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Comment by digdugdirk | Hacker News Reader

As someone who isn't a trained programmer (and has no background or understanding of lisp) that looks like you took something sensible and turned it into gibberish.

Is there a recommended "intro to understanding lisp" resource out there for someone like myself to dive in to?

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  (-> x (* 2) (+ 1) (- 3) (% x))

The part that is confusing if you don't know Clojure is (->). This a thread macro, and it passes "x" through a list of functions.

So it basically breaks this down into a list of instructions to do to x. You will multiply it by 2, add 1 to it, take 3 from it, then do the modulus by the original value of x (the value before any of these steps).

Clojurists feel like this looks more readable than the alternative, because you have a list of transformations to read left to right, vs this

  (% (- (+ (* x 2) 1) 3) x)

Which is the most unreadable of them all, to me.

this is very readable if you know to read the operations inside-out instead of left-to-right

But that reading requires looking back and forth to read the operator and the operand. The further you move out the more you shift your eyes and the harder it becomes to quickly jump back to the level of nesting that you are currently on at the other side.

Be that as it may, it's less readable than the other ones, to me

I feel like there's a joke in here somewhere about a backwards Forth dialect, but this also reminds of of chaining idioms used in other languages.

Currying with OOP:

  res = x
    .mult(2)
    .add(1)
    .sub(3)
    .mod(x)

Currying with assignment operators:

  res = x
  res *= 2
  res += 1
  res -= 3
  res %= x

Naming things instead of Currying:

  scale = 2
  offset = 1 - 3
  with_scale = x * scale
  with_offset = with_scale + offset
  result = with_offset % x

Or aping that in Scheme:

  (let* ((scale 2)
         (offset (- 1 3))
         (with_scale (* x scale))
         (with_offset (+ with_scale offset)))
       (remainder with_offset x))

  '(
    1. This is a list: ( ). Everything is a list. Data structures are all lists.
    2. A program is a list of function calls
    3. Function calls are lists of instructions and parameters. For (A B C), A is the function name, B and C are parameters.
    4. If you don't want to execute a list as a function, but as data, you 'quote' it using a single quote mark '(A B C)
    5. Data is code, code is data.)

> Is there a recommended "intro to understanding lisp" resource out there for someone like myself to dive in to?

Practical Common Lisp - https://gigamonkeys.com/book/

Casting SPELs in Lisp - http://www.lisperati.com/casting.html

Structure and Interpretation of Computer Programs - https://mitp-content-server.mit.edu/books/content/sectbyfn/b...

One of many prior discussions here on HN: https://news.ycombinator.com/item?id=22913750

... amongst many resources for learning LISP.

The language fundamentals of "Clojure for the Brave and True" (best intro to Clojure book IMO) is excellent (if you consider Clojure a lisp). I find the author's style/humor engaging.

https://www.braveclojure.com/clojure-for-the-brave-and-true/

It is gibberish. The expression means:

  (1 + (x * 2)) - (3 % x)

So the correct S-exp (let's use mod for modulo rather than %):

  (+ 1
     (* x 2)
     (- (mod 3 x)))

It's a sum of three terms, which are 1, (* x 2) and something negated (- ...), which is (mod 3 x): remainder of 3 modulo x.

The expression (% (- (+ (* x 2) 1) 3) x) corresponds to the parse

  ((x * 2 + 1) - 3) % x

I would simplify that before anything by folding the + 1 - 3:

  (x * 2 - 2) % x

Thus:

  (% (- (* 2 x) 2) x).

Also, in Lisps, numeric constants include the sign. This is different from C and similar languages where -2 is a unary expression which negates 2: two tokens.

So you never need this: (- (+ a b) 3). You'd convert that to (+ a b -3).

Trailing onstant terms in formulas written in Lisp need extra brackets around a - function call.

In real Lisp code you'd likely indent it something like this:

    (% 
      (-
        (+ (* x 2)
           1)
        3)
      x)

This makes the structure clearer, although it's still wasteful of space, and you still have to read it "inside-out". The thread macro version would be:

  (-> x
    (* 2)
    (+ 1)
    (- 3)
    (% x))

It's more compact, there's no ambiguity about order-of-operations, and we can read it in order, as a list of instructions:

"take x, times it by 2, add one, subtract 3, take modulus with the original x".

It's pretty much how you'd type it into a calculator.

EDIT: care to explain the downvote?

> In real Lisp code you'd likely indent it something like this:

Not only would that not be idiomatic, the operator for modulus in Common LISP is mod not %. and the brackets you and the parent used in the s-expr are around the wrong groups of symbols. So you're more likely to see:

(mod (* (+ 1 x) (- 2 3)) x)

or maybe with some limited indentation, such as:

(mod

      (* (+ 1 x) (- 2 3)) 

      x)

Nobody said it had to be Common Lisp. I'm going by the notation the grandparent commenter used. My point was that indentation can clarify the structure of nested sexps vs putting them on one line. And that is actually what people do. "mod" vs "%" hasn't the least to do with it. This isn't even really about arithmetic; those are just at-hand examples the GP commenter chose. Could just as well have been

  (foo 
    (bar
      (baz (bax x 2)
           "hello")
      "world")
    "!")

>the brackets you and the parent used in the s-expr are around the wrong groups of symbols

No they're not. Yours is wrong. Multiplication has higher priority than addition so the order of evaluation begins with (x * 2) not (1 + x).

> No they're not. Yours is wrong. Multiplication has higher priority than addition so the order of evaluation begins with (x * 2) not (1 + x).

OK, I shouldn't have gone that far.

FWIW, modulus has the same operator precedence as multiplication and division.

So really, it is more like:

(- (+ 1 (* x 2))

   (mod 3 x))

or using the increment function:

(- (1+ (* x 2))

   (mod 3 x))

Alright well I guess this is an object lesson in why infix notation is bad -- nobody can remember all the precedence rules once you get beyond +-*/^.

We'll call it even :-)

For what it's worth (speaking only for myself), I could not live without the threading macros (-> and ->>) in Clojure. Below is an example of some very involved ETL work I just did. For me this is very readable, and I understand if others have other preferences.

(defn run-analysis [path]

  ;load data from converted arrow file
  (let [data (load-data path)]

    (-> data
        ;; Calc a Weeknumber, Ad_Channel, and filter Ad_Channel for retail
        add-columns
        ;; Agg data by DC, Store, WeekNum, Item and sum qty and count lines
        rolled-ds
        ;; Now Agg again, this time counting the weeks and re-sum qty and lines
        roll-again)))

;;Run the whole thing

(time (run-analysis arrow-path))

This code processes 27 million lines in 75 seconds (shout out to https://techascent.github.io/tech.ml.dataset/100-walkthrough... library)

If such an expression gets indented at all, it would be more like this:

  (% (- (+ (* x 2)
           1)
        3)
     x)

Interestingly the second form is just infix notation where every operator has the same precedence and thus is evaluated left to right. That says to me that it's not infix notation that's inherently weird but instead it's the operator precedence rules of mathemetical infixes that are weird.

> that looks like you took something sensible and turned it into gibberish

This is the main thing I use Lisp (well, Guile Scheme) for. I used to use bc for little scratch pad calculations, now I usually jump into Scheme and do calculations. I don't recall if I thought it looked like gibberish at first but it's intuitive to me now.

https://en.wikipedia.org/wiki/Polish_notation#Explanation

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