ast-util should not raise strings, but should raise message-exceptions

[blerner]

Every use of raise("some string") renders an error box with

"some string"
(Show program execution trace...)

It would be better to raise(message-exception("some string")), which would render an error box as

some string
(Show program execution trace...)

(And it would be better still for the compiler to raise proper error values rather than just message exceptions, but this would be a good start.

[jswrenn]

The reason raise(EXPR) renders quotes is because any kind of value can be raised (and frequently is). Accordingly, the user-exception variant is used to wrap the user's value.

There's a few ways to implement the behavior you're describing. The simplest is to treat this as purely a rendering issue and have the rendering of user-exception differ depending on the type of its value field:

| user-exception(value :: Any) with:
  method render-fancy-reason(self, _, _):
    self.render-reason()
  end,
  method render-reason(self): [ED.error: [ED.para: ED.embed(self.value)]] end,
  method _output(self):
    if is-string(self.value):
      VS.vs-str(self.value)
    else:
      VS.vs-value(self.value)
    end
  end
end

[blerner]

We've talked elsewhere about having user-exception behave differently for strings vs other data, for this reason. But the broader point here is, the compiler ought not to just be raising raw strings, which was the subject of this bug report.

[jswrenn]

Oh, sorry, I missed your point; I agree. The right thing thing to do would be to create CompileError variants for these (or find one that's already applicable).

That said, there's about ~274 instances in horizon where string literals are raised (ast-util is a relatively minor offender). For internal errors the user should never see, internal-error is more applicable (and useful) than message-exception.