On Tuesday, July 30, 2019 at 7:47:05 PM UTC+1, Axel Wagner wrote:
>
> On Tue, Jul 30, 2019 at 8:31 PM <[email protected] <javascript:>> wrote:
>
>> My suggestion was that you can't use a pointer type as a type parameter
>> if the latter is subject to a contract.
>>
>
> I'm not sure I understand you. Wouldn't that preclude using a generic map
> with pointers as keys?
>
No, it wouldn't preclude that but the key would need to expressed as a *K
rather than a K, if K were subject to a contract. As a pointer type it
would automatically follow that *K was comparable.
>
>
>> In the case you mention, the contract could be expressed as a disjunction
>> of value and pointer methods:
>>
>> contract stringer(T) {
>> T String() string, *T String() string
>> }
>>
>
> Currently, Disjunctions only apply to a single type. You can't form
> expressions like this.
> IMO that's a good restriction to maintain. Because the more powerful the
> contract language becomes, the harder it'll be to make it useful.
>
Well, currently you can't use *T as a method receiver type in a contract so
this would be a necessary exception to that rule if my suggestion were
adopted.
However, I agree with your general point that the restriction should be
maintained in all other circumstances.
>
>
>> On the other hand and more generally, not knowing whether the type
>> parameter represented a pointer or a value might lead to some awkward
>> coding. For example, you wouldn't be able to de-reference the type argument
>> as it might not be a pointer.
>>
>
> If a generic function wants to de-reference an argument, it should specify
> that as a pointer: func f(type T) (p *T)
> This is the same as with slices, maps, channels, functions or any
> composite type - you can't express "type parameter T should be a slice of
> some kind", because you are instead expected to just specify []T if you
> want a slice.
>
Yes, but if T happened to be a pointer to some type, then *T would be a
double pointer to that type. As the design currently stands, you'd have no
way of knowing whether T was a pointer or not unless the contract specified
that it was one of the predefined types.
What I was trying to suggest here is that it would be helpful in some
circumstances to know whether T was or was not a pointer type which would
be a by-product of my suggestion.
Alan
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