Why Does Go Throw a 'Cannot Use Function as Type in Argument' Error, and How Can I Fix It?

Cannot Use Function as Type in Argument: Troubleshooting

This error occurs when attempting to pass a function as an argument to another function, but the function signature does not match the expected type. Particularly in the case of Go programming, this issue arises when you try to pass a function that takes a specific structured parameter and returns an interface and error as a requirement for another function.

Understanding the Problem

The error message "cannot use function (type func()) as type in argument" means that the function you are trying to pass does not match the type required by the argument. In this case, the argument expects an implementation of the MessageHandler interface, which has a specific function signature.

Fixing the Issue

To resolve this error, you must ensure that your function adheres to the expected signature of the MessageHandler interface:

type MessageHandler func(MessageDelivery) (interface{}, error)

This means that your function must:

• Take a single argument of the MessageDelivery type.

• Return a tuple with two values:

  • The first value should be of the interface type.
  • The second value should be an error.

Example:

Modify your functions to align with the required signature:

func UpperCaseHandler(md MessageDelivery) (interface{}, error) {
     s.Reply(MessageTest{strings.ToUpper(md.Message.(string))}, md.Delivery)
     return nil, nil
}

func RepeatHandler(md MessageDelivery) (interface{}, error) {
     s.Reply(MessageTest{strings.Repeat(md.Message.(string), 5)}, md.Delivery)
     return nil, nil
}

Now, these functions are compatible with the MessageHandler interface and can be passed as arguments to the ConsumerFunc() method.

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