my head hurts

withZip.swift

/**
 Accepts three functions, where the sole argument of the first two functions is a closure which each accept some ephemeral value and return some value.
 The third function accepts two parameters, the first parameter being the parameter of function A, and the second belonging to B. It's combinant value is returned.
 
 - Parameter sourceA: The first function to zip
 - Parameter sourceB: The second function to zip
 - Parameter combiner: The third function to receive the argument of both A and B
 - Returns: The return value
 */
@discardableResult
func withZip<FirstAccessorParameter, SecondAccessorParameter, ReturnValue>(
    _ firstAccessor: @escaping ((FirstAccessorParameter) throws -> ReturnValue) throws -> ReturnValue?,
    _ secondAccessor: @escaping ((SecondAccessorParameter) throws -> ReturnValue) throws -> ReturnValue,
    _ combine: (FirstAccessorParameter, SecondAccessorParameter) throws -> ReturnValue
) throws -> ReturnValue {
    // first accessor does not return nil
    guard let result = try firstAccessor ({ firstParameter in
        // second accessor does
        try secondAccessor { secondParameter in
            try combine(firstParameter, secondParameter)
        }
    }) else {
        throw CombinantError.firstFailure
    }
    
    return result
}

@discardableResult
func withZip<FirstAccessorParameter, SecondAccessorParameter, ReturnValue>(
    _ firstAccessor: @escaping ((FirstAccessorParameter) throws -> ReturnValue) throws -> ReturnValue,
    _ secondAccessor: @escaping ((SecondAccessorParameter) throws -> ReturnValue) throws -> ReturnValue?,
    _ combine: (FirstAccessorParameter, SecondAccessorParameter) throws -> ReturnValue
) throws -> ReturnValue {
    // first accessor does not return nil
    try firstAccessor { firstParameter in
        // second accessor does
        guard let secondResult = try secondAccessor ({ secondParameter in
            try combine(firstParameter, secondParameter)
        }) else {
            throw CombinantError.secondFailure
        }
        
        return secondResult
    }
}

@discardableResult
func withZip<FirstAccessorParameter, SecondAccessorParameter, ReturnValue>(
    _ firstAccessor: @escaping ((FirstAccessorParameter) throws -> ReturnValue) throws -> ReturnValue,
    _ secondAccessor: @escaping ((SecondAccessorParameter) throws -> ReturnValue) throws -> ReturnValue,
    _ combine: (FirstAccessorParameter, SecondAccessorParameter) throws -> ReturnValue
) rethrows -> ReturnValue {
    // first accessor does not return nil
    try firstAccessor { firstParameter in
        // second accessor does
        try secondAccessor { secondParameter in
            try combine(firstParameter, secondParameter)
        }
    }
}