[Haskell-cafe] A Thought: Backus, FP, and Brute Force Learning

[OWP]

When I said "I stare at a particular section of the code for a while", I
meant it as an idiom for deeply studying that particular code alone.  It's
just me and the code and whatever debugging tools I have readily
available.

Are you familiar with the difficulty in maintaining legacy platforms
written by a team which no longer exists?  In some cases, it might be
required to completely rewrite the entire platform in another code base
(like Haskell) while maintaining the original behavior (including any and
all bugs or undocumented features) of the legacy software.  In those cases,
"staring at the code" may be one of the better option because the code will
tell you most of what you need to know to recreate that software in another
platform.

Going back to the original thought, my curiosity has more to do about
Backus (he is cited quite a lot) and how much of his theory made it into
Haskell and other commonly used functional languages.


On Thu, Mar 21, 2013 at 12:36 AM, Albert Y. C. Lai <trebla at vex.net> wrote:

> On 13-03-20 06:54 PM, OWP wrote:
>
>> For me personally, one thing I enjoy about a typical procedural program
>> is that it allows me to "Brute Force Learn".
>>
> [...]
>
> 1. I believe that you can also stare at functional programs and figure out
> as much as what you can with procedural programs.
>
> It only requires knowing the language and the libraries. And you can no
> longer argue that functional languages are more declarative or higher level
> than procedural languages. Once upon a time, it was true, with
> parametricity, algebraic data types, higher-order functions, and list
> comprehensions; now procedural languages have them too or competitive
> alternatives.
>
> 2. I doubt how much you can learn from staring, be it functional programs
> or procedural programs.
>
> I posit that at most you're just reading aloud in your native tongue how
> to execute the program. But then you're just competing with a computer at
> its job. You barely know what requirement the program satisfies, much less
> why the program satisfies that requirement.
>
> (With the exception that the program contains no iteration or recursion,
> or contains just boring iteration or recursion such as walking over an
> array.)
>
> I do not mean that you lack jargons like "gradient" and "matrix", no. You
> can explain in your own words, if you know the right idea but just not the
> jargon. I am positing this: apart from telling me how to execute the
> program, you cannot explain the purpose of the program, not even in your
> own words, because you do not know.
>
> Here is an example I learned recently. It is ingenious.
>
> Let A, B be arrays of the same length N. Their elements are numbers. They
> use 0-based indexing. They are the input.
>
> int h=0, i=0, j=0;
> bool answer;
>
> while (h<N && i<N && j<N) {
>     int Aih = A[(i+h) % N], Bjh = B[(j+h) % N];
>
>     if (Aih == Bjh) {
>         h = h + 1;
>     } else if (Aih > Bjh) {
>         i = i + h + 1;
>         h = 0;
>     } else { /* Aih < Bjh */
>         j = j + h + 1;
>         h = 0;
>     }
> }
> answer = (h >= N);
>
> answer is the output. What does answer say about the input?
>
> The algorithm is no different in Haskell (it makes me use lowercase a, b,
> n):
>
> answer = go 0 0 0
> go h i j =
>     if h<n && i<n && j<n then
>         case compare (a!((i+h) `mod` n)) (b!((j+h) `mod` n)) of
>             EQ -> go (h+1) i j
>             GT -> go 0 (i+h+1) j
>             LT -> go 0 i (j+h+1)
>     else h>=n
>
> 3. I completely refuse to believe that you literally do not know what
> you're doing before you start.
>
> If it were true, it must be like this: You throw dice 500 times to
> generate a 500-character file. If the compiler doesn't like it, you throw
> dice more times to decide what to mutate in that file. Eventually the
> compiler surrenders. Now, and only now, you stare at the file for a few
> minutes, and discover: it implements doubly linked list! It will be useful
> when you write your own web browser later, it can help provide the "back"
> button and the "forward" button...
>
> No, it has to be the other way round. You have to decide to attempt a web
> browser project or whatever in the first place. You are vague about
> details, what not to include, what to include, how to do them... but you
> know vaguely it's a web browser. After some time, you have to decide which
> part, however small, you start to code up. Maybe you decide to code up a
> doubly linked list first. Now you type up something. You type up something
> knowing that the short term goal is doubly linked list, and the long term
> goal is some kind of web browser or whatever project it is. This much you
> know. And while you type, every key you type you strive to get closer to a
> doubly linked list in good faith. Maybe sometimes you're creative, maybe
> sometimes you make mistakes, maybe you write clever code and I can't
> understand it, but it is not random typing, you know the purpose, you have
> reasons, you understand, and you don't just stare.
>
>
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