Notes (OFVB 13/31): Putting Things in Boxes

see boxes.ml

This mix of functional and imperative is really interesting.

There is an erratum in the the file_statistics declaration: file_statistics_channel should be channel_statistics.

I kind of like the way mutation and array syntax works. It’s very imperative, but very accessible. In Haskell, we’d do something with Vector, MVector or MVar, which are all a little more conceptually complicated. I suspect that complexity is actually just inherent in mutation, so learning the various Haskell mutation types IORef, MVar, TVar, are actually really useful, whereas I suspect the OCaml abstraction might be trading off immediate ease of use for complexity wrinkles later on. That’s only a supposition though, we’ll see how that plays out as I learn more.

Questions

1

References x and y have been created. Initial values are !x = 1, !y = 2, final values are !x = 2, !y = 4. The expression evaluates to 6 with type int.

2

The difference is that in the first case we have a list of two different references, whereas the second is a list of two identical references. So updating either reference in the second list changes the value the other ref points to, because both refs point to the same place.

utop # updateRefListHead twoRef 6;;
- : int ref list = [{contents = 6}; {contents = 5}]
utop # updateRefListHead oneRef 6;;
- : int ref list = [{contents = 6}; {contents = 6}]

3

We could just use recursion or a while with an explicit iteration variable.

4

int array
bool array
int array array
int list array
int
unit

5

So we could do this in a pure way with a fold, but it feels like OCaml wants me to use a ref and a for with an imperative style and I don’t want to fight the language too much while I’m learning it.

7

Interesting! Array.make is a reference! So, Array.make n (Array.make n 0) actually only makes an array of references to the same array! So we’ll use Array.make_matrix instead.

8

Ascii upper and lower characters only differ by a single bit (the second high bit), which we can flip by adding or subtracting by 2^5 = 32.

9

This channel_statistics function is going to undercount the number of words if words don’t end with spaces, but with tabs, newlines or an EOF.

10

So to fix the word undercounting we can count the words per line with (List.length (String.split_on_char ' ' line)).