README.md: add REPL mode notes about independent command evaluation

2 files changed, 33 insertions, 0 deletions

diff --git a/README.md b/README.md
index cb7d9a5..15571d9 100644
--- a/README.md
+++ b/README.md
@@ -134,6 +134,22 @@ perc calc 1 2 3 show      # Show stack without modifying
 # → 1 2 3
 ```
 
+### REPL Mode Notes
+
+In REPL mode, each line is evaluated independently. To chain operations:
+- Use the `rpn` subcommand for multi-step expressions: `rpn 2 3 + 4 -`
+- Or use the `rpn` command to set variables: `rpn x 5 =` then `rpn x x +`
+- The `show` command can display the final stack state
+
+Example REPL session:
+```
+perc> rpn 2 3 4 +        # Push 2, 3, 4; add last two
+2 7
+perc> rpn show           # Show full stack (but we start fresh per command)
+```
+
+Note: Each REPL command is evaluated independently, so you cannot chain operations like `2 3 4 +` then `-` on separate lines. Use `rpn 2 3 4 + -` for a single expression instead.
+
 ## Building
 
 Using mage:
diff --git a/internal/repl/repl.go b/internal/repl/repl.go
index ed39755..0bcc4cc 100644
--- a/internal/repl/repl.go
+++ b/internal/repl/repl.go
@@ -18,6 +18,23 @@ import (
 
 const historyFile = ".perc_history"
 
+// RPNState holds the state for RPN operations in REPL
+type RPNState struct {
+	vars *rpn.Variables
+}
+
+var rpnState *RPNState
+
+// getRPNState returns or creates the RPN state
+func getRPNState() *RPNState {
+	if rpnState == nil {
+		rpnState = &RPNState{
+			vars: rpn.NewVariables().(*rpn.Variables),
+		}
+	}
+	return rpnState
+}
+
 // executor runs a calculation command and returns the result
 func executor(input string) {
 	input = strings.TrimSpace(input)