From: cross@spitfire.i.gajendra.net
In article <10fig54$1n41a$3@dont-email.me>,
Arne Vajhøj wrote:
>On 11/18/2025 2:04 PM, Arne Vajhøj wrote:
>> On 11/15/2025 9:16 PM, Dan Cross wrote:
>>>> Also note the availability of the 'Valid attribute to make sure that
>>>> what
>>>> is in the variable after the unsafe conversion is actually a valid
>>>> value.
>>>
>>> Sum types make this trivial:
>>>
>>> impl SomeType {
>>> fn try_from(i: i32) -> Option {
>>> // if valid, return `Some(whatever`),
>>> // else return `None`.
>>> }
>>> }
>> (assuming Option in Rust is what it is in other languages)
>>
>> Option and Ada Valid are somewhat different.
>>
>> Option is a way for a function/method to either return
>> a value or return the fact that there is no value.
>>
>> A much better way to do that than traditional
>> return null or -1 or whatever to indicate there
>> is no value.
>>
>> Ada valid attribute is a runtime check on the result from
>> an unsafe conversion to see if it meets the constraints
>> of the data type.
>
>Option is in fashion in recent years.
Algebraic data types have been used in functional languages
since the 1970s, starting with the "Hope" language from
Edinburgh. ML took them from Hope and popularized them, and
they leaked into Miranda, Haskell, and OCaml from there. Now
many languages support them; even C++ (`std::optional`).
Hope was first described in a paper in 1980, but the work of
course predated that.
That makes them about as old as VMS, and older than Ada. In
other words, they've been "In Fashion" since the 70s, which as
far as all things fashion goes, is a pretty good run;
particularly considering some of the things that were popular in
that decade.
>There are not that many languages on VMS supporting
>Option.
>
>Scala does - Option[T].
>
>And Java 8 Optional is almost the same.
More's the pity.
- Dan C.
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