Roger Peppe, 2018-09-05
In my previous post, I talked about an issue with a real world use case for contracts - the fact that all type parameters must be mentioned every time a contract is used in a definition. In this post, I introduce an idea for a possible way to fix this.
The most important feature of contracts, in my view, is that they bind together several types into one unified generic relationship. This means that we can define a function with several type parameters from a contract and we can be assured that they all work together as we expect.
In some sense, those type parameters really are bundled together by a contract. So... what if we changed contracts to be a little more similar to that familiar construct that bundles things together: a struct?
Here's the graph example from the design doc rewritten with this idea in mind. I've rewritten the contract to use interfaces conversions, because I think it's clearer and less ambiguous that way, but it's not crucial to this idea.
package graph
type Edger(Edge) interface {
Edges() []Edge
}
type Noder(Node) interface {
Nodes() (n1, n2 Node)
}
contract Contract(n Node, e Edge) {
Edger(n)
Noder(e)
}
type Graph(type G Contract) struct { ... }
func New(type G Contract)(nodes []G.Node) *Graph(G) { ... }
func (*Graph(G)) ShortestPath(from, to G.Node) []G.Edge { ... }
Instead of declaring all the type parameters positionally every time
we use a contract, we now use a single identifier.
This acts as a kind of "type struct" - we can select different
types from it by name using the familiar .
operator.
It's as if the contract is a type that contains other types,
which seems to me to fit nicely with its role as a
"bundler" of several types and their relationship to one another.
Note that when a contract is defined, its parameters are a little different from function parameters - not only do the parameters need to be distinct, but the type names do too, so there's no potential for ambiguity here.
Using named rather than positional type parameters means that we don't need to remember which order the type parameters are in. Passing the contract around as a whole makes the code easier and shorter.
It has the additional advantage that there's now no syntactic need for type parameters to use only one contract.
What we haven't covered yet is how we might actually create an instance of G in the first place. We can use a similar syntax to struct literals, except here we're associating type members of the contract with their actual types.
g := graph.New(graph.Contract{
Node: myNodeType,
Edge: myEdgeType,
})([]myNodeType{...})
We could even define this as its own type:
type myGraph = graph.Contract{
Node: myNodeType,
Edge: myEdgeType,
}
g := graph.New(myGraph)([]myNodeType{...})
In cases where type unification fails (for example when we need to specify a type that's in a return parameter), this should allow us to avoid the burden of passing the types each time.
Let's rewrite the example from my previous post to use this new syntax.
func PrintVariousDetails(type Mgo SessionContract)(s Mgo.Session) {
db := s.DB("mydb")
PrintBobDetails(db)
}
func PrintBobDetails(type Mgo SessionContract)(db Mgo.Database) {
iter := db.C("people").Find(bson.M{
"name": "bob",
})
... etc
}
I hope we can agree that this seems quite a bit nicer than the original.
I'm more meaning that in
the fact that
myGraph1
andmyGraph2
are not identical to each other is kind of useless. It only really makes sense to alias contract instances. The reason this matters is that if you can declare (rather than alias) contract instances, then it's no longer possible to infer the unique contract instance based on the inputs to a function, since there can be more than one instance with the same underlying type. I suppose you could just specify that the inference algorithm always infers the literal contract instance since it doesn't really make a difference.You could make them both inputs in that case. Or apply the contract to a
Graph
type and have the nodes and edges as outputs. Or just have a different contract depending on which way you need the type inference to work, likeSince you're already defining generic interfaces for each, those could probably serve in place of these contracts, with the type parameters of the interface being the outputs of the corresponding contract.
I mean that you can't infer the session type based in the database type. The type implication only goes one way, where you can be given a session type and can infer the database type based on it, but not vice versa. If your function accepts a database as its input (like
PrintBobDetails
does), but accepts a session type as its type parameter, you'll never be able to infer the type parameter representing the session type and it will have to be explicitly provided by the caller. But the body of the function doesn't actually use the session type anyways. I'm pointing out that the way around this is to have a distinctDatabaseContract
which doesn't require aSession
type in the first place.