What is the Purpose of the Blank Identifier in Go's Interface Assertion?

Purpose of the Blank Identifier in Variable Assignment

In variable declarations such as var _ PropertyLoadSaver = (*Doubler)(nil), the blank identifier serves a specific purpose. This construct is a compile-time assertion that the *Doubler type implements the PropertyLoadSaver interface.

In Go, a type implements an interface when it offers a method set that exceeds or matches the method set of the interface. If the *Doubler type doesn't meet this criterion, the compiler will throw an error akin to:

prog.go:21: cannot use (*Doubler)(nil) (type *Doubler) as type PropertyLoadSaver in assignment:
*Doubler does not implement PropertyLoadSaver (missing Save method)

This blank identifier technique involves declaring an unnamed variable of type PropertyLoadSaver, then assigning it a nil value of type Doubler via the expression (*Doubler)(nil). This assignment is only valid if Doubler implements the PropertyLoadSaver interface.

The blank identifier underscores the fact that this variable will not be referenced elsewhere in the package. Similarly, the following line achieves the same result but with a non-blank identifier:

var assertStarDoublerIsPropertyLoadSaver PropertyLoadSaver = (*Doubler)(nil)

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