#+TITLE: Go layer

#+TAGS: general|layer|multi-paradigm|programming

[[file:img/go.png]]

* Table of Contents                     :TOC_5_gh:noexport:
- [[#description][Description]]
  - [[#features][Features:]]
- [[#install][Install]]
  - [[#layer][Layer]]
  - [[#choosing-a-backend][Choosing a backend]]
  - [[#pre-requisites][Pre-requisites]]
- [[#configuration][Configuration]]
  - [[#execution][Execution]]
  - [[#indentation][Indentation]]
  - [[#autoformat][Autoformat]]
  - [[#linting][Linting]]
  - [[#tests][Tests]]
  - [[#coverage][Coverage]]
  - [[#guru][Guru]]
  - [[#auto-completion][Auto-completion]]
  - [[#lsp-backend][LSP backend]]
- [[#working-with-go][Working with Go]]
  - [[#go-commands-start-with-m][Go commands (start with =m=):]]
  - [[#go-guru][Go Guru]]
  - [[#refactoring][Refactoring]]

* Description
This layer adds extensive support for the [[https://golang.org][Go]] programming language.

** Features:
- Run [[https://golang.org/cmd/gofmt/][gofmt]] / [[https://godoc.org/golang.org/x/tools/cmd/goimports][goimports]] on file save (see [[#autoformat][Autoformat]])
- Auto-completion using [[https://github.com/nsf/gocode/tree/master/emacs][go-autocomplete.el]], via the =auto-completion= layer (see [[#auto-completion][Auto-completion]])
- Source analysis using [[https://docs.google.com/document/d/1_Y9xCEMj5S-7rv2ooHpZNH15JgRT5iM742gJkw5LtmQ][go-guru]] (see [[#guru][Guru]])
- Refactoring with [[http://gorefactor.org/][godoctor]]
- Edit struct field tags with [[https://github.com/fatih/gomodifytags][gomodifytags]]
- Linting with flycheck's built-in checkers or [[https://github.com/golangci/golangci-lint][golangci-lint]] (see [[#linting][Linting]])
- Test generation via [[https://github.com/s-kostyaev/go-gen-test][go-gen-test]] (see [[#tests][Tests]])
- Coverage profile visualization (see [[#coverage][Coverage]])
- List packages faster with [[https://github.com/haya14busa/gopkgs][gopkgs]]
- Fill a structure with default values using the [[https://github.com/davidrjenni/reftools/tree/master/cmd/fillstruct][fillstruct]]
- LSP backend (see [[#lsp-backend][LSP backend]])

* Install
** Layer
To use this configuration layer, add it to your =~/.spacemacs=. You will need to
add =go= to the existing =dotspacemacs-configuration-layers= list in this file.

** Choosing a backend
To choose a default backend set the layer variable =go-backend=:

#+BEGIN_SRC elisp
  (go :variables go-backend 'go-mode)
#+END_SRC

Alternatively the =lsp= backend will be automatically chosen if the layer =lsp=
is used and you did not specify any value for =go-backend=.

Backend can be chosen on a per project basis using directory local variables
(files named =.dir-locals.el= at the root of a project), an example to use the
=lsp= backend:

#+BEGIN_SRC elisp
  ;;; Directory Local Variables
  ;;; For more information see (info "(emacs) Directory Variables")

  ((go-mode (go-backend . lsp)))
#+END_SRC

*Note:* you can easily add a directory local variable with ~SPC f v d~.

** Pre-requisites
You will need =gocode=, =gogetdoc=, =godef=, =godoctor= and many others
to get all the goodies of this layer:

#+BEGIN_SRC sh
  GO111MODULE=on go get -v golang.org/x/tools/gopls@latest
  GO111MODULE=on CGO_ENABLED=0 go get -v -trimpath -ldflags '-s -w' github.com/golangci/golangci-lint/cmd/golangci-lint
  go get -u -v golang.org/x/tools/cmd/godoc
  go get -u -v golang.org/x/tools/cmd/goimports
  go get -u -v golang.org/x/tools/cmd/gorename
  go get -u -v golang.org/x/tools/cmd/guru
  go get -u -v github.com/cweill/gotests/...
  go get -u -v github.com/davidrjenni/reftools/cmd/fillstruct
  go get -u -v github.com/fatih/gomodifytags
  go get -u -v github.com/godoctor/godoctor
  go get -u -v github.com/haya14busa/gopkgs/cmd/gopkgs
  go get -u -v github.com/josharian/impl
  go get -u -v github.com/mdempsky/gocode
  go get -u -v github.com/rogpeppe/godef
  go get -u -v github.com/zmb3/gogetdoc
#+END_SRC

Make sure that the executables are in your search =$PATH=. Note that they are
installed on =$(go env GOPATH)/bin=, or =$GOBIN= if that environment variable is
defined. For information about setting up =$PATH=, check out the corresponding
section in the FAQ (~SPC h f $PATH RET~).

For best results, make sure that the =auto-completion= and =syntax-checking=
layers are enabled as well.

* Configuration
** Execution
To run the current =main= package with command line arguments, set the value of
=go-run-args= as a file local variable, e.g.

#+BEGIN_SRC emacs-lisp
  // Local Variables:
  // go-run-args: "--output run.log"
  // End:
#+END_SRC

** Indentation
By default, the tab width in Go mode is 8 spaces. To use a different value, set
the layer variable =go-tab-width=, e.g.

#+BEGIN_SRC emacs-lisp
  (go :variables go-tab-width 4)
#+END_SRC

If you're using =.editorconfig= in your project, set the value to nil to avoid
conflicts, i.e.

#+BEGIN_SRC emacs-lisp
  (go :variables go-tab-width nil)
#+END_SRC

** Autoformat
To run =gofmt= before save, set the value to a non-nil, i.e.

#+BEGIN_SRC emacs-lisp
  (go :variables go-format-before-save t)
#+END_SRC

To use a different formatter, set the value of =gofmt-command=, e.g.

#+BEGIN_SRC emacs-lisp
  (go :variables gofmt-command "goimports")
#+END_SRC

** Linting
If you wish to use =golangci-lint=, set the following layer variable to non-nil:

#+BEGIN_SRC emacs-lisp
  (go :variables go-use-golangci-lint t)
#+END_SRC

Check [[https://github.com/golangci/golangci-lint][golangci-lint]] and [[https://github.com/weijiangan/flycheck-golangci-lint][flycheck-golangci-lint]] for more details.

** Tests
If you're using =gocheck= or =testify= in your project you can use the
=go-use-gocheck-for-testing= or =go-use-testify-for-testing= variable to enable
suite testing and to get single function testing to work.

Tests are run in a compilation buffer displayed in a popup window that can be
closed by pressing ~C-g~ from any other window. The variable =go-test-buffer-name=
can be customized to set the output buffer name.

To provide additional arguments to =go test=, specify =go-use-test-args=.

#+BEGIN_SRC emacs-lisp
  (go :variables go-use-test-args "-race -timeout 10s")
#+END_SRC

** Coverage
=go-coverage-display-buffer-func= controls how =go-coverage= should display
the coverage buffer. See [[https://www.gnu.org/software/emacs/manual/html_node/elisp/Choosing-Window.html][display-buffer]] for a list of possible functions.
The default value is =display-buffer-reuse-window=.

** Guru
If you would like to use the =Go Guru= bindings in your work, in your project you
will need to set the scope with ~SPC m f o~. The scope is a comma separated set
of packages, and Go's recursive operator is supported. In addition, you can
prefix it with =-= to exclude a package from searching.

** Auto-completion
For auto-completion there are actually two choices. First there is the classic
=gocode=. This has been around for quite a long time now, however =gocode= has many
shortcomings, like not being able to show documentation for built-in objects or
being fully dependent on installed binary files to provide its suggestions.

A more modern and complete solution is provided by =gogetdoc=, which is able to
precisely detect all documentations in your go projects independently from where
they have been added. This is also the recommended choice from =go-mode.el=.

To choose =gocode= nothing more needs to be done. To use =gogetdoc= you need to set
the layer variable:

#+BEGIN_SRC emacs-lisp
  (go :variables godoc-at-point-function 'godoc-gogetdoc)
#+END_SRC

If you choose to use =gocode= there are some suggestions to improve its results.
As =gocode= uses the output from installed binary files to provide its suggestions.
You have a few options to ensure you always get up to date suggestions:
- Run =go install ./...= in your package directory when you make a file change.
- Run =gocode set autobuild true= to have gocode attempt to run =go install ./...=
  for you.
- You can configure your task runner to run the =go install ./...= command on every
  file change.

** LSP backend
To enable the LSP backend, ensure that the =lsp= layer is enabled, and set the
layer variable =go-backend=:

#+BEGIN_SRC elisp
  (go :variables go-backend 'lsp)
#+END_SRC

You can check [[https://www.github.com/emacs-lsp/lsp-mode/][lsp-mode]] for the gory details.

The backend can be chosen on a per project basis using directory local variables
(files named =.dir-locals.el= at the root of a project), an example is to use the
=go-mode= backend:

#+BEGIN_SRC elisp
  ;;; Directory Local Variables
  ;;; For more information see (info "(emacs) Directory Variables")

  ((go-mode (go-backend . go-mode)))
#+END_SRC

*Note:* you can easily add a directory local variable with ~SPC f v d~.

* Working with Go
** Go commands (start with =m=):

| Key binding   | Description                                                                           |
|---------------+---------------------------------------------------------------------------------------|
| ~SPC m =~     | run "go fmt"                                                                          |
| ~SPC m e b~   | go-play buffer                                                                        |
| ~SPC m e d~   | download go-play snippet                                                              |
| ~SPC m e r~   | go-play region                                                                        |
| ~SPC m g a~   | jump to matching test file or back from test to code file                             |
| ~SPC m g c~   | open a clone of the current buffer with a coverage info (=go tool cover -h= for help) |
| ~SPC m g g~   | go jump to definition                                                                 |
| ~SPC m h h~   | godoc at point                                                                        |
| ~SPC m i a~   | add import                                                                            |
| ~SPC m i g~   | goto imports                                                                          |
| ~SPC m i r~   | remove unused import                                                                  |
| ~SPC m r n~   | go rename                                                                             |
| ~SPC m t P~   | run "go test" for the current package and all packages under it                       |
| ~SPC m t g f~ | generate tests for all exported functions                                             |
| ~SPC m t g F~ | generate tests for all functions                                                      |
| ~SPC m t g g~ | DWIM generate test for the function in the active region                              |
| ~SPC m t p~   | run "go test" for the current package                                                 |
| ~SPC m t s~   | run "go test" for the suite you're currently in (requires gocheck)                    |
| ~SPC m t t~   | run "go test" for the function you're currently in (while you're in a _.test.go file) |
| ~SPC m x x~   | run "go run" for the current 'main' package                                           |

** Go Guru

| Key binding | Description                                          |
|-------------+------------------------------------------------------|
| ~SPC m f <~ | go-guru show possible callers                        |
| ~SPC m f >~ | go-guru show call targets                            |
| ~SPC m f c~ | go-guru show channel sends/receives                  |
| ~SPC m f d~ | go-guru describe symbol at point                     |
| ~SPC m f e~ | go-guru show possible contants/types for error value |
| ~SPC m f f~ | go-guru show free variables                          |
| ~SPC m f i~ | go-guru show implements relation                     |
| ~SPC m f j~ | go-guru jump to symbol definition                    |
| ~SPC m f o~ | go-guru set analysis scope                           |
| ~SPC m f p~ | go-guru show what the select expression points to    |
| ~SPC m f r~ | go-guru show referrers                               |
| ~SPC m f s~ | go-guru show callstack                               |

** Refactoring

| Key binding | Description                                                     |
|-------------+-----------------------------------------------------------------|
| ~SPC m r d~ | Add comment stubs                                               |
| ~SPC m r e~ | Extract code as new function                                    |
| ~SPC m r f~ | Add field tags (with =gomodifytags=)                            |
| ~SPC m r F~ | Remove field tags (with =gomodifytags=)                         |
| ~SPC m r i~ | Generate method stubs for implementing an interface (=go-impl=) |
| ~SPC m r n~ | Rename (with =godoctor=)                                        |
| ~SPC m r N~ | Rename (with =go-rename=)                                       |
| ~SPC m r s~ | Fill structure with default values (with =go-fillstruct=)       |
| ~SPC m r t~ | Toggle declaration and assignment                               |