Complete arguments to "echo -to-file"

Including this here because grandparent parent commit broke completions
for "edit -fifo".

7 files changed, 157 insertions, 123 deletions

diff --git a/src/command_manager.cc b/src/command_manager.cc
index caae2776..22b365ea 100644
--- a/src/command_manager.cc
+++ b/src/command_manager.cc
@@ -831,7 +831,12 @@ Completions CommandManager::complete(const Context& context,
                         : Completions{start+1, cursor_pos, std::move(switches), Completions::Flags::Menu};
             }
             case ParametersParser::State::SwitchArgument:
-                return Completions{};
+            {
+                const auto& switch_desc = command.param_desc.switches.get(raw_params.at(raw_params.size() - 2).substr(1_byte));
+                if (not *switch_desc.arg_completer)
+                    return Completions{};
+                return offset_pos(requote((*switch_desc.arg_completer)(context, flags, raw_params.back(), pos_in_token), token.type), start);
+            }
             case ParametersParser::State::Positional:
                 break;
         }
diff --git a/src/commands.cc b/src/commands.cc
index 5dc5f3f7..6edc308b 100644
--- a/src/commands.cc
+++ b/src/commands.cc
@@ -128,6 +128,27 @@ auto filename_completer = make_completer(
                                             cursor_pos, FilenameFlags::Expand),
                                             menu ? Completions::Flags::Menu : Completions::Flags::None}; });
 
+template<bool menu>
+auto filename_arg_completer =
+    [](const Context& context, CompletionFlags flags, StringView prefix, ByteCount cursor_pos) -> Completions
+    { return { 0_byte, cursor_pos,
+               complete_filename(prefix,
+                                 context.options()["ignored_files"].get<Regex>(),
+                                 cursor_pos, FilenameFlags::OnlyDirectories),
+               menu ? Completions::Flags::Menu : Completions::Flags::None }; };
+
+auto client_arg_completer =
+    [](const Context& context, CompletionFlags flags, StringView prefix, ByteCount cursor_pos) -> Completions
+    { return { 0_byte, cursor_pos,
+               ClientManager::instance().complete_client_name(prefix, cursor_pos),
+               Completions::Flags::Menu }; };
+
+auto arg_completer = [](auto candidates) -> PromptCompleter {
+    return [=](const Context& context, CompletionFlags flags, StringView prefix, ByteCount cursor_pos) -> Completions {
+        return Completions{ 0_byte, cursor_pos, complete(prefix, cursor_pos, candidates), Completions::Flags::Menu };
+    };
+};
+
 template<bool ignore_current = false>
 static Completions complete_buffer_name(const Context& context, CompletionFlags flags,
                                         StringView prefix, ByteCount cursor_pos)
@@ -444,12 +465,12 @@ void edit(const ParametersParser& parser, Context& context, const ShellContext&)
 }
 
 ParameterDesc edit_params{
-    { { "existing", { false, "fail if the file does not exist, do not open a new file" } },
-      { "scratch",  { false, "create a scratch buffer, not linked to a file" } },
-      { "debug",    { false, "create buffer as debug output" } },
-      { "fifo",     { true,  "create a buffer reading its content from a named fifo" } },
-      { "readonly", { false, "create a buffer in readonly mode" } },
-      { "scroll",   { false, "place the initial cursor so that the fifo will scroll to show new data" } } },
+    { { "existing", { {}, "fail if the file does not exist, do not open a new file" } },
+      { "scratch",  { {}, "create a scratch buffer, not linked to a file" } },
+      { "debug",    { {}, "create buffer as debug output" } },
+      { "fifo",     { {filename_arg_completer<true>},  "create a buffer reading its content from a named fifo" } },
+      { "readonly", { {}, "create a buffer in readonly mode" } },
+      { "scroll",   { {}, "place the initial cursor so that the fifo will scroll to show new data" } } },
       ParameterDesc::Flags::None, 0, 3
 };
 const CommandDesc edit_cmd = {
@@ -477,17 +498,17 @@ const CommandDesc force_edit_cmd = {
 
 const ParameterDesc write_params = {
     {
-        { "sync", { false, "force the synchronization of the file onto the filesystem" } },
-        { "method", { true, "explicit writemethod (replace|overwrite)" } },
-        { "force", { false, "Allow overwriting existing file with explicit filename" } }
+        { "sync", { {}, "force the synchronization of the file onto the filesystem" } },
+        { "method", { {arg_completer(Array{"replace", "overwrite"})}, "explicit writemethod (replace|overwrite)" } },
+        { "force", { {}, "Allow overwriting existing file with explicit filename" } }
     },
     ParameterDesc::Flags::None, 0, 1
 };
 
 const ParameterDesc write_params_except_force = {
     {
-        { "sync", { false, "force the synchronization of the file onto the filesystem" } },
-        { "method", { true, "explicit writemethod (replace|overwrite)" } },
+        { "sync", { {}, "force the synchronization of the file onto the filesystem" } },
+        { "method", { {arg_completer(Array{"replace", "overwrite"})}, "explicit writemethod (replace|overwrite)" } },
     },
     ParameterDesc::Flags::None, 0, 1
 };
@@ -872,8 +893,8 @@ const CommandDesc rename_buffer_cmd = {
     "rename-buffer <name>: change current buffer name",
     ParameterDesc{
         {
-            { "scratch",  { false, "convert a file buffer to a scratch buffer" } },
-            { "file",  { false, "convert a scratch buffer to a file buffer" } }
+            { "scratch",  { {}, "convert a file buffer to a scratch buffer" } },
+            { "file",  { {}, "convert a scratch buffer to a file buffer" } }
         },
         ParameterDesc::Flags::None, 1, 1
     },
@@ -997,7 +1018,7 @@ const CommandDesc add_highlighter_cmd = {
     "    <path> is a '/' delimited path or the parent highlighter, starting with either\n"
     "   'global', 'buffer', 'window' or 'shared', if <name> is empty, it will be autogenerated",
     ParameterDesc{
-        { { "override", { false, "replace existing highlighter with same path if it exists" } }, },
+        { { "override", { {}, "replace existing highlighter with same path if it exists" } }, },
         ParameterDesc::Flags::SwitchesOnlyAtStart, 2
     },
     CommandFlags::None,
@@ -1097,9 +1118,9 @@ const CommandDesc add_hook_cmd = {
     "            (and any window for that buffer)\n"
     "  * window: hook is executed only for the current window\n",
     ParameterDesc{
-        { { "group", { true, "set hook group, see remove-hooks" } },
-          { "always", { false, "run hook even if hooks are disabled" } },
-          { "once", { false, "run the hook only once" } } },
+        { { "group", { ArgCompleter{}, "set hook group, see remove-hooks" } },
+          { "always", { {}, "run hook even if hooks are disabled" } },
+          { "once", { {}, "run the hook only once" } } },
         ParameterDesc::Flags::None, 4, 4
     },
     CommandFlags::None,
@@ -1335,19 +1356,19 @@ const CommandDesc define_command_cmd = {
     "def",
     "define-command [<switches>] <name> <cmds>: define a command <name> executing <cmds>",
     ParameterDesc{
-        { { "params",                   { true,  "take parameters, accessible to each shell escape as $0..$N\n"
+        { { "params",                   { ArgCompleter{},  "take parameters, accessible to each shell escape as $0..$N\n"
                                                  "parameter should take the form <count> or <min>..<max> (both omittable)" } },
-          { "override",                 { false, "allow overriding an existing command" } },
-          { "hidden",                   { false, "do not display the command in completion candidates" } },
-          { "docstring",                { true,  "define the documentation string for command" } },
-          { "menu",                     { false, "treat completions as the only valid inputs" } },
-          { "file-completion",          { false, "complete parameters using filename completion" } },
-          { "client-completion",        { false, "complete parameters using client name completion" } },
-          { "buffer-completion",        { false, "complete parameters using buffer name completion" } },
-          { "command-completion",       { false, "complete parameters using kakoune command completion" } },
-          { "shell-completion",         { false, "complete parameters using shell command completion" } },
-          { "shell-script-completion",  { true,  "complete parameters using the given shell-script" } },
-          { "shell-script-candidates",  { true,  "get the parameter candidates using the given shell-script" } } },
+          { "override",                 { {}, "allow overriding an existing command" } },
+          { "hidden",                   { {}, "do not display the command in completion candidates" } },
+          { "docstring",                { ArgCompleter{},  "define the documentation string for command" } },
+          { "menu",                     { {}, "treat completions as the only valid inputs" } },
+          { "file-completion",          { {}, "complete parameters using filename completion" } },
+          { "client-completion",        { {}, "complete parameters using client name completion" } },
+          { "buffer-completion",        { {}, "complete parameters using buffer name completion" } },
+          { "command-completion",       { {}, "complete parameters using kakoune command completion" } },
+          { "shell-completion",         { {}, "complete parameters using shell command completion" } },
+          { "shell-script-completion",  { ArgCompleter{},  "complete parameters using the given shell-script" } },
+          { "shell-script-candidates",  { ArgCompleter{},  "get the parameter candidates using the given shell-script" } } },
         ParameterDesc::Flags::None,
         2, 2
     },
@@ -1408,7 +1429,7 @@ const CommandDesc complete_command_cmd = {
     "complete-command [<switches>] <name> <type> [<param>]\n"
     "define command completion",
     ParameterDesc{
-        { { "menu",                     { false, "treat completions as the only valid inputs" } }, },
+        { { "menu",                     { {}, "treat completions as the only valid inputs" } }, },
         ParameterDesc::Flags::None, 2, 3},
     CommandFlags::None,
     CommandHelper{},
@@ -1426,11 +1447,11 @@ const CommandDesc echo_cmd = {
     nullptr,
     "echo <params>...: display given parameters in the status line",
     ParameterDesc{
-        { { "markup", { false, "parse markup" } },
-          { "quoting", { true, "quote each argument separately using the given style (raw|kakoune|shell)" } },
-          { "to-file", { true, "echo contents to given filename" } },
-          { "to-shell-script", { true, "pipe contents to given shell script" } },
-          { "debug", { false, "write to debug buffer instead of status line" } } },
+        { { "markup", { {}, "parse markup" } },
+          { "quoting", { {arg_completer(Array{"raw", "kakoune", "shell"})}, "quote each argument separately using the given style (raw|kakoune|shell)" } },
+          { "to-file", { {filename_arg_completer<false>}, "echo contents to given filename" } },
+          { "to-shell-script", { ArgCompleter{}, "pipe contents to given shell script" } },
+          { "debug", { {}, "write to debug buffer instead of status line" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart
     },
     CommandFlags::None,
@@ -1666,8 +1687,8 @@ const CommandDesc set_option_cmd = {
     "<scope> can be global, buffer, window, or current which refers to the narrowest "
     "scope the option is set in",
     ParameterDesc{
-        { { "add",    { false, "add to option rather than replacing it" } },
-          { "remove", { false, "remove from option rather than replacing it" } } },
+        { { "add",    { {}, "add to option rather than replacing it" } },
+          { "remove", { {}, "remove from option rather than replacing it" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart, 2, (size_t)-1
     },
     CommandFlags::None,
@@ -1776,8 +1797,8 @@ const CommandDesc declare_option_cmd = {
     "    range-specs: list of range specs\n"
     "    str-to-str-map: map from strings to strings\n",
     ParameterDesc{
-        { { "hidden",    { false, "do not display option name when completing" } },
-          { "docstring", { true,  "specify option description" } } },
+        { { "hidden",    { {}, "do not display option name when completing" } },
+          { "docstring", { ArgCompleter{},  "specify option description" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart, 2, (size_t)-1
     },
     CommandFlags::None,
@@ -1862,7 +1883,7 @@ const CommandDesc map_key_cmd = {
     nullptr,
     "map [<switches>] <scope> <mode> <key> <keys>: map <key> to <keys> in given <mode> in <scope>",
     ParameterDesc{
-        { { "docstring", { true,  "specify mapping description" } } },
+        { { "docstring", { ArgCompleter{},  "specify mapping description" } } },
         ParameterDesc::Flags::None, 4, 4
     },
     CommandFlags::None,
@@ -1920,11 +1941,11 @@ template<size_t... P>
 ParameterDesc make_context_wrap_params_impl(Array<HashItem<String, SwitchDesc>, sizeof...(P)>&& additional_params,
                                             std::index_sequence<P...>)
 {
-    return { { { "client",     { true,  "run in given client context" } },
-               { "try-client", { true,  "run in given client context if it exists, or else in the current one" } },
-               { "buffer",     { true,  "run in a disposable context for each given buffer in the comma separated list argument" } },
-               { "draft",      { false, "run in a disposable context" } },
-               { "itersel",    { false, "run once for each selection with that selection as the only one" } },
+    return { { { "client",     { {client_arg_completer},  "run in given client context" } },
+               { "try-client", { {client_arg_completer},  "run in given client context if it exists, or else in the current one" } },
+               { "buffer",     { {complete_buffer_name<false>},  "run in a disposable context for each given buffer in the comma separated list argument" } },
+               { "draft",      { {}, "run in a disposable context" } },
+               { "itersel",    { {}, "run once for each selection with that selection as the only one" } },
                std::move(additional_params[P])...},
         ParameterDesc::Flags::SwitchesOnlyAtStart, 1
     };
@@ -2089,9 +2110,9 @@ const CommandDesc execute_keys_cmd = {
     "exec",
     "execute-keys [<switches>] <keys>: execute given keys as if entered by user",
     make_context_wrap_params<3>({{
-        {"save-regs",  {true, "restore all given registers after execution (default: '/\"|^@:')"}},
-        {"with-maps",  {false, "use user defined key mapping when executing keys"}},
-        {"with-hooks", {false, "trigger hooks while executing keys"}}
+        {"save-regs",  {ArgCompleter{}, "restore all given registers after execution (default: '/\"|^@:')"}},
+        {"with-maps",  {{}, "use user defined key mapping when executing keys"}},
+        {"with-hooks", {{}, "trigger hooks while executing keys"}}
     }}),
     CommandFlags::None,
     CommandHelper{},
@@ -2113,9 +2134,9 @@ const CommandDesc evaluate_commands_cmd = {
     "eval",
     "evaluate-commands [<switches>] <commands>...: execute commands as if entered by user",
     make_context_wrap_params<3>({{
-        {"save-regs",  {true, "restore all given registers after execution (default: '')"}},
-        {"no-hooks", { false, "disable hooks while executing commands" }},
-        {"verbatim", { false, "do not reparse argument" }}
+        {"save-regs",  {ArgCompleter{}, "restore all given registers after execution (default: '')"}},
+        {"no-hooks", { {}, "disable hooks while executing commands" }},
+        {"verbatim", { {}, "do not reparse argument" }}
     }}),
     CommandFlags::None,
     CommandHelper{},
@@ -2151,18 +2172,18 @@ const CommandDesc prompt_cmd = {
     "prompt [<switches>] <prompt> <command>: prompt the user to enter a text string "
     "and then executes <command>, entered text is available in the 'text' value",
     ParameterDesc{
-        { { "init", { true, "set initial prompt content" } },
-          { "password", { false, "Do not display entered text and clear reg after command" } },
-          { "menu", { false, "treat completions as the only valid inputs" } },
-          { "file-completion", { false, "use file completion for prompt" } },
-          { "client-completion", { false, "use client completion for prompt" } },
-          { "buffer-completion", { false, "use buffer completion for prompt" } },
-          { "command-completion", { false, "use command completion for prompt" } },
-          { "shell-completion", { false, "use shell command completion for prompt" } },
-          { "shell-script-completion", { true, "use shell command completion for prompt" } },
-          { "shell-script-candidates", { true, "use shell command completion for prompt" } },
-          { "on-change", { true, "command to execute whenever the prompt changes" } },
-          { "on-abort", { true, "command to execute whenever the prompt is canceled" } } },
+        { { "init", { ArgCompleter{}, "set initial prompt content" } },
+          { "password", { {}, "Do not display entered text and clear reg after command" } },
+          { "menu", { {}, "treat completions as the only valid inputs" } },
+          { "file-completion", { {}, "use file completion for prompt" } },
+          { "client-completion", { {}, "use client completion for prompt" } },
+          { "buffer-completion", { {}, "use buffer completion for prompt" } },
+          { "command-completion", { {}, "use command completion for prompt" } },
+          { "shell-completion", { {}, "use shell command completion for prompt" } },
+          { "shell-script-completion", { ArgCompleter{}, "use shell command completion for prompt" } },
+          { "shell-script-candidates", { ArgCompleter{}, "use shell command completion for prompt" } },
+          { "on-change", { ArgCompleter{}, "command to execute whenever the prompt changes" } },
+          { "on-abort", { ArgCompleter{}, "command to execute whenever the prompt is canceled" } } },
         ParameterDesc::Flags::None, 2, 2
     },
     CommandFlags::None,
@@ -2226,9 +2247,9 @@ const CommandDesc menu_cmd = {
     "menu [<switches>] <name1> <commands1> <name2> <commands2>...: display a "
     "menu and execute commands for the selected item",
     ParameterDesc{
-        { { "auto-single", { false, "instantly validate if only one item is available" } },
-          { "select-cmds", { false, "each item specify an additional command to run when selected" } },
-          { "markup", { false, "parse menu entries as markup text" } } }
+        { { "auto-single", { {}, "instantly validate if only one item is available" } },
+          { "select-cmds", { {}, "each item specify an additional command to run when selected" } },
+          { "markup", { {}, "parse menu entries as markup text" } } }
     },
     CommandFlags::None,
     CommandHelper{},
@@ -2285,7 +2306,7 @@ const CommandDesc on_key_cmd = {
     "on-key [<switches>] <command>: wait for next user key and then execute <command>, "
     "with key available in the `key` value",
     ParameterDesc{
-        { { "mode-name", { true, "set mode name to use" } } },
+        { { "mode-name", { ArgCompleter{}, "set mode name to use" } } },
         ParameterDesc::Flags::None, 1, 1
     },
     CommandFlags::None,
@@ -2312,10 +2333,10 @@ const CommandDesc info_cmd = {
     nullptr,
     "info [<switches>] <text>: display an info box containing <text>",
     ParameterDesc{
-        { { "anchor", { true, "set info anchoring <line>.<column>" } },
-          { "style", { true, "set info style (above, below, menu, modal)" } },
-          { "markup", { false, "parse markup" } },
-          { "title", { true, "set info title" } } },
+        { { "anchor", { ArgCompleter{}, "set info anchoring <line>.<column>" } },
+          { "style", { {arg_completer(Array{"above", "below", "menu", "modal"})}, "set info style (above, below, menu, modal)" } },
+          { "markup", { {}, "parse markup" } },
+          { "title", { ArgCompleter{}, "set info title" } } },
         ParameterDesc::Flags::None, 0, 1
     },
     CommandFlags::None,
@@ -2518,9 +2539,9 @@ const CommandDesc select_cmd = {
     "\n"
     "selection_desc format is <anchor_line>.<anchor_column>,<cursor_line>.<cursor_column>",
     ParameterDesc{{
-            {"timestamp", {true, "specify buffer timestamp at which those selections are valid"}},
-            {"codepoint", {false, "columns are specified in codepoints, not bytes"}},
-            {"display-column", {false, "columns are specified in display columns, not bytes"}}
+            {"timestamp", {ArgCompleter{}, "specify buffer timestamp at which those selections are valid"}},
+            {"codepoint", {{}, "columns are specified in codepoints, not bytes"}},
+            {"display-column", {{}, "columns are specified in display columns, not bytes"}}
         },
         ParameterDesc::Flags::SwitchesOnlyAtStart, 1
     },
@@ -2642,7 +2663,7 @@ const CommandDesc enter_user_mode_cmd = {
     nullptr,
     "enter-user-mode [<switches>] <name>: enable <name> keymap mode for next key",
     ParameterDesc{
-        { { "lock", { false, "stay in mode until <esc> is pressed" } } },
+        { { "lock", { {}, "stay in mode until <esc> is pressed" } } },
         ParameterDesc::Flags::None, 1, 1
     },
     CommandFlags::None,
@@ -2672,7 +2693,7 @@ const CommandDesc provide_module_cmd = {
     nullptr,
     "provide-module [<switches>] <name> <cmds>: declares a module <name> provided by <cmds>",
     ParameterDesc{
-        { { "override", { false, "allow overriding an existing module" } } },
+        { { "override", { {}, "allow overriding an existing module" } } },
         ParameterDesc::Flags::None,
         2, 2
     },
diff --git a/src/highlighters.cc b/src/highlighters.cc
index 9d733521..5a84a522 100644
--- a/src/highlighters.cc
+++ b/src/highlighters.cc
@@ -678,10 +678,10 @@ const HighlighterDesc wrap_desc = {
     "Parameters: [-word] [-indent] [-width <max_width>] [-marker <marker_text>]\n"
     "Wrap lines to window width",
     { {
-        { "word",   { false, "wrap at word boundaries instead of codepoint boundaries" } },
-        { "indent", { false, "preserve line indentation of the wrapped line" } },
-        { "width",  { true, "wrap at the given column instead of the window's width" } },
-        { "marker", { true, "insert the given text at the beginning of the wrapped line" } }, },
+        { "word",   { {}, "wrap at word boundaries instead of codepoint boundaries" } },
+        { "indent", { {}, "preserve line indentation of the wrapped line" } },
+        { "width",  { ArgCompleter{}, "wrap at the given column instead of the window's width" } },
+        { "marker", { ArgCompleter{}, "insert the given text at the beginning of the wrapped line" } }, },
         ParameterDesc::Flags::None, 0, 0
     }
 };
@@ -1047,11 +1047,11 @@ const HighlighterDesc show_whitespace_desc = {
     "Parameters: [-tab <separator>] [-tabpad <separator>] [-lf <separator>] [-spc <separator>] [-nbsp <separator>]\n"
     "Display whitespaces using symbols",
     { {
-        { "tab",    { true, "replace tabulations with the given character" } },
-        { "tabpad", { true, "append as many of the given character as is necessary to honor `tabstop`" } },
-        { "spc",    { true, "replace spaces with the given character" } },
-        { "lf",     { true, "replace line feeds with the given character" } },
-        { "nbsp",   { true, "replace non-breakable spaces with the given character" } } },
+        { "tab",    { ArgCompleter{}, "replace tabulations with the given character" } },
+        { "tabpad", { ArgCompleter{}, "append as many of the given character as is necessary to honor `tabstop`" } },
+        { "spc",    { ArgCompleter{}, "replace spaces with the given character" } },
+        { "lf",     { ArgCompleter{}, "replace line feeds with the given character" } },
+        { "nbsp",   { ArgCompleter{}, "replace non-breakable spaces with the given character" } } },
         ParameterDesc::Flags::None, 0, 0
     }
 };
@@ -1131,11 +1131,11 @@ const HighlighterDesc line_numbers_desc = {
     "Parameters: [-relative] [-hlcursor] [-separators <separator|separator:cursor|cursor:up:down>] [-min-digits <cols>]\n"
     "Display line numbers",
     { {
-        { "relative", { false, "show line numbers relative to the main cursor line" } },
-        { "separator", { true, "string to separate the line numbers column from the rest of the buffer (default '|')" } },
-        { "cursor-separator", { true, "identical to -separator but applies only to the line of the cursor (default is the same value passed to -separator)" } },
-        { "min-digits", { true, "use at least the given number of columns to display line numbers (default 2)" } },
-        { "hlcursor", { false, "highlight the cursor line with a separate face" } } },
+        { "relative", { {}, "show line numbers relative to the main cursor line" } },
+        { "separator", { ArgCompleter{}, "string to separate the line numbers column from the rest of the buffer (default '|')" } },
+        { "cursor-separator", { ArgCompleter{}, "identical to -separator but applies only to the line of the cursor (default is the same value passed to -separator)" } },
+        { "min-digits", { ArgCompleter{}, "use at least the given number of columns to display line numbers (default 2)" } },
+        { "hlcursor", { {}, "highlight the cursor line with a separate face" } } },
         ParameterDesc::Flags::None, 0, 0
     }
 };
@@ -1777,9 +1777,9 @@ const HighlighterDesc higlighter_group_desc = {
     "Parameters: [-passes <passes>]\n"
     "Creates a group that can contain other highlighters",
     { {
-        { "passes", { true, "flags(colorize|move|wrap) "
-                            "kind of highlighters can be put in the group "
-                            "(default colorize)" } } },
+        { "passes", { ArgCompleter{}, "flags(colorize|move|wrap) "
+                                       "kind of highlighters can be put in the group "
+                                       "(default colorize)" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart, 0, 0
     }
 };
@@ -1795,9 +1795,9 @@ const HighlighterDesc ref_desc = {
     "Parameters: [-passes <passes>] <path>\n"
     "Reference the highlighter at <path> in shared highlighters",
     { {
-        { "passes", { true, "flags(colorize|move|wrap) "
-                            "kind of highlighters that can be referenced "
-                            "(default colorize)" } } },
+        { "passes", { ArgCompleter{}, "flags(colorize|move|wrap) "
+                                      "kind of highlighters that can be referenced "
+                                      "(default colorize)" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart, 1, 1
     }
 };
@@ -1885,8 +1885,8 @@ const HighlighterDesc region_desc = {
     "highlighter as defined by <type> and eventual <params>...\n"
     "The region starts at <begin> match and ends at the first <end>",
     { {
-        { "match-capture", { false, "only consider region ending/recurse delimiters whose first capture group match the region beginning delimiter" } },
-        { "recurse",       { true, "make the region end on the first ending delimiter that does not close the given parameter" } } },
+        { "match-capture", { {}, "only consider region ending/recurse delimiters whose first capture group match the region beginning delimiter" } },
+        { "recurse",       { ArgCompleter{}, "make the region end on the first ending delimiter that does not close the given parameter" } } },
         ParameterDesc::Flags::SwitchesOnlyAtStart | ParameterDesc::Flags::IgnoreUnknownSwitches,
         3
     }
diff --git a/src/input_handler.cc b/src/input_handler.cc
index 2564529a..69ef3385 100644
--- a/src/input_handler.cc
+++ b/src/input_handler.cc
@@ -941,7 +941,7 @@ public:
         {
             on_next_key_with_autoinfo(context(), "explicit-completion", KeymapMode::None,
                 [this](Key key, Context&) {
-                    m_explicit_completer = PromptCompleter{};
+                    m_explicit_completer = ArgCompleter{};
 
                     if (key.key == 'f')
                         use_explicit_completer([](const Context& context, StringView token) {
@@ -968,7 +968,7 @@ public:
         }
         else if (key == ctrl('o'))
         {
-            m_explicit_completer = PromptCompleter{};
+            m_explicit_completer = ArgCompleter{};
             m_auto_complete = not m_auto_complete;
 
             if (m_auto_complete)
diff --git a/src/main.cc b/src/main.cc
index 3e5ba370..6e4415cf 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -1071,23 +1071,23 @@ int main(int argc, char* argv[])
     set_signal_handler(SIGTTOU, SIG_IGN);
 
     const ParameterDesc param_desc{
-        SwitchMap{ { "c", { true,  "connect to given session" } },
-                   { "e", { true,  "execute argument on client initialisation" } },
-                   { "E", { true,  "execute argument on server initialisation" } },
-                   { "n", { false, "do not source kakrc files on startup" } },
-                   { "s", { true,  "set session name" } },
-                   { "d", { false, "run as a headless session (requires -s)" } },
-                   { "p", { true,  "just send stdin as commands to the given session" } },
-                   { "f", { true,  "filter: for each file, select the entire buffer and execute the given keys" } },
-                   { "i", { true, "backup the files on which a filter is applied using the given suffix" } },
-                   { "q", { false, "in filter mode, be quiet about errors applying keys" } },
-                   { "ui", { true, "set the type of user interface to use (terminal, dummy, or json)" } },
-                   { "l", { false, "list existing sessions" } },
-                   { "clear", { false, "clear dead sessions" } },
-                   { "debug", { true, "initial debug option value" } },
-                   { "version", { false, "display kakoune version and exit" } },
-                   { "ro", { false, "readonly mode" } },
-                   { "help", { false, "display a help message and quit" } } }
+        SwitchMap{ { "c", { ArgCompleter{},  "connect to given session" } },
+                   { "e", { ArgCompleter{},  "execute argument on client initialisation" } },
+                   { "E", { ArgCompleter{},  "execute argument on server initialisation" } },
+                   { "n", { {}, "do not source kakrc files on startup" } },
+                   { "s", { ArgCompleter{},  "set session name" } },
+                   { "d", { {}, "run as a headless session (requires -s)" } },
+                   { "p", { ArgCompleter{},  "just send stdin as commands to the given session" } },
+                   { "f", { ArgCompleter{},  "filter: for each file, select the entire buffer and execute the given keys" } },
+                   { "i", { ArgCompleter{}, "backup the files on which a filter is applied using the given suffix" } },
+                   { "q", { {}, "in filter mode, be quiet about errors applying keys" } },
+                   { "ui", { ArgCompleter{}, "set the type of user interface to use (terminal, dummy, or json)" } },
+                   { "l", { {}, "list existing sessions" } },
+                   { "clear", { {}, "clear dead sessions" } },
+                   { "debug", { ArgCompleter{}, "initial debug option value" } },
+                   { "version", { {}, "display kakoune version and exit" } },
+                   { "ro", { {}, "readonly mode" } },
+                   { "help", { {}, "display a help message and quit" } } }
     };
 
     try
diff --git a/src/parameters_parser.cc b/src/parameters_parser.cc
index 633cb092..75575323 100644
--- a/src/parameters_parser.cc
+++ b/src/parameters_parser.cc
@@ -11,14 +11,14 @@ String generate_switches_doc(const SwitchMap& switches)
     if (switches.empty())
         return res;
 
-    auto switch_len = [](auto& sw) { return sw.key.column_length() + (sw.value.takes_arg ? 5 : 0); };
+    auto switch_len = [](auto& sw) { return sw.key.column_length() + (sw.value.arg_completer ? 5 : 0); };
     auto switches_len = switches | transform(switch_len);
     const ColumnCount maxlen = *std::max_element(switches_len.begin(), switches_len.end());
 
     for (auto& sw : switches) {
         res += format("-{} {}{}{}\n",
                       sw.key,
-                      sw.value.takes_arg ? "<arg>" : "",
+                      sw.value.arg_completer ? "<arg>" : "",
                       String{' ', maxlen - switch_len(sw) + 1},
                       sw.value.description);
     }
@@ -69,7 +69,7 @@ ParametersParser::ParametersParser(ParameterList params, const ParameterDesc& de
             }
             switch_seen[switch_index] = true;
 
-            if (it->value.takes_arg)
+            if (it->value.arg_completer)
             {
                if (++i == params.size())
                {
@@ -80,7 +80,7 @@ ParametersParser::ParametersParser(ParameterList params, const ParameterDesc& de
                m_state = State::SwitchArgument;
             }
 
-            m_switches[switch_name.str()] = it->value.takes_arg ? params[i] : StringView{};
+            m_switches[switch_name.str()] = it->value.arg_completer ? params[i] : StringView{};
         }
         else // positional
         {
diff --git a/src/parameters_parser.hh b/src/parameters_parser.hh
index 680c6f93..86ba88ce 100644
--- a/src/parameters_parser.hh
+++ b/src/parameters_parser.hh
@@ -10,6 +10,8 @@
 #include "string.hh"
 #include "string_utils.hh"
 
+#include <functional>
+
 namespace Kakoune
 {
 
@@ -37,9 +39,15 @@ struct wrong_argument_count : public parameter_error
     wrong_argument_count() : parameter_error("wrong argument count") {}
 };
 
+class Context;
+struct Completions;
+enum class CompletionFlags;
+using ArgCompleter = std::function<Completions (const Context&, CompletionFlags,
+                                                StringView, ByteCount)>;
+
 struct SwitchDesc
 {
-    bool takes_arg;
+    Optional<ArgCompleter> arg_completer;
     String description;
 };