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x/tools/gopls: support implementation LSP request #32973

litleleprikon opened this issue Jul 8, 2019 · 7 comments


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commented Jul 8, 2019

What version of Go are you using (go version)?

$ go version
go version go1.12.6 darwin/amd64

Does this issue reproduce with the latest release?


What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/sh/vcq1w7n1547584p7d_kh_vf00000gn/T/go-build605958996=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Trying to get implementations of interface in go

What did you expect to see?

gopls returns the list of places with structures which implements interface

What did you see instead?

textDocument/implementation request is not implemented

@gopherbot gopherbot added this to the Unreleased milestone Jul 8, 2019


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commented Jul 8, 2019

As this feature is very valuable for my everyday development process I want to work on it if maintainers do not have any reasons against it.

@stamblerre stamblerre changed the title x/tools/gopls: Support for textDocument/implementation request x/tools/gopls: support implementation LSP request Jul 8, 2019


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commented Jul 8, 2019

@litleleprikon: If you are interested in working on this feature, please go ahead! We will be happy to review any CLs.


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commented Jul 28, 2019

@stamblerre that response was cutting a corner a bit :) because I think it gave a false sense of how difficult it is to implement :P But then again I could be totally wrong here

The good news I think is we can copy paste stuff from godoc src code who apparently has this feature already

Would appreciate for example more specific references to parts in the gopls source code to take a look at that may help in the right direction.

Also some references in the source code where godoc analysis (see link above) does it's interface implement list magic.

And maybe some CL boilerplate so somebody calling this feature can already see "Hello world" appearing as ouput and basically has a crash course example how gopls works and know where to start by just looking at the boilerplate CL code provided


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commented Aug 5, 2019

A lot of the code for this feature can actually be taken from guru, which has an implements feature (see

I intend to compile a document that explains how to begin contributing to gopls, so I won't write in much detail here, but is a good place to start looking to see how other language features are implemented in gopls. I would also suggest reading the LSP spec for textDocument/implementation, as well as taking a look here to see how to expose the feature via the LSP.

For those with specific questions, I'm available via Slack on the #gopls channel or via DM.

@stamblerre stamblerre added help wanted and removed Suggested labels Aug 8, 2019


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commented Aug 16, 2019

Actually, the author of bingo have already done this work and took the guru implementation.
He now manages the fork of gopls at
I don't know if @saibing wishes to integrate his work into the main repo, but I can try to merge the code on top of the current master.


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commented Sep 4, 2019

I think the implementation command should support function types and interface methods too. @litleleprikon Maybe add it to the issue description? Or maybe we should add separate issues for each case?

Actually, the author of bingo have already done this work and took the guru implementation.
He now manages the fork of gopls at

I've tried this and it seems to work just like "go to defintion". So, not what we want. Or probably it just doesn't work that well, and it falls back to "go to definition" if no implementation is found.

In any case, from a quick glance, guru (and bingo) seem to work on a per-request basis by:

  • Fetching all named types reachable from the current package (yes, all of them).
  • Trying to mach each one of them with the specified type.

This should be optimized so that gopls indexes this stuff, ie. as gopls typechecks (which I guess doesn't happen in every request, but incrementally in every file change), keep mappings from:

  • Named type definitions to the interfaces they implement.
  • Function declarations and literals to the function types they can be assigned to.

And direct requests to those mappings.

That tells us which types/values could implement a type. Alternatively, or additionally, it might be interesting to keep track of which values are actually assigned to the type. So, if you do this:

/* myfile.go:13 */ type myImpl struct { ... } // implements MyInterface

/* myfile.go:42 */ var i MyInterface = myImpl{}

then a implementation request on MyInterface could return both myfile.go:13 (as guru would do, only faster) and myfile.go:42.

This may be more interesting than the find-all-possible-implementors approach because if an interface or function is simple enough, many types may implement it accidentally, while only in a few places we find actual implementations.

E. g.:

type Decoder func(interface{}) error

func Foo(decoder Decoder) { ... }

// actually create a Decoder value, so return this position in a `implementation`
// response
Foo(func(interface{}) error {

// this function could also be used as a Decoder, but it isn't, and it would be
// noisy that it shows up in an `implementation` response
func Inspect(interface{}) error 

What do you think?


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commented Sep 8, 2019

Hi everyone in this chat! Thank you all for your suggestions. Excuse me for long silence. For personal reasons I had a lack of time but now I am ready to continue development of this feature.

@tcard I think your suggestion is pretty good and caching types and implementations will reduce the time for implementations search. I am right now not sure about a ways how to implement it and probably I need some time to be more familiar with caching in gopls.

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