On Dec 9, 2007 10:16 PM, Conal Elliott <conal@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@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@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@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@gmail.com> wrote:
On Dec 3, 2007 6:36 PM, Ben Franksen < ben.franksen@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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