refactor(repl): introduce RPNCalculator interface to fix DIP violation (task rj)

Handlers accessed repl.rpnState.rpnCalc — three levels of concrete types
violating DIP and Law of Demeter. Define an RPNCalculator interface
capturing only the methods handlers actually need, and expose it via
REPL.RpnCalculator() so handlers depend on the interface, not the
concrete chain of *rpn.RPN.

1 files changed, 10 insertions, 0 deletions

diff --git a/internal/repl/repl.go b/internal/repl/repl.go
index 98ff23e..2ecd9ea 100644
--- a/internal/repl/repl.go
+++ b/internal/repl/repl.go
@@ -87,6 +87,16 @@ type REPL struct {
 	logWriter     io.WriteCloser
 }
 
+// RpnCalculator returns the RPN calculator behind this REPL as an interface.
+// This lets handlers depend on the RPNCalculator interface (DIP) rather than
+// reaching through repl.rpnState.rpnCalc into concrete types.
+func (r *REPL) RpnCalculator() RPNCalculator {
+	if r.rpnState == nil {
+		return nil
+	}
+	return r.rpnState.rpnCalc
+}
+
 // ReadlinePrompt provides an interactive prompt using chzyer/readline.
 // It supports:
 //   - Ctrl+R for reverse history search