Go Concepts - Lexical Structure

Internal

• Go Concepts

Source Files

Go code goes into source files. They have the .go extension.

Each source file is mandatory declared to belong to a package.

They can then optionally import code from other packages, using the import keyword.

Each source file has its own file block.

Thera are no restrictions on the file name, it is not required to have any relationship with the name of the package it contains code for.

Comments

// to the end of the line

/*
  Multiline
  line 1
  ...
  line n
*/

Whitespace

Whitespace in Go are carriage returns (u+000D), newlines (u+000A), spaces (u+0020) and tabs (u+0009).

Semicolons

Go programs may omit most of the semicolons at the end of line if the following conditions apply:

1. When the input is broken into tokens, a semicolon is automatically inserted into the token stream immediately after a line's final token if that token is:
   1. an identifier
   2. an integer, floating-point, imaginary, rune, or string literal
   3. one of the keywords break, continue, fall through, or return
   4. one of the operators and delimiters ++, --, ), ], or }
2. To allow complex statements to occupy a single line, a semicolon may be omitted before a closing ")" or "}".

For more details, see the specification:

https://golang.org/ref/spec#Semicolons

Blocks

A block is a sequence of declarations and statements within matching curly braces. The block may be empty. The most common blocks are those defined in the source code - they define block lexical scopes (see variable scopes). Aside from those, Go recognizes the following implicit blocks:

• The universe block which contains all Go source text.
• The package block.
• The file block that contains all the source text in the file.
• Each if, for and switch statements is considered to be in its own implicit block.
• Each clause in a switch and select statements acts as an implicit block.

Identifiers

Identifiers name program entities, such as constants, variables, types, functions, etc. An identifier is a sequence of one more letter and digits. The first character must be a letter. The letters are unicode letters or underscore "_".

Identifiers can be exported, and the way the first letter is capitalized is relevant to whether the identifier is exported or not. See exported identifiers.

It is idiomatic to use camel case style for Go identifiers.

Pre-Declared Identifiers

Pre-declared identifiers are available by default, without needing to import anything. They are implicitly declared in the universe block.

Pre-declared types: bool, byte, complex64, complex128, float32, float64, int, int8, int16, int32, int64, rune, string, uint, uint8, uint16, uint32, uint64, uintptr and error.

Pre-declared constants: true, false, iota.

Pre-declared functions: append, cap, close, complex, copy, delete, imag, len, make, new, panic, print, println, real, recover. Also see built-in functions.

Pre-declared zero value: nil.

The Blank Identifier

"_" is the blank identifier.

Literals

A literal is a notation for representing a fixed value in source code. Go provides literals for booleans, integers, floating-point values, strings, built-in types such as arrays, slices and maps, user-defined types (structs), functions, etc. The compiler is always able to infer the literal's type.

Types

The Type System

Functions

Functions

Operators

Operators

Declarations

A declaration binds an identifier to a constant, type, variable, function, label or package. Every identifier in the program must be declared. No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block.

Variables

A variable is a storage location in memory. Go being a statically typed language, a variable always has a specific type. A variable also has a name, which is referred to as variable identifier.

var <variable_identifier> <variable_type_literal> = <initial_value_literal>

Example:

var a string = "blue"
var b = "red"
var c string

Declaring the type is optional if an initial value is specified, the compiler is able to infer the type of the literal, and the variable will be built to have that type.

Assigning a value is optional, if no value is explicitly assigned, the variable is initialized to the type's zero value. However, the type must be specified in this case.

Another form of variable declaration that always assigns an initial value and also infers the variable type based on the literal type is the following:

<variable_identifier> := <initial_value_literal>

Example:

a := "blah"

Note that this form cannot be used for the declaration of a package-level variable.

Generally, the shorter value should be used whenever possible. Some books claim that the long value is not "idiomatic". For more details on variable names, see identifiers.

Variable Scopes

The range of places where a variable is visible from it is called the scope of the variable.

Go is lexically scoped using blocks. A variable is visible within the block delimited by the nearest curly braces {}, but not outside them.

Package-Level Variable

An unexported variable declared at the package level (outside any function) is visible throughout the package.

An exported ariable declared at the package level is visible throughout the package and in all packages that import the exporting package.

Pass by Value vs Pass by Reference

In Go, all variables are passed by value. Even for pointer variables, since the value of the pointer is a memory address, passing pointer variables is still considered pass by value.

Pointers

Pointer variable.

Statements

• Go Language Specification https://golang.org/ref/spec#Statements

Statements control execution flow within a function.

Expressions

Operators combine operands into expressions.

Keywords

• Go Specification - Keywords: https://golang.org/ref/spec#Keywords

range	type	func	.	.
package	go	.	.	.
defer	chan	import	.	.