WolframLang: Expression

WolframLang source code is made of expressions. Every part in source code, is expression. (there is no such thing as “statements”.) Everything is a expression.

Atom

An atom is a indivisible expression.
An atom is one of: Number, Symbol, String , Special Character. [see Atomic Expression]

Expression

Expression is either a single Atom , or a sequence of atoms in the form

e0[e1, e2 etc]

Each of the e is a expression.

Head of expression

The e0 in e0[…] is called the Head of expression. [see Head of Expression]

List

A list is a expression whose head is List. [see List]

FullForm

Many commonly used functions has syntax shortcuts. For example, you can write

FullForm	Shortcut
Plus[3,4]	3+4
Rational[3,4]	3/4
List[3,4]	{3,4}
Equal[x,y]	x == y
SameQ[x,y]	x === y
Greater[x,y]	x > y
Set[x,3]	x = 3

• The form e0[…] is called FullForm.
• Atomic Expression such as 3, x, "abc", are in FullForm by themselves.
• The WolframEngine convert shortcut syntax to FullForm internally before eval.
• All Pattern Matching are done against FullForm.

You can use FullForm[Hold[expr]] to show the FullForm. [see WolframLang Syntax/Operators]