Covariance and contravariance

:: physics, maths, pointers

Physicists seem still to be taught about tensors as being, essentially, multidimensional arrays with special transformation rules which must be learned by rote. So I wrote a document which tries to present a more useful approach.

Road worn

:: music, photography

I play the guitar. Something that has been fashionable for some time is what are often called ‘road worn’ guitars. In other words new (but vintage-spec) guitars which have been aggressed in various ways to make them look old.

Macros in Racket, part one

:: computer, lisp

I’ve written in Lisp for a long time, but I’ve never used a hygienic macro system in any way other than the most simple. Here are some initial notes on my experiences learning Racket’s macro system.