Assignment operator in Go language
SyntaxGoSyntax Problem Overview
Lately I was playing with google's new programming language Go and was wondering why the assignment operator := has a colon in front of the equal sign =.
Is there a particular reason why the authors of the language wanted to use name := "John" instead of name = "John"?
Syntax Solutions
Solution 1 - Syntax
The := notation serves both as a declaration and as initialization.
foo := "bar"
is equivalent to
var foo = "bar"
Why not using only foo = "bar" like in any scripting language, you may ask ? Well, that's to avoid typos.
foo = "bar"
fooo = "baz" + foo + "baz" // Oops, is fooo a new variable or did I mean 'foo' ?
Solution 2 - Syntax
name := "John"
is just syntactic sugar for
var name string
name = "John"
Go is statically typed, so you have to declare variables.
Solution 3 - Syntax
:= is not the assignment operator. It's a short variable declaration. = is the assignment operator.
> Short variable declarations
>
> A short variable declaration uses the syntax:
>
> ShortVarDecl = IdentifierList ":=" ExpressionList .
>
> It is a shorthand for a regular variable declaration with initializer
> expressions but no types:
>
> "var" IdentifierList = ExpressionList .
>
> Assignments
>
> Assignment = ExpressionList assign_op ExpressionList .
>
> assign_op = [ add_op | mul_op ] "=" .
In Go, name := "John" is shorthand for var name = "John".
Solution 4 - Syntax
Rob Pike explains why Go has := during his talk "Origins of Go" (2010).
:= was a pseudo operator in another language codesigned by Pike called Newsquek (1989).
Which had Pascal-ish notation and ability to infer type for declare and initialize idiom (page 15)
// variable: [type] = value
x: int = 1
x := 1
Marginal note: Robert Griesemer brings up := operator answering the question "What would be one thing you take out from Go?" during QA session at Google I/O 2013. Referring to it as convenient but problematic.
Solution 5 - Syntax
There is at least one subtle difference between
name := "John"
and
var name = "John"
The former is a non-declaration statement and not allowed outside of a function body, whereas the latter is a valid statement at the package level.
Solution 6 - Syntax
Both are the different technique of variable declaration in Go language.
var name = "John" // is a variable declaration
AND
name := "John" // is a short variable declaration.
A short variable declaration is a shorthand for a regular variable declaration with initializer expressions but no types.
Read below for detail:
Solution 7 - Syntax
Important Context for the Answer:
:= is a shorthand operator for initializing a variable. In Go, the following operations are equivalent:
var myNumb String = "one"
myNumb := "one"
Answer:
The implied question now is: "Why did go design the shorthand notation := to have a : before the =?". The reason is to prevent prevalent typos. If the shorthand assignment operator was just =, then you could have the following situation:
var myNumb String = "one"
myNumb = "two"
Now did the user who created that code intend to reassign two to myNumb, or did he mistype myNumb instead of correctly typing myNumbTwo? By including the colon in :=, the programmer would have to commit two errors (forget the colon and forget the var) in order to have a bug, hence decreasing the probability of doing so drastically.