Golang中的字符串和通用函数的映射

Question

I'm trying to create a map that will map strings to functions. Not all functions have the same signature. For example, I'd like to have something like this:

rf := map[string]func(...interface{}) error{
    "FirstName":        validateExistence(a.FirstName, "FirstName is required."),
    "Postcode":         validateMatchPattern(a.Postcode, `^\d{5}$`, "Could not match pattern for postcode."),
    "Address":          validateLength(a.Address, 0, 35, "Address must have up to 35 chars."),
}

I'm getting this error:

cannot use validateExistence("FirstName is required.") (type func(string) error) as type func(...interface {}) error in map value

If I change the map declaration to map[string]func(f string, m string) error the error for the FirstName is solved, but I get other errors for the other two functions:

cannot use validateMatchPattern(a.Postcode, "^\\d{5}$", "Could not match pattern for postcode.") (type error) as type func(string) error in map value
cannot use validateLength(a.Address, 0, 35, "Address must have up to 35 chars.") (type error) as type func(string) error in map value

I understand the issue is in the map declaration, in the func(...interface{}) to be more precise. This part must have the same signatures as the function I'm using as key.

So, my question is: is there any other way to declare the map that can hold functions with different signatures?

Answer 1

If I was designing this, I'd break this problem down like so:

1. I want a map of validators, each of which takes a string and returns a human-readable error.
2. I want to be able to pair a simple validator that returns a yes-or-no answer with an error message.
3. I need a set of simple validators.

Note that in all of these cases you need functions that return functions. The length validator, for example, can be:

func validateLength(min, max int) func(string) bool {
        return func(s string) bool {
                return len(s) >= min && len(s) <= max
        }
}

Then you can wrap this in a function that produces the error

func makeValidator(f func(string) bool, msg string) func(string) error {
        return func(s string) error {
                if !f(s) {
                        return errors.New(msg)
                }
                return nil
        }
}

Now you have a couple of layers that build the functions that go into the map, so you can create

rt := map[string]func(string) error {
        "Address": makeValidator(
                validateLength(0, 35),
                "Address must have up to 35 chars."
        )
}

This doesn't pair it with the specific field (and indeed the suggestion from comments to just write this in code is probably a good one) but if you needed this to really be generic you could use reflection to look at the contents of a structure.