Resolved: Insert "!" is a bad fixit

Evolution Pitches
55

I was continuing with @QuinceyMorris's example. Let me try to further clarify what I mean.

Assume a programmer writes this:

let x = optionalReturningFunc(/*args*/)
// Do some stuff
let y = optionalReturningFunc2(/*args*/)
// Do some more stuff
let z = optionalReturningFunc3(/*args*/)
// Do the rest of the stuff

Then the programmer types this, and compiler offers to insert ! for x, y and z:

let t = function(x,y,z)

The programmer accepts compiler suggestion and it becomes:

let t = function(x!,y!,z!)

Then the programmer runs the code, and it stops with a runtime error on the above line. A novice programmer will have a hard time figuring out which one of x, y, z or function is causing the error.

Now imagine if the compiler went further and after the above fix-it was applied, generated a new warning suggesting the programmer move "I know it is not nil" assertion where this assertion actually materializes here:

let x =  optionalReturningFunc(/*args*/) // Warning: ...

The new warning could read something like this:

Warning: x is asserted not-nil at use site and is not checked or manipulated after this point.

With the following "fix it" suggestions:

  1. Put ! after the function call to assert the function result can't be nil.
  2. Put explicit optional type to silence this warning.
    (It could go even further and suggest handling nil result with if let or guard let)

If the programmer accepts the first option, the code ultimately becomes:

let x = optionalReturningFunc(/*args*/)!
// Do some stuff
let y = optionalReturningFunc2(/*args*/)!
// Do some more stuff
let z = optionalReturningFunc3(/*args*/)!
// Do the rest of the stuff
let t = function(x,y,z)

This time, when the programmer runs the code, it fails on the declaration site of either x, y, or z and our imaginary programmer will have a better idea what went wrong and probably add a check to see if the function actually returned a value.

We could go further and make things even more intelligent, but I hope you get the basic idea of putting assertions that programmer makes where the assertion actually materializes.

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