Please answer these questions before submitting your issue. Thanks!

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

go version go1.7 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/niels/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/bb/xxdvfvm103n8p4vktz66d7d40000gn/T/go-build040892158=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"

I'm using go-guru with Emacs 24.5.1 from emacsformacosx.com, here's my M-x version RET output:

GNU Emacs 24.5.1 (x86_64-apple-darwin13.4.0, NS apple-appkit-1265.21) of 2015-04-10 on builder10-9.porkrind.org

1. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good.
   A link on play.golang.org is best.

• Open Emacs with go-guru.el from this commit loaded
• Type M-x customize-group RET go-guru RET
• Type M-x eval-expression RET (setq go-guru-build-tags "one") RET
• Type M-x toggle-debug-on-error RET
• Run any go-guru command that calls go-guru--command

1. What did you expect to see?

Normal guru output.

1. What did you see instead?

The following stacktrace is produced:

Debugger entered--Lisp error: (wrong-type-argument sequencep 111)
  mapconcat(identity "one" ",")
  (format "-tags=%s" (mapconcat (quote identity) go-guru-build-tags ","))
  eval((format "-tags=%s" (mapconcat (quote identity) go-guru-build-tags ",")) nil)
  eval-expression((format "-tags=%s" (mapconcat (quote identity) go-guru-build-tags ",")) nil)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)

This seems to have been broken by this commit. Note that the stacktrace is only produced if go-guru-build-tags is a non-empty string, which explains why this bug was not immediately noticed.

I see a few possible solutions.

1. Change the following line in go-guru--command from

(format "-tags=%s" (mapconcat 'identity go-guru-build-tags ","))

back to just

"-tags" go-guru-build-tags

If choosing this solution it would probably be good to expand the docstring for go-guru-build-tags to indicate that the value must be comma-separated when including multiple build tags.

1. Change the declaration for go-guru-build-tags to a list of strings

(defcustom go-guru-build-tags '()
  "Build tags passed to guru."
  :type '(repeat string)
  :group 'go-guru)

and leave go-guru--command as it currently is. This solution seems better except that it would break anyone currently setting go-guru-build-tags as a string. Therefore, I'm not sure which one should be preferred.

/cc @alandonovan, @dominikh