What's the Difference Between To_A and To_Ary

What's the difference between :: and +:

The difference is that +: is an abstraction of ::, which operates on generalized Seqs rather than only Lists. For instance, +: also works on Streams:

scala> 3 +: Stream(1,2)
res0: scala.collection.immutable.Stream[Int] = Stream(3, ?)

When you use it with a List, it is functionally the same as ::.

What's the difference between [ ] and { } when defining a variable (Python)?

Curly Braces creates a Set (https://www.w3schools.com/python/python_sets.asp) or Dictionary, while square braces creates List (https://www.w3schools.com/python/python_lists.asp).

A set is an unordered data structure, best for finding items fast, while a list is an ordered data structure

What is the difference between :: . and - in c++

1.-> for accessing object member variables and methods via pointer to object

Foo *foo = new Foo();
foo->member_var = 10;
foo->member_func();

2.. for accessing object member variables and methods via object instance

Foo foo;
foo.member_var = 10;
foo.member_func();

3.:: for accessing static variables and methods of a class/struct or namespace. It can also be used to access variables and functions from another scope (actually class, struct, namespace are scopes in that case)

int some_val = Foo::static_var;
Foo::static_method();
int max_int = std::numeric_limits<int>::max();

What is the difference between :: and . in Rust?

. is used when you have a value on the left-hand-side. :: is used when you have a type or module.

Or: . is for value member access, :: is for namespace member access.

Difference between : and :: in CSS

Pseudo-classes
The pseudo-class concept is introduced to permit selection based on information that lies outside of the document tree or that cannot be expressed using the other simple selectors.
A pseudo-class always consists of a "colon" (:) followed by the name of the pseudo-class and optionally by a value between parentheses.

ref: http://www.w3.org/TR/css3-selectors/#pseudo-classes

Pseudo-elements
Pseudo-elements create abstractions about the document tree beyond those specified by the document language. For instance, document languages do not offer mechanisms to access the first letter or first line of an element's content. Pseudo-elements allow authors to refer to this otherwise inaccessible information. Pseudo-elements may also provide authors a way to refer to content that does not exist in the source document (e.g., the ::before and ::after pseudo-elements give access to generated content).
A pseudo-element is made of two colons (::) followed by the name of the pseudo-element.

ref: http://www.w3.org/TR/css3-selectors/#pseudo-elements

We typically used to use just a single colon for everything, but now the best practice is to follow the W3C's guidelines, though I would add a caveat that you want to ensure the browsers you are supporting understands the syntax before you start making wholesale changes to your stylesheets.

PHP: Difference between - and ::

$name = $foo->getName();

This will invoke a member or static function of the object $foo, while

$name = $foo::getName();

will invoke a static function of the class of $foo. The 'profit', if you wanna call it that, of using :: is being able to access static members of a class without the need for an object instance of such class. That is,

$name = ClassOfFoo::getName();

What's the difference between `=` and ` -` in R?

From here:

The operators <- and = assign into the environment in which they are evaluated. The operator <- can be used anywhere, whereas the operator = is only allowed at the top level (e.g., in the complete expression typed at the command prompt) or as one of the subexpressions in a braced list of expressions.