Question: Go provides two ways of handling errors, but I'm not sure which one to use.

Assuming I'm implementing a classic `ForEach` function which accepts a slice or a map as an argument. To check whether an iterable is passed in, I could do:

```
func ForEach(iterable interface{}, f interface{}) {
    if isNotIterable(iterable) {
        panic("Should pass in a slice or map!")
    }
}

```

or 

```
func ForEach(iterable interface{}, f interface{}) error {
    if isNotIterable(iterable) {
        return fmt.Errorf("Should pass in a slice or map!")
    }
}

```

I saw some discussions saying `panic()` should be avoided, but people also say that if program cannot recover from error, you should `panic()`.

Which one should I use? And what's the main principle for picking the right one?

Answer: 
**Use `panic`.**

Because your use case is to **catch a bad use of your API**. This should never happen at runtime if the program is calling your API properly.

In fact, any program calling your API with correct arguments will behave in the same way *if the test is removed*. The test is there only to fail early with an error message helpful to the programmer that did the mistake. Ideally, the panic might be reached once during development when running the testsuite and the programmer would fix the call even before committing the bad code, and that incorrect use would never reach production.

See also [this reponse](https://stackoverflow.com/a/22865084/328115) to question *Is function parameter validation using errors a good pattern in Go?*.
Ask yourself these questions:

* Do you expect the exceptional situation to occur, regardless how well would you code your app? Do you think it should be useful to make the user aware of such condition as part of the normal usage of your app? Handle it as an error, because it concerns the application as working normally.
* Should that exceptional situation NOT occur if you code appropriately (and somewhat defensively)? (example: dividing by zero, or accessing an array element out of bounds) Is your app totally clueless under that error? Panic.
* Do you have your API and want to ensure users use it appropriately? Panic. Your API will seldom recover if used incorrectly.