[swift-evolution] [Pitch] Introducing the "Unwrap or Die" operator to the standard library

[Paul Cantrell]

> On Jun 28, 2017, at 9:50 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Wed, Jun 28, 2017 at 8:54 PM, Paul Cantrell <paul at bustoutsolutions.com <mailto:paul at bustoutsolutions.com>> wrote:
> 
>> On Jun 28, 2017, at 8:32 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> I would like to see an example where this string plausibly makes the difference between having to hunt down the code and not have to do so. I do not believe that "array must not be empty" or "array guaranteed non-empty" is such an example, and I cannot myself imagine another scenario where it would make such a difference.
> 
> You needn’t imagine. There was one up-thread:
> 
>   let paramData = params.data(using: String.Encoding.ascii)!
> 
> Huh? Why is force unwrap safe here? OK, the code plainly says the author thinks that `params` must already be ASCII, but why is that a safe assumption? What reasoning lead to that? What other sections of the code does that reasoning depend on? If we get a crash on this line of code, what chain of assumptions should we follow to discover the change that broke the original author’s reasoning behind the force unwrap?
> 
> This is a job for a comment:
> 
>   let paramData = params.data(using: String.Encoding.ascii)!  // params is URL-escaped, thus already ASCII
> 
> Aha, it’s URL escaped.
> 
> That comment does not repeat information already stated in the code itself. It does what any good comment does: it explains intent, context, and rationale. It doesn’t restate _what_, but rather explains _why_.
> 
> For those who appreciate comments like that, this proposal simply allows them to surface at runtime:
> 
>   let paramData = params.data(using: String.Encoding.ascii) !! "params is URL-escaped, thus already ASCII"
> 
> And those who see no value in such a runtime message — and thus likely also see no value such a comment — are free not to use either.
> 
> If this is the most convincing example, then I'd actually be adamantly _against_ such an operator (where now I'm merely skeptical and would like to see evidence of usefulness). This example is, quite simply, _wrong_. Here's why:
> 
> First, if force unwrapping fails, the message should explain why it failed: the reason why it failed is _not_ because it's URL-escaped and _not_ because it's ASCII, but rather because it's **not** ASCII.

Fine, then:

    let paramData = params.data(using: String.Encoding.ascii) !! “params must be URL-escaped, and thus ASCII"

…or format the runtime message to fit that style of phrasing:

    fatal error: unexpectedly found nil while unwrapping an Optional value
    Failing underlying assumption:
        params is URL-escaped, thus already ASCII
    Current stack trace:
        …

> Second, even supposing the wording were fixed, it's at best not more useful than `!` and at worst misleading.
…
> If the error message is "params not URL-escaped," then it's misleading, as that's not at all what the LHS is actually asserting: it can be unwrapped *whether or not* it's URL-escaped and it only matters that it's ASCII.

Yes, of _course_ it’s not what the LHS is actually asserting. That is precisely the point of having a message. There is pertinent information not already present in the code.

The message describes an invariant not captured by the type system. In other words, the author of this code believes they have guaranteed something that the compiler itself cannot check. Thus this statement is exactly backwards:

> You **absolutely cannot** proceed from this point in the code assuming that `paramData` is a URL-escaped string.

The author of this code is telling you they have _already_ proceeded from this point assuming that `paramData` is URL-escaped. They may have assumed wrong, but even if they did, this insight into the developer’s thinking is valuable.

Indeed, though we try to minimize it, there will inevitably be code that relies on assumptions the compiler cannot check. That is why the language has ! in the first place. Those uncheckable assumptions are not always self-evident. This is why languages provide comments and diagnostic messages.

Literalism nit-picking over the semantics of the RHS deliberately miss the original argument: the RHS is information-bearing.

Cheers, P

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