Why Does Type Assertion Fail When Casting to a Function Type Alias in Go?

Type Casting in Go: Understanding Type Assertions and Identity

In Go, the inability to perform a type cast from a type alias to its underlying function type has puzzled many developers. Let's delve into the reasons behind this behavior.

Go offers two mechanisms for type manipulation: type assertions and type conversions. Type assertions, used in the provided snippet, ensure that the value stored in an interface type is of a specific type. However, unlike type conversions, type assertions strictly adhere to type identity. This means that the dynamic type of the interface value must be identical to the asserted type, with no room for compatibility or inheritance relationships.

In the provided code, the type alias somethingFuncy represents a function with an int parameter and a bool return type. While assigning the function funcy to an interface variable a succeeds thanks to Go's ability to dynamically bind functions, asserting that a is of type somethingFuncy fails due to the requirement for exact type identity.

To summarize, in Go, type assertions prioritize exact type matching rather than function signatures or compatibility relationships. Therefore, casting to a type alias that represents a function requires an explicit type declaration to ensure the asserted interface value's dynamic type matches the alias definition.

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