Parsec Patterns

At a meeting of the Toronto Haskell User Group a few months back (Jan 9, 2013), after going over the basics of the combinatoric parsing library Parsec, I talked about some clever tricks that made my work with it much cleaner. (See also Alber Lai’s write up of his talk, Parsec Generally.)

Note that I don’t have deep knowledge of Parsec. I’ve just used it for a few things and noticed that some code patterns really cleaned up my code. I’d love to hear about other people’s tricks for writing awesome parsers with Parsec!

(Read More On Github)

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Monads For The Terrified

It seems like everyone writes a monad tutorial in the Haskell community… Well, here’s mine.

People learning Haskell generally seem to get tripped up about monads. There are good reasons for this.

1. Monads are, really, needed to write a serious Haskell program. As a consequence, people generally try to learn them fairly shortly after beginning to use Haskell.
2. Monads are defined in Haskell and have a very short definition. As such, it is very tempting to not just teach someone how to use monads, but to also try to teach them the underlying definition. The definition involves a bunch of rather advanced Haskell. In combination with (1), this is a big problem.
3. Monads are a very abstract idea. I’m not really sure that the motivation can be properly grasped by anything other than using them.
4. Monads are generally interweaved with the idea of an IO object.

This introduction attempts to avoid some of these problems. In particular, it is only going to teach you how to use monads and not what they are. This avoids (2) and, to a lesser extent, (3). I’ve tested this approach on a number of people in the last few months, and it seems to be quite effective. That said, I understand that some people may prefer an introduction focusing on the what and why, in which case you may wish to look Chris Smith’s Why Do Monad’s Matter?.

Some day, you will need to learn more. But, if you find you understand the contents of this tutorial, I’d encourage you to wait and play around with monads for a while. Then, when the time comes to learn the in full, they’ll be natural and obvious ideas. We can talk more about that later. For now: onwards!

(Read more on github)

HaskSymb: An Experiment in Haskell Symbolic Algebra

One interesting detour I took this last month was writing a toy Haskell Symbolic Algebra library, HaskSymb. It takes advantage of Quasi-Quoters and View Patterns to do awesome math pattern matching and enable one to write beautiful code.

An example of its use:

> -- Let's make some variables!
> let (a,b)  = (V "a", V "b")
> -- Basic Expression manipulation
> (a+1)^3
(a + 1)³
> expand $ (a+1)^3
a³ + a² + a² + a² + a + a + a + 1
> collectTerms $ expand $ (a+1)^3
a³ + 3a² + 3a + 1

The interesting part, however, is the code for expand, collectTerms and friends. It’s extremely pretty. For example:

expand [m| a+b |] = expand a + expand b
expand [m|a*(b+c)|] = expand (a*b) + expand (a*c)
expand a = a

Read more on github. You can also find some discussion on the Haskell reddit.

ImplicitCAD 0.0.1 Release

I’m pleased to announce the second release of ImplicitCAD: 0.0.1. (The first release was 0.0.0 because 0 is the true first ordinal.)

The point of this release is somewhat arbitrarily chosen. We were over due for one and no clean break was in sight. Then I woke up with a nasty headache and couldn’t seem to code, so I thought I’d do a release instead.

I’m going to try and martial together my thoughts and discuss changes in this release and what’s coming up. The TL;DR is that ImplicitCAD is going exciting places and if you are willing to tolerate bugs and file bug reports, you should become a beta user for it.