Rework how the mode is reported in fish_vi_mode

Add a new function fish_mode_prompt which (if it is defined) has its output
prepended to the left prompt. Rather than replacing the prompt wholesale, make
fish_vi_mode enable this function by setting a variable __fish_vi_mode. This
enables vi mode to interoperate nicely with custom prompts. Users who want
to change how the mode is reported can either redefine this function or
erase it entirely. Fixes #1988.

reader.cpp

@@ -128,6 +128,10 @@ commence.
 #define RIGHT_PROMPT_FUNCTION_NAME L"fish_right_prompt"
 
 
+/* The name of the function for getting the input mode indicator */
+#define MODE_PROMPT_FUNCTION_NAME L"fish_mode_prompt"
+
+
 /**
    The default title for the reader. This is used by reader_readline.
 */
@@ -997,6 +1001,18 @@ static void exec_prompt()
     {
         proc_push_interactive(0);
 
+        // Prepend any mode indicator to the left prompt (#1988)
+        if (function_exists(MODE_PROMPT_FUNCTION_NAME))
+        {
+            wcstring_list_t mode_indicator_list;
+            exec_subshell(MODE_PROMPT_FUNCTION_NAME, mode_indicator_list, apply_exit_status);
+            // We do not support multiple lines in the mode indicator, so just concatenate all of them
+            for (size_t i = 0; i < mode_indicator_list.size(); i++)
+            {
+                data->left_prompt_buff += mode_indicator_list.at(i);
+            }
+        }
+
         if (! data->left_prompt.empty())
         {
             wcstring_list_t prompt_list;

share/functions/fish_mode_prompt.fish

@@ -0,0 +1,19 @@
+# The fish_mode_prompt function is prepended to the prompt
+function fish_mode_prompt --description "Displays the current mode"
+  # Do nothing if not in vi mode
+  if set -q __fish_vi_mode
+    switch $fish_bind_mode
+      case default
+        set_color --bold --background red white
+        echo '[N]'
+      case insert
+        set_color --bold --background green white
+        echo '[I]'
+      case visual
+        set_color --bold --background magenta white
+        echo '[V]'
+    end
+    set_color normal
+    echo -n ' '
+  end
+end

share/functions/fish_vi_mode.fish

@@ -1,6 +1,8 @@
 function fish_vi_mode
-  function fish_prompt
+  # Set the __fish_vi_mode variable
-    fish_vi_prompt
+  # This triggers fish_mode_prompt to output the mode indicator
-  end
+  set -g __fish_vi_mode 1
+  
+  # Turn on vi keybindings
   set -g fish_key_bindings fish_vi_key_bindings
 end

share/functions/fish_vi_prompt.fish

@@ -1,51 +0,0 @@
-function fish_vi_prompt_cm --description "Displays the current mode"
-  echo -n " "
-  switch $fish_bind_mode
-    case default
-      set_color --bold --background red white
-      echo "[N]"
-    case insert
-      set_color --bold --background green white
-      echo "[I]"
-    case visual
-      set_color --bold --background magenta white
-      echo "[V]"
-  end
-  set_color normal
-end
-
-function fish_vi_prompt --description "Simple vi prompt"
-
-	# Just calculate these once, to save a few cycles when displaying the prompt
-	if not set -q __fish_prompt_hostname
-		set -g __fish_prompt_hostname (hostname|cut -d . -f 1)
-	end
-
-	if not set -q __fish_prompt_normal
-		set -g __fish_prompt_normal (set_color normal)
-	end
-
-	switch $USER
-
-		case root toor
-
-		if not set -q __fish_prompt_cwd
-			if set -q fish_color_cwd_root
-				set -g __fish_prompt_cwd (set_color $fish_color_cwd_root)
-			else
-				set -g __fish_prompt_cwd (set_color $fish_color_cwd)
-			end
-		end
-
-		echo -n -s "$USER" @ "$__fish_prompt_hostname" ' ' "$__fish_prompt_cwd" (prompt_pwd) "$__fish_prompt_normal" (fish_vi_prompt_cm) '# '
-
-		case '*'
-
-		if not set -q __fish_prompt_cwd
-			set -g __fish_prompt_cwd (set_color $fish_color_cwd)
-		end
-
-		echo -n -s "$USER" @ "$__fish_prompt_hostname" ' ' "$__fish_prompt_cwd" (prompt_pwd) "$__fish_prompt_normal" (fish_vi_prompt_cm) '> '
-
-	end
-end