Thanks for the suggestion. I agree that this would be helpful, and should theoretically not be that difficult to add.

Transferred to the Go issue tracker, as this would be implemented by Gopls.

This sounds fun, do you mind if I give it a try?
If I understood this correctly, when asking for code actions while on an undefined symbol that is a function call we should give the option to create an empty function with a signature that satisfies what that symbol is trying to do? Arguments would be what's being passed to the function, do we want to look at the "left" side too for return values?

And re: argument names: if variables are passed to the function call like in @enkeyz example we could use those in the signature, what if instead this was the case?

sum(1, 2)
// or
reverse("string to reverse")

I guess for non primitive types we could use a camel case version of the type name? (e.g. net.Conn => conn)
And for primitive types maybe the initial char? (e.g. string => s)
And for multiple arguments with the same type maybe append a number? For the sum(1, 2) example that would be

func sum(i1, i2 int) {}

Sorry jumping ahead a bit maybe, just trying to think of edge cases to keep that generated function compilable.

sum(1, 2) or sum([1, 2]): this also can be a variadic function: func sum(nums ...int) int {}
and func sum(nums ...{}interface) int {}

There are so many edge cases.

@rentziass: Yes, you can definitely give this a try! You should be able to match on the error code that says that the interface isn't implemented, and then you will need to extract the expected function signature from the error message. I wouldn't focus on edge cases to start--the implementation doesn't need to be perfect, since it's a user-initiated action. You're right to say that this should be a code action for a given diagnostic.

Hey @stamblerre, it's been a while! Cool, I'll give this shot (although I'll probably come back with more questions). As per the signature for the time being I'd focus on arguments rather than returned values, it sounds easier although I might be wrong again.

@rentziass, I'd recommend starting this outside of the gopls codebase, using just go/ast and go/types. Given a package with a single undeclared name error, can you generate a stub that makes the error go away? This is the crux of the problem, but integrating with gopls' APIs for type information and diagnostics can be distracting. If you can solve the problem outside of gopls, we can help you integrate it as a quickfix.

@findleyr thanks a lot, that's very useful!

So I've put together something that works (or seems to work). I'm probably gathering the data I need to generate those stubs in a poor manner, so please let me know how it can be improved.

I've added a bunch of test cases I used to validate what I was doing (still staying well away from any possible values an undeclared func might be intended to return).

For arguments names, if an identifier was used the name of that identifier is reused, otherwise names are inferred by the type of the argument, e.g.:

• int -> i
• error -> err
• net.Conn -> conn
• []int -> i

Also, if two arguments share the same identifier and/or type, argument name uniqueness is ensured, for example:

c("some", "strings")

// results in
func c(s1 string, s2 string) {}

For the sake of this conversation, I'm reporting below what code fixed by generating function stubs for each case looks like right now, so whoever is interested in checking the outputs doesn't have to dive into code:

package missingfunction

func a(s string) {
        u(s)
}

func u(s string) {}

package missingfunction

import "io"

func a(r io.Reader) {
        u(r)
}

func u(r io.Reader) {}

package missingfunction

type T struct {}

func a() {
        pointer := &T{}
        var value T
        u(pointer, value)
}

func u(pointer *T, value T) {}

package missingfunction

func a(s string, i int) {
        u(s, i, s)
}

func u(s1 string, i int, s2 string) {}

package missingfunction

type T struct {}

func a() {
        u("hey compiler", T{}, &T{})
}

func u(s string, t1 T, t2 *T) {}

package missingfunction

import "io"

func a() {
        u(io.MultiReader())
}

func u(reader io.Reader) {}

package missingfunction

func a() {
        var err error
        u(err)
}

func u(err error) {}

package missingfunction

func a() {
        u(b())
}

func b() (string, error) {
        return "", nil
}

func u(s string, err error) {}

package missingfunction

import "time"

func a() {
        u(10 * time.Second)
}

func u(duration time.Duration) {}

package missingfunction

func a() {
        u([]int{1, 2})
}

func u(i []int) {}

If what's there is actually usable/useful, what's the plan now? :) also thanks for having me doing this, I'm having a blast!

This looks awesome, thanks for sharing this update @rentziass! I think the next step here would be to integrate this logic into the gopls codebase. This should be a "type error analyzer", which is an analysis that provides quick fixes for type errors. You can find some examples here: https://github.com/golang/tools/tree/master/internal/lsp/analysis. Fillreturns (https://github.com/golang/tools/blob/master/internal/lsp/analysis/fillreturns/fillreturns.go) may be a good one to look at. Basically, you will want to create another folder in this directory named something like "implementmissing" and put the logic in an analyzer. You can add your test cases there as well, and then you can add your type error analyzer here: https://github.com/golang/tools/blob/a55d5155d907b5d1114e99daae56070cc0253114/internal/lsp/source/options.go#L1165.

With regards to naming parameters, you can also look at the code that does this in autocompletion here.

Would it be too big to ask, if you could implement this for methods too, not just functions?

go s.scannerWorker(jobs, result)

to

func (s *domainScanner) scannerWorker(jobs <-chan string, result chan<- *scanReport) {}

for example.

Anyway, thank you for your work @rentziass

@enkeyz that's something I personally would have done after this piece, to keep this relatively small and reasonable. Another thing I'd like to look at as well is returned values expected for the function, i.e.

var err error
err = myFunc() # undeclared myFunc

should generate

func myFunc() error {}

But again, as iterations on top of this.

@stamblerre I'm finally getting to integrate this, although I noticed a todo in analysis/undeclaredname to handle the calling of undeclared functions. Given that undeclared name is the same type error I'd be handling here should I just add the ability to handle undeclared functions in that package to keep all of that in the same place? I'll start doing that, if you come back with a denial extracting to its own separate package shouldn't be that big of a deal :)

Nope, I get it now, suggested fixes don't come out paired with undeclaredname diagnostics, separate package it is :D

Change https://golang.org/cl/348829 mentions this issue: internal/lsp/analysis/implementmissing: add analyzer

This went in gopls/v0.7.3, should this issue remain open until the generated function considers returned values too?

Thanks for adding this feature, but there's one issue. When doing TDD, you write the tests first, so sometimes the function, or method doesn't exists.

So when you use this auto implement feature, it creates the function or method in the test file, instead of the normal file, where you put the package code.

Is this something you can solve?

@rentziass Also, yeah, it should consider return values imo. Awesome job btw.

@enkeyz just a year late, sorry 😅 not sure this is something you can solve, test code is still part of a package and depending on whether you're using a _test package or not it might even be the same package as your non-test code.

@findleyr is it ok if I start looking into adding expected returned values too? After sometime spent as a user of this analyzer it really is a bit of a missing feature.

@rentziass sounds good, please let us know how we can help.

I change the title to reflect the remaining missing functionality: the 'undeclared' analyzer currently implements the missing function, but does not yet infer its return types from the context of the call.

Change https://go.dev/cl/560475 mentions this issue: gopls/internal/analysis/undeclaredname: improve fix name

Change https://go.dev/cl/623156 mentions this issue: gopls/internal/analysis/undeclaredname: merge into CodeAction