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.

2 files changed, 36 insertions, 2 deletions

diff --git a/scm/mu-scm.cc b/scm/mu-scm.cc
index ab162fb..068c2c8 100644
--- a/scm/mu-scm.cc
+++ b/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
 
 /*
diff --git a/scm/mu-scm.hh b/scm/mu-scm.hh
index e213331..96cd72c 100644
--- a/scm/mu-scm.hh
+++ b/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
 	 *