scm: support --eval

With the ~--eval~ option you can evaluate an expression in mu/scm
environment. For example:

$ mu scm --eval \
  '(format #t "found ~d match(es)\n" (length (mfind "hello")))'
found 7173 match(es)

Add command-line parameter and implement.

man/mu-scm.1.org

@@ -38,6 +38,15 @@ prefixed by *UNIX_CONNECT:* and ending with a newline. For instance:
 UNIX-CONNECT:/run/user/1000/mu-scm-socket-4eb5db40
 #+end_example
 
+** --eval
+
+With the ~--eval~ option you can evaluate an expression in mu/scm environment. For
+example:
+#+begin_example
+$ mu scm --eval '(format #t "found ~d match(es)\n" (length (mfind "hello")))'
+found 7173 match(es)
+#+end_example
+
 #+include: "muhome.inc" :minlevel 2
 
 #+include: "common-options.inc" :minlevel 1

mu/mu-cmd.cc

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2010-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2010-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -75,7 +75,9 @@ cmd_scm(const Store& store, const Options& opts)
 #else
 	if (opts.scm.script_path)
 		return Mu::Scm::run_script(store, opts, *opts.scm.script_path);
-	else
+	else if (opts.scm.eval)
+		return Mu::Scm::run_eval(store, opts, *opts.scm.eval);
+		else
 		return Mu::Scm::run_repl(store, opts, opts.scm.socket_path.value_or(""));
 #endif /*BUILD_SCM*/
 }

mu/mu-options.cc

@@ -650,7 +650,9 @@ sub_scm(CLI::App& sub, Options& opts)
 		->excludes("--listen");
 	sub.add_option("script-args", opts.scm.params, "Parameters for script")
 		->type_name("<parameters>");
-
+	sub.add_option("--eval", opts.scm.eval, "Expression to evaluate")
+		->excludes("--listen")
+		->excludes("script-path");
 }
 
 static void

mu/mu-options.hh

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2022-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2022-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -261,6 +261,7 @@ struct Options {
 		bool listen; /**< Whether to start listening on a socket */
 		Option<std::string> socket_path; /**< path for the '--listen'
 						  * Unix domain socket */
+		Option<std::string> eval; /**< expression to evaluate */
 	} scm;
 
 

scm/mu-scm.cc

@@ -1,5 +1,5 @@
 /*
-** Copyright (C) 2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+** Copyright (C) 2025-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 **
 ** This program is free software; you can redistribute it and/or modify it
 ** under the terms of the GNU General Public License as published by the
@@ -272,6 +272,23 @@ Mu::Scm::run_script(const Mu::Store& store, const Mu::Options& opts,
 	return Ok();
 }
 
+Result<void>
+Mu::Scm::run_eval(const Mu::Store& store, const Mu::Options& opts, const std::string& expr)
+{
+	if (const auto res = prepare_run(opts, scm_args); !res)
+		return Err(res.error());
+
+	scm_args.emplace_back("-c");
+	// a little hacky...
+	scm_args.emplace_back("(use-modules ((mu))) " + expr);
+
+	run_scm(store, opts);
+
+	return Ok();
+}
+
+
+
 #ifdef BUILD_TESTS
 
 /*

scm/mu-scm.hh

@@ -63,7 +63,8 @@ namespace Mu::Scm {
 	 * based on the configuration.
 	 *
 	 * @param store a Store object
-	 * @param opts options; opts.scm.script_path must set
+	 * @param opts options
+	 * @param a path to the script
 	 *
 	 * @return Ok() or some error
 	 */
@@ -71,6 +72,22 @@ namespace Mu::Scm {
 				const std::string& script_path);
 
 
+
+	/**
+	 * Evaluate a Guile/SCM expression
+	 *
+	 * Initialize the Scm sub-system, then start evaluate some expression
+	 * based on the configuration.
+	 *
+	 * @param store a Store object
+	 * @param opts options
+	 * @param expr some expression to evaluate
+	 *
+	 * @return Ok() or some error
+	 */
+	Result<void> run_eval(const Mu::Store& store, const Mu::Options& opts,
+			      const std::string& expr);
+
 	/**
 	 * Helpers
 	 *