Why Use the Blank Identifier in Go's Compile-Time Interface Assertions?

Purpose of the Blank Identifier in Variable Assignment

When encountering variable assignments like the following, you might wonder why a blank identifier is used:

var _ PropertyLoadSaver = (*Doubler)(nil)

This blank identifier serves a crucial purpose in performing compile-time assertions, ensuring that a specific type satisfies the requirements of an interface.

In this example, the *Doubler type is checked against the PropertyLoadSaver interface. If *Doubler does not implement all the necessary methods defined in the interface, compilation will fail with an error stating that *Doubler is missing a specific method.

The code assigns an untyped nil value to a variable of type PropertyLoadSaver using (*Doubler)(nil). This assignment is only valid if *Doubler implements the PropertyLoadSaver interface. If it doesn't, compilation will end with an error message similar to:

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

The blank identifier _ is used because the variable doesn't need to be referenced anywhere else within the package. An alternative approach using a non-blank identifier is also possible:

var assertStarDoublerIsPropertyLoadSaver PropertyLoadSaver = (*Doubler)(nil)

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