为什么在golang中仅初始化定义中的一个变量会失败

Question

In calling a library function with the following signature:

func New() (*sql.DB, Sqlmock, error)

like this:

suite.db, suite.mock, err := sqlmock.New() // inside a suite method

I get error

expected identifier on left side of :=

However, when I change to this

var err error
suite.db, suite.mock, err = sqlmock.New()

the error disappears! Why does declaring k < n variables in a := assignment fail?!

Answer 1

:= is not assignment, it is a short variable declaration. Assignment uses e.g. the simple = operator.

As its name says: it is to declare variables. suite.db is not a variable, it is an expression, more specifically a primary expression; a selector to be exact.

The short variable declaration uses the syntax:

ShortVarDecl = IdentifierList ":=" ExpressionList .

Where IdentifierList is:

IdentifierList = identifier { "," identifier } .

So you must list identifiers. One "exception" to this "declare new variables rule" is the redeclaration:

Unlike regular variable declarations, a short variable declaration may redeclare variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original.

So you are allowed to use existing variables in a short variable declaration if they were declared in the same block, and you also provide new identifiers (not just existing ones–in which case you would have to use assignment instead).

See related: Why there are two ways of declaring variables in Go, what's the difference and which to use?

When you declare err prior and change := to = it works, because the assignment does not require identifiers on the left of the assignment operator, but expressions:

Assignment = ExpressionList assign_op ExpressionList .

And as detailed above, suite.db is an expression, just like existing variables (identifiers).