your program is, here and
there it will be interacting with the rest of the world.

  -- Lennart

On Dec 9, 2007 10:16 PM, Conal Elliott <conal at conal.net> wrote:

> Thanks.  If I'm tracking, your real point is that imperative programming
> in Haskell is still useful enough to keep around.  I agree.
>
> I'm still puzzled.  Did you understand something I said, or maybe someone
> else said, as suggesting that imperative programming be removed from Haskell
> any time soon?
>
> > It's also important to teach people to stay away from IO whenever
> possible, but it's simply not always possible.
>
> How can we possibly teach them to stay away from IO where possible if
> we're also telling them that they can't write *any* real program without
> using IO?
>
> Cheers, - Conal
>
>
> On Dec 9, 2007 12:02 PM, Lennart Augustsson <lennart at augustsson.net >
> wrote:
>
> > Conal,
> >
> > It's true that you can avoid using IO (except for a wrapper) for certain
> > kinds of programs.
> > For instance, if all you want is a String->String function, or some GUI
> > program (you forgot to mention fudgets, which was the first wrapper of this
> > kind) then you can ignore IO and just use a nice wrapper.
> >
> > But if someone asks me how to traverse a directory tree, invoking the
> > 'file' program for each ',o' file and then renaming it if it's a text file,
> > then what should I answer?  "Sorry, you can't do that in Haskell."  or "You
> > need to use the IO monad."?
> > I prefer the latter answer, and I think people who learn Haskell need to
> > learn something about how you do some of the things that are easy in other
> > languages.
> >
> > It's also important to teach people to stay away from IO whenever
> > possible, but it's simply not always possible.
> >
> >   -- Lennart
> >
> >
> > On Dec 9, 2007 5:31 PM, Conal Elliott <conal at conal.net > wrote:
> >
> > > > IO is important because you can't write any real program without
> > > using it.
> > >
> > > Ouch!  I get awfully discouraged when I read statements like this
> > > one.  The more people who believe it, the more true it becomes.  If you want
> > > to do functional programming, instead of imperative programming in a
> > > functional language, you can.  For instance, write real, interactive
> > > programs in FRP, phooey, or TV.  And if you do, you'll get semantic
> > > simplicity, powerful & simpler reasoning, safety and composability.
> > >
> > >   - Conal
> > >
> > > On Dec 8, 2007 1:26 AM, Lennart Augustsson <lennart at augustsson.net >
> > > wrote:
> > >
> > > > I agree with Dan here.
> > > >
> > > > IO is important because you can't write any real program without
> > > > using it.
> > > > So why not teach enough of it to get people off the ground straight
> > > > away?
> > > >
> > > > People who hang around long enough to do some more Haskell
> > > > programming
> > > > will run into the other monads sooner or later.  But IO is an
> > > > unavoidable step to
> > > > writing Haskell programs.
> > > >
> > > >
> > > >
> > > >
> > > > On Dec 4, 2007 5:11 AM, Dan Piponi < dpiponi at gmail.com> wrote:
> > > >
> > > > > On Dec 3, 2007 6:36 PM, Ben Franksen < ben.franksen at online.de>
> > > > > wrote:
> > > > > > then the special features of IO
> > > > > > will remain associated with monads in general, leading to a
> > > > > whole jumble of
> > > > > > completely wrong ideas about them.
> > > > >
> > > > > As I only learnt about monads a couple of years ago, the process
> > > > > is
> > > > > still fresh in my mind. I wasted quite a bit of time labouring
> > > > > under
> > > > > the impression that monads were primarily about sequencing. But
> > > > > that
> > > > > wasn't because I incorrectly generalised from IO. It was because
> > > > > countless people out there explicitly said they were about
> > > > > sequencing.
> > > > > I suspect that if courses started with the List monad there'd be
> > > > > countless blogs telling people that monads are a way to eliminate
> > > > > loops from your code like the way list comprehensions are used in
> > > > > Python.
> > > > >
> > > > > > This is yet another problem with IO as the standard example for
> > > > > monads: its
> > > > > > effect base is huge and poorly structured.
> > > > >
> > > > > You don't teach *all* of IO to students in one go!
> > > > >
> > > > > > This again makes it difficult to
> > > > > > see exactly which intuitions about IO can be generalized to
> > > > > arbitrary
> > > > > > monads and which not.
> > > > >
> > > > > That's true of any monad. IO is unique. [] is unique. Cont is
> > > > > unique.
> > > > > All of them can lead you down the garden path. You need to see
> > > > > multiple monads, and it helps if you can sneak an example under a
> > > > > student's nose so they can already reason about monads before they
> > > > > even know what a monad is.
> > > > >
> > > > > > What is pointless about failure and how to handle it?
> > > > >
> > > > > It's pointless when you're still trying to make your first tweaks
> > > > > to
> > > > > "Hello, World!" work.
> > > > > --
> > > > > Dan
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> > > >
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> > >
> >
>
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