Friday, January 15, 2010

When types and definitions aren't enough.

Was in #haskell on freenode a bit this morning, and someone mentioned something about how they were not exactly excited about the new rules for code formatting on if-then-else expressions.

I mentioned that I try to avoid if-then-else and case as much as possible by using types like Maybe that have 2 kinds of constructors, namely Nothing and "Just a" (for Maybe a).

I said that I can use the MonadPlus instance for Maybe a to get a lot of what is available in if-then-else clauses.

let x = someExpression
in if x == Nothing
then 9
else fromJust x

could be written as

let x = someExpression
in fromJust $ x `mplus` Just 9

mplus is defined for Maybe as evaluating the first parameter, and if it is not Nothing, it returns it, otherwise it will return the second parameter. It's essentially an "or" operator.

However, someone pointed out that there's absolutely no requirement for mplus to be written this way. It can still live up to all the rules and restrictions of MonadPlus by short-circuiting evaluation on the second argument instead of the first. Sure, it's sort of a de-facto first then second sequencing of evaluation, but it is not as safe as say "if-then-else".

I wonder now about the Applicative module as well, and specifically Alternative for the Maybe class.

I could just as easily write

let x = someExpression
in fromJust $ x <|> Just 9

But do we fall into the same trap of no guarantees? Is there a rule in Applicative enforcing the short-circuit of the first argument before the second?

Much code is written in the Applicative style for Parsec, so I really hope this is well defined.

Monday, January 11, 2010

What's Missing in the Haskell Community?

Documentation is often sighted as possibly the #1 item that needs to be improved with respect to Haskell. It depends on what modules we use, but I have to agree. It's quite difficult to uphold the claim that you don't need to understand Category Theory in order to employ a Monoid or Monad, but when you run into Monoid instances like "endo" and don't know what to make of it because the documents don't really describe how to use it, you're probably going to suffer in that that implementation of Monoid is likely useful for some kind of programming that you want to do, and you'll end up struggling with a solved problem. Some people have been stepping up to improve the documentation, and that's really wonderful, but I think there's still some work to be done there.

The best way to learn Haskell in general for me has been to get the great books that are available out there. Real World Haskell is freely available online. (But please support the authors and get a copy if you're finding it useful). Search for Haskell on and you'll find that the reviews are a really good guide to picking which ones might be right for you. If you're really new to the language. Dr. Graham Hutton's book is outstanding. There's even been a series on MSDN's channel 9 walking through the chapters of this book, explaining how to solve some problems and think like a functional programmer.

To keep up to date with Haskell developments, reddit has been invaluable. You'll find blog posts, updates about new Haskell packages, and general community news and related discussion topics there.

So what's still missing?

I can tell you that over the years I've been messing around with Haskell, trying to understand how it works, why it's appropriate for certain kinds of problem solving, and to really get a good appreciation of why people seem to really like it so much, that I feel like the community has been pretty amazing with respect to fueling the flames of curiosity.

Where I think we might be needing a little more help is in the following areas of Haskell.

Explaining where laziness or non-strict evaluation is an advantage over strict evaluation. Perhaps this requires learning to think about the code we write differently, in much the same way it can be a leap to get to recursive programming, I feel this might be a slightly wider gap to cross mentally. (But then again maybe I'm just getting old... )

Show more examples of unintended data-growth or space leakage due to the lack of strict evaluation. In languages like C, you're in direct control of when memory is allocated or deallocated. This is generally considered a "bad thing" for a lot of tasks, including systems programming if you are signed up with the Go camp. A side effect of non-strict by default seems to be that you have to understand how the code you're writing will be evaluated from a bit of a wider view than you might need to care about malloc and free, or new or delete. It seems that unless you've somehow been taught how to recognize the patterns that could cause a a space leak, you're basically doomed to run into some sharp corners that others already seem to understand how to avoid.

Real World Haskell has a great chapter on optimization, but perhaps it's time for an "Optimizing Haskell" book too? There's lots of good advice scattered all over the web, and the experts are not shy to offer you help should you ask. Sometimes I think it's difficult to even ask the right questions when you're confused though, and I suspect this may turn some folks off to Haskell.

Wednesday, January 6, 2010

L4 and Plan 9 or Inferno or both?

Some folks already started in on a Plan 9 port to L4 it seems (PDF), but I'm not sure how far they've gotten with it. I've been sort of peeping at L4 on and off for a long time now, and OKL4 has been successfully shipped in a few ARM phones, successfully being used to host Linux and a Qualcomm OS that drives the phone's radios and such. Neat stuff.

Been wondering if Plan 9 or Inferno and L4 are really a good marriage and what benefits could be added by leveraging Plan 9's namespace based resource management and L4's powerful IPC mechanisms.

As I've had a little more spare time lately, I've been digging around looking into L4 again, and I'm interested in exploring some ideas a little more deeply.

There's a few different implementations of L4 to look at...

OKL4 is now basically an ARM only platform in it's latest releases. Fiasco is a Pentium targeted L4 implementation that has a userland implementation that might be fun to work with (Fiasco-UX), and Pistachio is still being worked by at least a few people, with the latest changes coming in as of yesterday. Pistachio also supports a lot of architectures.

I'm tempted to play with each of these, but my problem has always been one of focus when it comes to these spare time projects as free time for me is usually at a premium.

Tim Newsham has been following the seL4 stuff, and OKL4 is migrating towards those APIs. His vote is I shoot for OKL4, so I believe that's where I'm going to start. There's a good community around that implementation, and a commercial pressure to keep things working nicely.

But as they say, talk is cheap... Let's see what I learn.


Looks like a lot of progress was made by others who have already started this work!