[Haskell-cafe] Do real programs need IO? (was IO is a bad example for Monads)

Conal Elliott conal at conal.net
Sun Dec 9 14:26:39 EST 2007

On Dec 9, 2007 10:07 AM, Daniel Fischer <daniel.is.fischer at web.de> wrote:

> Interactive programmes without using IO? Cool :)

And how!

> I think you misunderstood Lennart.

Thanks for checking.  In this case, I think I understood Lennart fine and
that he was saying what you're saying.

> Would you deny that any useful programme has to do at least some of the
> -accept programme arguments at invocation
> -get input, be it from a keyboard, mouse, reading files, pipes...
> -output a result or state info, to the monitor, a file, a pipe...

If by "programme", you mean the code I write, then I'm happy to deny that my
programme has to do these things.  Examples below.  If you include a
stateful RTS, then no I don't deny it.

> I think Lennart was referring to that, you HAVE to know a little IO to
> programmes, at least getArgs, getLine, putStr(Ln), readFile, writeFile,
> appendFile. And therefore some use of the IO monad has to be taught
> relatively early.

Explicit imperative programming is just one way to deal with input & output,
not the only way.  As proof, see FRP, Pan, or TV programs, which contain
uses of none of these functions.  (Nor could they, as these libraries are
functional, having IO-free types and semantics.)  Moreover, use of
imperative programming sacrifices some of the semantic simplicity &
composability that makes FP so appealing.  That's why I'd like to see this
belief in its necessity dispelled.

That said, I don't think the existing functional (non-IO) approaches to
interaction are quite there yet with the flexibility of imperative
programming.  It will take more work to get them there, and that work is
mostly likely to be pursued by people who doubt the necessity of IO for
writing "real programs".  In that sense, Lennart's and your statements are
self-fulfilling prophechies, as are mine.

BTW, if you haven't seen it already, please check out
http://haskell.org/haskellwiki/TV .  The TV (tangible values) approach
includes a simple algebra of interfaces (input/output) and keeps separable
from the core computation.  The separability allows the interface parts to
be composed in parallel with the core part.  For instance, when two
function-valued TVs are composed, the interfaces are peeled off, so that the
core functions can be composed directly.  The output half of one interface
and the matching input half of the other are discarded.  The remaining input
and output halves are recombined into a new interface, which is used as the
interface of the composed TV.  The core interface algebra can be used for
text stream i/o, GUIs, and many other possible styles of information

I mention TV, because it's an example of combining the purity &
composability I love about FP with the usability a "real" app.  For more
about this combination, please see my Google tech talk "Tangible Functional
Programming: a modern marriage of usability and composability" (
That talk focus on end-user composability, but the essential points apply as
well to explicit programming.  As I mentioned before, TV (a) is currently
less flexible than imperative/IO programming, and (b) has the composability,
guaranteed safety, and amenability to reasoning of pure functional

Cheers,  - Conal

Am Sonntag, 9. Dezember 2007 18:31 schrieb Conal Elliott:
> > > 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>

> > [...]

> > IO is important because you can't write any real program without using
> > So why not teach enough of it to get people off the ground straight

> > 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.
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