> Am 06.06.2016 um 00:53 schrieb Charles Srstka <[email protected]>:
>
>> On Jun 5, 2016, at 5:41 PM, Michael Peternell via swift-evolution
>> <[email protected]> wrote:
>>
>>> Am 05.06.2016 um 20:26 schrieb Антон Жилин via swift-evolution
>>> <[email protected]>:
>>>
>>> The following names were suggested: NoReturn, Bottom, None, Never.
>>> I would pick None, because it looks like opposite to Any and fits nicely in
>>> generic types.
>>
>> I would like to call the type `Void`. `Void` is a type with zero possible
>> values. The current `Void` would have to be renamed to `Unit`, the data type
>> with exactly one possible value. Less C, More Haskell :) But wait, that's
>> not really Haskell-ish, and it's not C-ish either.
>
> That is the most confusing possible thing that this could possibly be called.
> Seeing a Void return will cause anyone coming from a C, C++, ObjC, etc.
> background to think of something that is definitely *not* a no-return
> function.
I agree. For this reason, my email continued after that paragraph. I
deliberately ended the last sentence of that paragraph with "But wait, that's
not really..."
There is a really really good reason why Haskell doesn't have a named "bottom
type" like "None" or "Void". The bottom type has type `forall a. a`,
corresponding to the mathematical fact that from "false" follows anything.
Assigning a bottom type to a variable makes only sense in Haskell, because it
uses lazy evaluation. With strict evaluation, a bottom type is just confusing
at best. And a `Nothing` type corresponds to Haskell's `Void` type, not to its
bottom type - a really exotic type that is seldom used. Its not consistent to
have a @noreturn-function return `Nothing` instead, because `Nothing` that's
not a proper type. You can write a foo-function that is really unintuitive:
func foo() -> NoReturn {
let x = fatalError("crash please")
print("1+1=2")
return x
}
Shouldn't we all just try to understand the rationale for the current language
behavior before we try to propose changes?
E.g. with the proposal, the following function:
@noreturn func error<T>(msg: String = "") -> T {
fatalError("bottom: \(msg)")
}
has to be written as
func error<T>(msg: String = "") -> T {
fatalError("bottom: \(msg)")
}
It still returns bottom, but there is no way to say so in the declaration. The
proposal just made the language less expressive!
See also:
https://en.wikibooks.org/wiki/Haskell/Denotational_semantics
for a more in-depth explanation why "bottom" is not an ordinary type with a
name.
-Michael
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