[Fix #1061] Add cider-find-ns

Provide a completing read of available namespaces and jump to the file
containing its definition.

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 ### New features
 
+* [#1061](https://github.com/clojure-emacs/cider/issues/1061) New command `cider-find-ns` which prompts for an ns and jumps to the corresponding source file.
 * [#1019](https://github.com/clojure-emacs/cider/pull/1019): New file, cider-debug.el.
   Provides a new command, `cider-debug-defun-at-point`, bound to <kbd>C-u C-M-x</kbd>.
   Interactively debug top-level clojure forms.

cider-interaction.el

@@ -214,7 +214,7 @@ if the candidate is not namespace-qualified."
 (defvar cider-required-nrepl-ops
   '("apropos" "classpath" "complete" "eldoc" "format-code" "format-edn" "info"
     "inspect-pop" "inspect-push" "inspect-refresh"
-    "macroexpand" "ns-list" "ns-vars" "refresh"
+    "macroexpand" "ns-list" "ns-vars" "ns-path" "refresh"
     "resource" "stacktrace" "toggle-trace-var" "toggle-trace-ns" "undef")
   "A list of nREPL ops required by CIDER to function properly.
 
@@ -924,6 +924,31 @@ thing at point."
                  #'cider--find-var-other-window
                #'cider--find-var))))
 
+(defun cider-sync-request:ns-path (ns)
+  "Get the path to the file containing NS."
+  (-> (list "op" "ns-path"
+            "ns" ns)
+      nrepl-send-sync-request
+      (nrepl-dict-get "path")))
+
+(defun cider--find-ns (ns &optional other-window)
+  (-if-let (path (cider-sync-request:ns-path ns))
+      (cider-jump-to (cider-find-file path) nil other-window)
+    (error "Can't find %s" ns)))
+
+(defun cider-find-ns (&optional arg ns)
+  "Find the file containing NS.
+
+A prefix of `-` or a double prefix argument causes
+the results to be displayed in a different window."
+  (interactive "P")
+  (cider-ensure-op-supported "ns-path")
+  (if ns
+      (cider--find-ns ns)
+    (let* ((namespaces (cider-sync-request:ns-list))
+           (ns (completing-read "Find namespace: " namespaces)))
+      (cider--find-ns ns (cider--open-other-window-p arg)))))
+
 (define-obsolete-function-alias 'cider-jump-to-resource 'cider-find-resource "0.9.0")
 (define-obsolete-function-alias 'cider-jump-to-var 'cider-find-var "0.9.0")
 (defalias 'cider-jump-back 'pop-tag-mark)