I also blog frequently on the Yesod Web Framework blog, as well as the FP Complete blog.

Kids Coding, Part 3

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Eliezer asked me a few questions about programming yesterday, and I ended up demonstrating a little bit about pattern matching. He then had a lot of fun showing a friend of his how programming works. I may end up giving some lessons to some of my kids’ friends in the next few weeks, which should be interesting.

Kids IDE

I’ve started using a minimalistic “kids IDE” I threw together for teaching the kids. It’s helped us not worry about details like filepaths, saving, and running in a terminal. It’s not a great tool, but good enough. I’ve included links to better tools in the README, though this fits my needs for the moment.


I’ve set up AppVeyor to upload Windows executables to S3:


You’ll also need to install Stack.

Pattern matching strings

This morning, Eliezer and Gavriella both had their next “official” lesson. I started over with that pattern matching demonstration. First, I showed them this example:

main = do
  sayNickname "Eliezer"
  sayNickname "Gavriella"
  sayNickname "Yakov"
  sayNickname "Lavi"

sayNickname realname =
  putStrLn (nickname realname)

nickname "Eliezer" = "Elie Belly"
nickname "Gavriella" = "Galla Galla"
nickname "Lavi" = "Fat baby"
nickname "Yakov" = "Koko"

They got the basic idea of this. (And I ended up introducing putStrLn around here as well, which they were fine with.) However, as I had them typing out some of this on their own, they ended up with a lot of trouble around capitalization, which was a good introduction to Haskell’s rules around that. We’ll see how in a bit.

Pattern matching ints

After establishing that we could pattern match on strings, I switched the code to this:

main = do
  sayNickname 1
  sayNickname 2
  sayNickname 3
  sayNickname 4

sayNickname realname =
  putStrLn (nickname realname)

nickname 1 = "Elie Belly"
nickname "Gavriella" = "Galla Galla"
nickname "Lavi" = "Fat baby"
nickname "Yakov" = "Koko"

And gave them the challenge to rewrite nickname so that the code worked, which wasn’t too much of a problem. The misordering of Lavi and Yakov between main and nickname did cause some confusion, and then helped them understand better how pattern matching works. (I didn’t intentionally put that in, it somehow slipped in while the kids were working on rewriting things.)

Type signatures

I asked them what the type of nickname was, and they said function (yay!). And then explained to them that you can tell Haskell explicitly what a thing’s type is, and typed this in:

nickname :: ? -> ?

The funny syntax didn’t give them too much trouble, and then we got to fill in the question marks. I asked them what the output of the function was, pointing at the string. I was surprised: they said the type was “nickname” or “name.” They accepted that it was a string, but they didn’t like that. (New theory: humans’ brains are naturally strongly typed.)

I then asked what the input was, and they said “number.” I hadn’t taught them about Int yet, and didn’t know about integers from math, so I told them that integer is a kind of number, and that in Haskell we call it Int. Filling in the type signature was fine.

I pointed out that some things (like Int and String) were upper case, and some were lower case. I pointed out that concrete things that “actually exist” (maybe not the best terminology) are capitalized. We know what an Int is, for example. Variables are things we don’t know yet, and those are lowercase. And finally, you can put whatever you want inside a string, but it has to match exactly. That seemed to click fairly well for them.


I pointed out that referring to the kids as numbers isn’t good. (He responded that I actually do call them 1, 2, 4, and 8 sometimes…) Anyway, I said that a better type for the nickname function would be to take a Child as input. He said “can we say Child = Int,” which was a great insight. I showed up that, yes, we can do type Child = Int, but that there’s a better way.

I introduced the idea that data defines a new datatype, and then showed them:

data Child
  = Eliezer
  | Gavriella
  | Yakov
  | Lavi

Gavriella asked “what are those lines?” and I explained they mean “or.” Therefore, a child is Eliezer, or Gavriella, or Yakov, or Lavi. They got this.

Exercise: fix the following program, uncommenting the lines in main.

main = do
  sayNickname Eliezer
  --sayNickname Gavriella
  --sayNickname Yakov
  --sayNickname Lavi

sayNickname realname =
  putStrLn (nickname realname)

data Child
  = Eliezer
  | Gavriella
  | Yakov
  | Lavi

nickname :: Child -> String
nickname Eliezer = "Elie Belly"
--nickname 2 = "Galla Galla"
--nickname 3 = "Fat baby"
--nickname 4 = "Koko"

Some caveats where they got stuck:

  • Using lower case lavi instead of Lavi. I got to explain how pattern matching a variable works, and “wildcards.” They got this, though still needed to be coached on it.
  • I’d been on the fence about including syntax highlighting in the kids IDE, but it turns out that the different colors of Eliezer and lavi helped Gavriella realize her mistake. Syntax highlighting is a good thing here.
  • I did ultimately have to give her a hint: “Lavi is BIG but you made him small.”

I started to try and define data Person = Kid Child | Adult Parent, but realized quickly it was too complicated for now and aborted.


I did not teach this well, the children did not get the concepts correctly. However, they did understand the code as I wrote it.

main = print total

total = addUp 10

addUp 10 = 10 + addUp 9
addUp 9 = 9 + addUp 8
addUp 8 = 8 + addUp 7
addUp 7 = 7 + addUp 6
addUp 6 = 6 + addUp 5
addUp 5 = 5 + addUp 4
addUp 4 = 4 + addUp 3
addUp 3 = 3 + addUp 2
addUp 2 = 2 + addUp 1
addUp 1 = 1 + addUp 0
addUp 0 = 0

They really hated how repetitive typing that out was, which was exactly the goal. It was easy to convince them that this was stupid. I then changed total to addUp 11 and it broke. They understood why, and so we started working on something better.

main = print total

total = addUp 10

addUp 0 = 0
addUp x = x + addUp (x - 1)

I stepped through the executable of this, complete with pattern matching. Doing this a few times in the future is probably a good idea.

Eliezer asked what happens if we remove the addUp 0 = 0 case. We discussed it, he said it wouldn’t work, and said it would “keep going.” I told him it was called an infinite loop, and we got a stack overflow. Good insight.

Gavriella asked how long it took me to learn this stuff. I told her 12 years; after all, I only started learning Haskell in my twenties. It made them feel pretty good that they were learning this stuff earlier than I did.

I gave them an exercise to implement multTo instead of addUp. They didn’t understand this, or recursion, and I had to help them through the whole thing. Mea culpa completely.

Gavriella asked for another exercise:

main = do
  sayAge Eliezer
  sayAge Gavriella
  sayAge Yakov
  sayAge Lavi

data Child = ...

sayAge child = print (age child)

age ? = ?

She typed something like:

age ? = ?

I showed her that she needed:

age Eliezer = 10

And then she got the rest just fine, though with a few capitalization mistakes.

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