Ruby Syntax Question: Rational(A, B) and Rational.New!(A, B)

Ruby syntax question: Rational(a, b) and Rational.new!(a, b)

All you have to do is declare a global function with the same name as your class. And that is what rational.rb does:

def Rational(a, b = 1)
  if a.kind_of?(Rational) && b == 1
    a
  else
    Rational.reduce(a, b)
  end
end

to make the constructor private:

private :initialize

and similarly for the new method:

private_class_method :new

I suppose Rational.new could be kept public and made to do what Rational() does, but having a method that turns its arguments into instances is consistent with Array(), String(), etc. It's a familiar pattern that's easy to implement and understand.

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Startups, in general, take up all your waking time, are high-stress, require reasonably high-quality and creative work, and offer no guaranteed return. For something like that, most people need all the motivation they can get, and large quantities of money (not guaranteed) aren't enough. If you're not passionate about what you're doing, you're likely to fail, and almost certain to be miserable.

That being said, if what you're passionate about isn't something people will pay for, leave it as a hobby.

What does this symbol mean in JavaScript?

See the documentation on MDN about expressions and operators and statements.

Basic keywords and general expressions

this keyword:

• How does the "this" keyword work, and when should it be used?

var x = function() vs. function x()  —  Function declaration syntax

• var functionName = function() {} vs function functionName() {}

(function(){…})()  —  IIFE (Immediately Invoked Function Expression)

• What is the purpose?, How is it called?
• Why does (function(){…})(); work but function(){…}(); doesn't?
• (function(){…})(); vs (function(){…}());
• shorter alternatives:
  
  • !function(){…}(); - What does the exclamation mark do before the function?
  • +function(){…}(); - JavaScript plus sign in front of function expression
  • !function(){ }() vs (function(){ })(), ! vs leading semicolon
• (function(window, undefined){…}(window));

someFunction()()  —  Functions which return other functions

• Two sets of parentheses after function call

=>  —  Equal sign, greater than: arrow function expression syntax

• What's the meaning of "=>" (an arrow formed from equals & greater than) in JavaScript?

|>  —  Pipe, greater than: Pipeline operator

• What does the "|>" operator do in JavaScript?

function*, yield, yield*  —  Star after function or yield: generator functions

• What is "function*" in JavaScript?
• What's the yield keyword in JavaScript?
• Delegated yield (yield star, yield *) in generator functions

[…], [ value ], Array()  —  Square brackets: array notation

• What’s the difference between "Array()" and "[]" while declaring a JavaScript array?
• What is array literal notation in javascript and when should you use it?

If the square brackets appear on the left side of an assignment ([a] = ...), or inside a function's parameters, it's a destructuring assignment.

{…}, { key: value }, { [key]: value }  —  Curly brackets: object literal syntax (not to be confused with blocks)

• What do curly braces in JavaScript mean?
• Javascript object literal: what exactly is {a, b, c}?
• What do square brackets around a property name in an object literal mean?
• How does this object method definition work without the "function" keyword? (ES2015 Method definitions)

If the curly brackets appear on the left side of an assignment ({ a } = ...) or inside a function's parameters, it's a destructuring assignment.

`…${…}…`  —  Backticks, dollar sign with curly brackets: template literals

• What does this `…${…}…` code from the node docs mean?
• Usage of the backtick character (`) in JavaScript?
  
• What is the purpose of template literals (backticks) following a function in ES6?

/…/  —  Slashes: regular expression literals

• Meaning of javascript text between two slashes

$  —  Dollar sign in regex replace patterns: $$, $&, $`, $', $n

• JavaScript replace() method dollar signs

()  —  Parentheses: grouping operator

• MDN: Grouping operator

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Property-related expressions

obj.prop, obj[prop], obj["prop"]  —  Square brackets or dot: property accessors

• JavaScript property access: dot notation vs. brackets?

?., ?.[], ?.()  —  Question mark, dot: optional chaining operator

• Question mark after parameter
• Null-safe property access (and conditional assignment) in ES6/2015
• Optional Chaining in JavaScript
• Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?
• Is there a "null coalescing" operator in JavaScript?

::  —  Double colon: bind operator

• JavaScript double colon (bind operator)

new operator

• What is the 'new' keyword in JavaScript?
• What is "new.target"?

...iter  —  Three dots: spread syntax; rest parameters

• (...rest) => {}  —  What is the meaning of “…args” (three dots) in a function definition?
• fn(...args)  —  What is the meaning of “foo(…arg)” (three dots in a function call)?
• [...iter]  —  javascript es6 array feature […data, 0] “spread operator”
• {...props}  —  Javascript Property with three dots (…), What does the '…rest' stand for in this object destructuring?

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Increment and decrement

++, --  —  Double plus or minus: pre- / post-increment / -decrement operators

• ++someVariable vs someVariable++ in Javascript

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Unary and binary (arithmetic, logical, bitwise) operators

delete operator

• What is the purpose of the delete operator in Javascript?

void operator

• What does `void 0` mean?

+, -  —  Plus and minus: addition or concatenation, and subtraction operators; unary sign operators

• What does = +_ mean in JavaScript, Single plus operator in javascript
• What's the significant use of unary plus and minus operators?
• Why is [1,2] + [3,4] = "1,23,4" in JavaScript?
• Why does JavaScript handle the plus and minus operators between strings and numbers differently?

|, &, ^, ~  —  Single pipe, ampersand, circumflex, tilde: bitwise OR, AND, XOR, & NOT operators

• What do these JavaScript bitwise operators do?
• How to: The ~ operator?
• Is there a & logical operator in Javascript
• What does the "|" (single pipe) do in JavaScript?
• What does the operator |= do in JavaScript?
• What does the ^ (caret) symbol do in JavaScript?
• Using bitwise OR 0 to floor a number, How does x|0 floor the number in JavaScript?
• Why does ~1 equal -2?
• What does ~~ ("double tilde") do in Javascript?
• How does !!~ (not not tilde/bang bang tilde) alter the result of a 'contains/included' Array method call? (also here and here)

%  —  Percent sign: remainder operator

• What does % do in JavaScript?

&&, ||, !  —  Double ampersand, double pipe, exclamation point: logical operators

• Logical operators in JavaScript — how do you use them?
• Logical operator || in javascript, 0 stands for Boolean false?
• What does "var FOO = FOO || {}" (assign a variable or an empty object to that variable) mean in Javascript?, JavaScript OR (||) variable assignment explanation, What does the construct x = x || y mean?
• Javascript AND operator within assignment
• What is "x && foo()"? (also here and here)
• What is the !! (not not) operator in JavaScript?
• What is an exclamation point in JavaScript?

??  —  Double question mark: nullish-coalescing operator

• How is the nullish coalescing operator (??) different from the logical OR operator (||) in ECMAScript?
• Is there a null-coalescing (Elvis) operator or safe navigation operator in javascript?
• Is there a "null coalescing" operator in JavaScript?

**  —  Double star: power operator (exponentiation)

• x ** 2 is equivalent to Math.pow(x, 2)
• Is the double asterisk ** a valid JavaScript operator?
• MDN documentation

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Equality operators

==, ===  —  Equal signs: equality operators

• Which equals operator (== vs ===) should be used in JavaScript comparisons?
• How does JS type coercion work?
• In Javascript, <int-value> == "<int-value>" evaluates to true. Why is it so?
• [] == ![] evaluates to true
• Why does "undefined equals false" return false?
• Why does !new Boolean(false) equals false in JavaScript?
• Javascript 0 == '0'. Explain this example
• Why false == "false" is false?

!=, !==  —  Exclamation point and equal signs: inequality operators

• != vs. !==
• What is the difference between != and !== operators in JavaScript?

---

Bit shift operators

<<, >>, >>>  —  Two or three angle brackets: bit shift operators

• What do these JavaScript bitwise operators do?
• Double more-than symbol in JavaScript
• What is the JavaScript >>> operator and how do you use it?

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Conditional operator

…?…:…  —  Question mark and colon: conditional (ternary) operator

• Question mark and colon in JavaScript
• Operator precedence with JavaScript's ternary operator
• How do you use the ? : (conditional) operator in JavaScript?

---

Assignment operators

=  —  Equal sign: assignment operator

• What is the difference between the `=` and `==` operators and what is `===`? (Single, double, and triple equals)

This symbol is also used for default parameters or default values in a destructuring assignment:

• what does (state = {}) => state means
• What does ({"key": "value"} = {}) syntax mean inside a JavaScript function

%=  —  Percent equals: remainder assignment

• Having Confusion with Modulo operator

+=  —  Plus equals: addition assignment operator

• How does += (plus equal) work?

&&=, ||=, ??=  —  Double ampersand, pipe, or question mark, followed by equal sign: logical assignments

• What purpose do &&=, ||= and ??= serve?
• Replace a value if null or undefined in JavaScript
• Set a variable if undefined
• Ruby’s ||= (or equals) in JavaScript?
• Original proposal
• Specification

<<=, >>=, >>>=, &=, ^=, |= — Double less than, double greater than, triple greater than, ampersand, caret, or pipe followed by equal sign: bitwise assignments

• What do these JavaScript bitwise operators do?

Destructuring

• of function parameters: Where can I get info on the object parameter syntax for JavaScript functions?
• of arrays: Multiple assignment in JavaScript? What does `[ a, b, c ] = [ 1, 2, 3 ]` mean?
• of objects/imports: Javascript object bracket notation ({ Navigation } =) on left side of assign

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Comma operator

,  —  Comma operator (not to be confused with the comma used in variable declarations)

• What does the comma operator do in JavaScript?
• Comma operator returns first value instead of second in argument list?
• When is the comma operator useful?

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Control flow

{…}  — Curly brackets: blocks (not to be confused with object literal syntax)

• JavaScript curly braces with no function or json

Declarations

var, let, const  —  Declaring variables

• What is the difference between "let" and "var"?
• Are there constants in JavaScript?
• What is the temporal dead zone?
• var a, b;  —  Comma used in variable declarations (not to be confused with the comma operator): JavaScript variable definition: Commas vs. Semicolons

---

Label

label:  —  Colon: labels

• What does the JavaScript syntax foo: mean?
• What does ':' (colon) do in JavaScript?

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Other

123n  —  n after integer: BigInt

• What does character 'n' after numeric literal mean in JavaScript?

#  —  Hash (number sign): Private methods or private fields

• What does the # symbol do in JavaScript?

_  —  Underscore: separator in numeric literals

• Javascript numeric separators?
• Is there a Javascript equivalent to the Ruby syntax using underscores (e.g. 10_000 = 10000) to make larger integers human readable?

Regular expression to match a line that doesn't contain a word

The notion that regex doesn't support inverse matching is not entirely true. You can mimic this behavior by using negative look-arounds:

^((?!hede).)*$

Non-capturing variant:

^(?:(?!:hede).)*$

The regex above will match any string, or line without a line break, not containing the (sub)string 'hede'. As mentioned, this is not something regex is "good" at (or should do), but still, it is possible.

And if you need to match line break chars as well, use the DOT-ALL modifier (the trailing s in the following pattern):

/^((?!hede).)*$/s

or use it inline:

/(?s)^((?!hede).)*$/

(where the /.../ are the regex delimiters, i.e., not part of the pattern)

If the DOT-ALL modifier is not available, you can mimic the same behavior with the character class [\s\S]:

/^((?!hede)[\s\S])*$/

Explanation

A string is just a list of n characters. Before, and after each character, there's an empty string. So a list of n characters will have n+1 empty strings. Consider the string "ABhedeCD":

    ┌──┬───┬──┬───┬──┬───┬──┬───┬──┬───┬──┬───┬──┬───┬──┬───┬──┐
S = │e1│ A │e2│ B │e3│ h │e4│ e │e5│ d │e6│ e │e7│ C │e8│ D │e9│
    └──┴───┴──┴───┴──┴───┴──┴───┴──┴───┴──┴───┴──┴───┴──┴───┴──┘
    
index    0      1      2      3      4      5      6      7

where the e's are the empty strings. The regex (?!hede). looks ahead to see if there's no substring "hede" to be seen, and if that is the case (so something else is seen), then the . (dot) will match any character except a line break. Look-arounds are also called zero-width-assertions because they don't consume any characters. They only assert/validate something.

So, in my example, every empty string is first validated to see if there's no "hede" up ahead, before a character is consumed by the . (dot). The regex (?!hede). will do that only once, so it is wrapped in a group, and repeated zero or more times: ((?!hede).)*. Finally, the start- and end-of-input are anchored to make sure the entire input is consumed: ^((?!hede).)*$

As you can see, the input "ABhedeCD" will fail because on e3, the regex (?!hede) fails (there is "hede" up ahead!).

What do and stand for?

• < stands for the less-than sign: <
• > stands for the greater-than sign: >
• ≤ stands for the less-than or equals sign: ≤
• ≥ stands for the greater-than or equals sign: ≥

AND' vs '&&' as operator

If you use AND and OR, you'll eventually get tripped up by something like this:

$this_one = true;
$that = false;

$truthiness = $this_one and $that;

Want to guess what $truthiness equals?

If you said false... bzzzt, sorry, wrong!

$truthiness above has the value true. Why? = has a higher precedence than and. The addition of parentheses to show the implicit order makes this clearer:

($truthiness = $this_one) and $that

If you used && instead of and in the first code example, it would work as expected and be false.

As discussed in the comments below, this also works to get the correct value, as parentheses have higher precedence than =:

$truthiness = ($this_one and $that)

What is the difference between equality and equivalence?

Wikipedia: Equivalence relation:

In mathematics, an equivalence
relation is a binary relation between
two elements of a set which groups
them together as being "equivalent" in
some way. Let a, b, and c be arbitrary
elements of some set X. Then "a ~ b"
or "a ≡ b" denotes that a is
equivalent to b.

An equivalence relation "~" is reflexive, symmetric, and transitive.

In other words, = is just an instance of equivalence relation.

Edit: This seemingly simple criteria of being reflexive, symmetric, and transitive are not always trivial. See Bloch's Effective Java 2nd ed p. 35 for example,

public final class CaseInsensitiveString {
...
    // broken
    @Override public boolean equals(Object o) {
        if (o instance of CaseInsensitiveString)
            return s.equalsIgnoreCase(
                ((CaseInsensitiveString) o).s);
        if (o instanceof String) // One-way interoperability!
            return s.equalsIgnoreCase((String) o);
        return false;
    }  
}

The above equals implementation breaks the symmetry because CaseInsensitiveString knows about String class, but the String class doesn't know about CaseInsensitiveString.

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