Quoting Andy Balholm (2018-10-20 13:30:58)
> There are some contracts in the draft design that can’t be expressed with the
> “implements” syntax as it stands. Some would be fairly simple syntax
> extensions, but others describe the relationship between two types rather
> than just the operations on a single type. No extension of interfaces (except
> perhaps generic interfaces with mutually-referential type parameters) can
> handle those.
>
> The one called convertible is an example of one that would require a fairly
> trivial extension: adding a way to declare that a method implements a type
> conversion.
>
> For an example of the other kind, take G from the graph example. I don’t
> think any of the alternate proposals can handle that situation (at least not
> nearly as succinctly):
>
> contract G(n Node, e Edge) {
> var _ []Edge = n.Edges()
> var from, to Node = e.Nodes()
> }
The graph example is addressed in the context of my proposal here:
https://gist.github.com/zenhack/ad508d08c72fce6df945a49945ad826d#mutually-referential-type-parameters
Granted, it is slightly more verbose -- but I do not think that the
extra concision for this *one* example in the draft is worth introducing
an entirely new non-orthogonal concept.
The convertible example does not seem especially well motivated to me.
FormatUnsigned is defined like this:
func FormatUnsigned(type T convertible(uint64, T))(v T) string {
return strconv.FormatUint(uint64(v), 10)
}
And then used like:
s := FormatUnsigned(rune)('a')
But why not:
func FormatUnsigned(v uint64) string {
return strconv.FormatUint(v, 10)
}
// ...
s := FormatUnsigned(uint64('a'))
I'm quite willing to discard convertible without a more compelling
example.
> Even if we give up on the ability to define relationships between types,
> adding extensions for all the other operations that people will want support
> for will likely take away much of the initial simplicity that makes the
> “implements” syntax attractive—one trivial extension at a time. The
> “contract” proposal, on the other hand, is actually a simple, powerful idea;
> it's working out the implications that ties people’s brains in knots.
Eric has said this elsewhere, but his suggestion (and operator
overloading generally) is not a complete solution (nor is it
intended to be). However, it complements my own proposal (see
above) in a way that allows it to abstract over operators as well as
operators. I haven't seen any *compelling* use cases that can be dealt
with using contracts, but not interfaces + operator overloading.
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