What I Remember

(Note: Twitter links have screenshots next to them, as it’s notoriously difficult to predict what Twitter shows on the other end of a link.)

Update History:

• 2020-05-14: Added links to archive.org, showing that the CDC website used to say “don’t use masks unless sick”
• 2020-05-05: Added link to WHO saying “yes, it’s worse than the flu”
• 2020-04-26: Initial Release

What do I remember about the coronavirus pandemic?

I remember watching case numbers double and redouble throughout late February and early March, thinking “oh god, we’re only a few doublings behind Italy, and our leaders are going to drop the ball”.

The Power of Tiny DSLs

I was playing around with codeworld-api recently, and found myself with a pair of interesting problems:

• Place (move and scale) a grid (represented as a CodeWorld Picture) onto the canvas; and
• When responding to mouse clicks on the grid, reverse the transformation that placed the grid, so that I can determine which grid cell was clicked.

Placing the picture is easy — codeworld-api provides two functions that do exactly what I want:

• translated :: Double -> Double -> Picture -> Picture
• scaled :: Double -> Double -> Picture -> Picture

Going the other way is more difficult. After noodling around on paper trying to compute the inverse transform, I remembered that these transformations could be represented as 3x3 matrices (Wikipedia has some examples), and that inverting a 3x3 matrix is easy (provided that the affine transformation it represents hasn’t collapsed the space).

This means I have to compute the transformation twice: once as codeworld-api calls, and once as matrices. Or do I?