# Learning Clojure

## Friday, January 29, 2010

### How the Clojure dojo actually worked in practice.

We decided that I'd try out (the first half of) my Clojure dojo on the Chemistry Department.

The audience was the non-lispers in our group, so there were about eight very bright Java guys, one of whom had done some ML before, and one of whom had been through this very exercise with me in a pub some months before as his first contact with LISP since he met McCarthy in the sixties(!).

The dojo format worked very well, and I mainly sat back and let them get on with it.

I didn't say a great deal after posing the problem in the terms outlined at the start. When they completed a subproblem I gave them a new problem and maybe a hint or two. I was surprised how much they managed to infer from limited information.

At first everything went absolutely swimmingly, and the syntax didn't faze them at all. Questions like 'What's the sum of the squares of 3 and 4' were answered without breaking stride. They understood Heron's method immediately. When I suggested that they turned it into a function to improve guesses, they spontaneously put in a target variable instead of hard-coding 10, and started talking about recursion.

They couldn't figure out how to do it, so I suggested trying to write factorial. This they managed to work out for themselves.

There was then much excitement trying to turn that into a solver, but they couldn't do it.

I suggested trying to write good-enough?

They realised the need for an absolute value function, and to my utter amazement, guessed the syntax for calling Math/abs, and then for Math/sqrt!

They declared the problem solved.

So I asked them to generalize the solution to make a general root-finder, which got them back to trying to get Heron's method to work.

This was the last time I said anything helpful. Much more thinking, and they got on the right track. They defined an okguess function which took the tolerance as a parameter, and were well on the way to solving the problem, when the ML guy and the guy who'd done it before took the chairs.

I said they were only allowed a couple of minutes, but they had the thing done almost immediately.

I still wanted to go back and show them iterate and map, so I asked them to make a function that returned a function. Can we make a function that takes a number and returns a function which multiplies things by it?

I hinted that defn was a combination of def and fn, and that was enough

That solved, I asked them to make a function which took a target and gave back the relevant improver.

Then I told them to try (doc iterate), and see if they could figure out what it was for. And then I told them to look up map and take. They were playing happily with infinite sequences when our hour and a half ran out.