feedback from my first experiment with generic go code

[Jochen Voss]

Dear all,

Today I tried out the new generics support in Go for the first time by implementing Dijkstra's algorithm for the shortest path in a directed graph.  In case it's interesting to somebody, here are my impressions, code is at https://github.com/seehuhn/go-dijkstra .

The key parts of my code are:

type Vertex[edge any] interface {
comparable
Edges() []edge
}

type Edge[vertex any, length constraints.Ordered] interface {

From() vertex

To() vertex

Length() length

}

func ShortestPath[edge Edge[vertex, length], vertex Vertex[edge], length constraints.Ordered](start, end vertex) ([]edge, error) { ...}

My comments/questions:

• I found generics reasonably easy to use.  This was my first experience with generics, and I went wrong only once.  Overall, writing the code was straightforward.
• Since I am using the beta release, I have to type "go1.18beta2" to call go.  Sometimes I typed "go" accidentally, and I found the resulting error messages often confusing and unhelpful.  For example, "go test" says "package constraints is not in GOROOT".  I wonder whether in case of errors the go command should show a message if the go version in go.mod is newer than the current version.
• "go doc" is still sometimes confused about generics.  When I type "go1.18beta2 doc seehuhn.de/go/dijkstra Vertex" the output claims that Vertex "has unexported methods".  The message is misleading, it is cause by Vertex embedding "comparable".  Is this worth reporting as a bug?
• To me it is not intuitive why "comparable" is built intro the language but "Ordered" is in an external package.
• I have not managed to make it possible for ShortestPath() to automatically infer the type parameters.  Is there a trick I have missed?
• Currently I am using constraints.Ordered for the edge length.  This currently works, but I'm slightly cheating here since I am also using "+" on the edge lengths.  If in the future new ordered types were added to  constraints.Ordered, my code might break.  Is there a way to say "I need <= and +" (short of enumerating all types)?
• When checking for negative edge lengths, I need the zero value for my instance of constraints.Ordered.  Currently I am using a separate, uninitialised variable for this.  Is there a better way to say "x <= 0" when x is of a type which matches constraints.Ordered?
• I needed to reverse a slice in-place.  Would a function to do this make a good addition to golang.org/x/exp?
• The "func ShortestPath..." line is hard to read and quite long.  I didn't find a good place for a line break either.  Any ideas for making this more readable?

I'd be happy to receive feedback about my code.

All the best,

Jochen

[Jan Mercl]

On Wed, Feb 16, 2022 at 5:31 PM Jochen Voss <joche...@gmail.com> wrote:

> To me it is not intuitive why "comparable" is built intro the language but "Ordered" is in an external package.

Quoting rogpepe from elsewhere:

""""
because there's no way to define comparable as a type constraint, but
you can do that with Ordered because there are a known number of
underlying types that can implement Ordered
""""