feat(repl): improve support of multi-line statements (gnolang#1129)

![demo](https://github.com/gnolang/gno/assets/5792239/308e61bc-bdf9-498b-9fa7-cd756835f774)

This is a followup of gnolang#978. Instead of starting in multi-line mode prior
to submit, the line is parsed, and new inputs are appended to it as long
as the statment is not complete, as detected by the Go scanner.

This is simpler and more general than previous attempt. The secondary
prompt is "...", different from primary "gno>", similarly to many REPL
programs (node, python, bash, ...).

The "/editor" command is removed as not useful anymore. Note also that
it is now possible to exit using Ctrl-D.

Related issues: gnolang#446 gnolang#950

<!-- please provide a detailed description of the changes made in this
pull request. -->

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [x] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [x] Updated the official documentation or not needed
- [x] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [x] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

---------

Co-authored-by: Manfred Touron <94029+moul@users.noreply.github.com>

gnovm/cmd/gno/repl.go

@@ -2,10 +2,11 @@ package main
 
 import (
 	"bufio"
-	"bytes"
 	"context"
+	"errors"
 	"flag"
 	"fmt"
+	"go/scanner"
 	"os"
 	"strings"
 
@@ -88,91 +89,85 @@ func execRepl(cfg *replCfg, args []string) error {
 //   gno> import "gno.land/p/demo/avl"     // import the p/demo/avl package
 //   gno> func a() string { return "a" }   // declare a new function named a
 //   gno> /src                             // print current generated source
-//   gno> /editor                          // enter in editor mode to add several lines
+//   gno> /editor                          // enter in multi-line mode, end with ';'
 //   gno> /reset                           // remove all previously inserted code
 //   gno> println(a())                     // print the result of calling a()
-//   gno> /exit
+//   gno> /exit                            // alternative to <Ctrl-D>
 `)
 	}
 
 	return runRepl(cfg)
 }
 
 func runRepl(cfg *replCfg) error {
-	// init repl state
 	r := repl.NewRepl()
 
 	if cfg.initialCommand != "" {
 		handleInput(r, cfg.initialCommand)
 	}
 
-	var multiline bool
-	for {
-		fmt.Fprint(os.Stdout, "gno> ")
+	fmt.Fprint(os.Stdout, "gno> ")
 
-		input, err := getInput(multiline)
-		if err != nil {
-			return err
+	inEdit := false
+	prev := ""
+	liner := bufio.NewScanner(os.Stdin)
+
+	for liner.Scan() {
+		line := liner.Text()
+
+		if l := strings.TrimSpace(line); l == ";" {
+			line, inEdit = "", false
+		} else if l == "/editor" {
+			line, inEdit = "", true
+			fmt.Fprintln(os.Stdout, "// enter a single ';' to quit and commit")
+		}
+		if prev != "" {
+			line = prev + "\n" + line
+			prev = ""
+		}
+		if inEdit {
+			fmt.Fprint(os.Stdout, "...  ")
+			prev = line
+			continue
 		}
 
-		multiline = handleInput(r, input)
+		if err := handleInput(r, line); err != nil {
+			var goScanError scanner.ErrorList
+			if errors.As(err, &goScanError) {
+				// We assune that a Go scanner error indicates an incomplete Go statement.
+				// Append next line and retry.
+				prev = line
+			} else {
+				fmt.Fprintln(os.Stderr, err)
+			}
+		}
+
+		if prev == "" {
+			fmt.Fprint(os.Stdout, "gno> ")
+		} else {
+			fmt.Fprint(os.Stdout, "...  ")
+		}
 	}
+	return nil
 }
 
-// handleInput reads the input string and parses it depending if it
-// is a specific command, or source code. It returns true if the following
-// input is expected to be on more than one line.
-func handleInput(r *repl.Repl, input string) bool {
+// handleInput executes specific "/" commands, or evaluates input as Gno source code.
+func handleInput(r *repl.Repl, input string) error {
 	switch strings.TrimSpace(input) {
 	case "/reset":
 		r.Reset()
 	case "/src":
 		fmt.Fprintln(os.Stdout, r.Src())
 	case "/exit":
 		os.Exit(0)
-	case "/editor":
-		fmt.Fprintln(os.Stdout, "// Entering editor mode (^D to finish)")
-		return true
 	case "":
-		// avoid to increase the repl execution counter if sending empty content
-		fmt.Fprintln(os.Stdout, "")
-		return false
+		// Avoid to increase the repl execution counter if no input.
 	default:
 		out, err := r.Process(input)
 		if err != nil {
-			fmt.Fprintln(os.Stderr, err)
+			return err
 		}
 		fmt.Fprintln(os.Stdout, out)
 	}
-
-	return false
-}
-
-const (
-	inputBreaker = "^D"
-	nl           = "\n"
-)
-
-func getInput(ml bool) (string, error) {
-	s := bufio.NewScanner(os.Stdin)
-	var mlOut bytes.Buffer
-	for s.Scan() {
-		line := s.Text()
-		if !ml {
-			return line, nil
-		}
-
-		if line == inputBreaker {
-			break
-		}
-
-		mlOut.WriteString(line)
-		mlOut.WriteString(nl)
-	}
-
-	if err := s.Err(); err != nil {
-		return "", err
-	}
-
-	return mlOut.String(), nil
+	return nil
 }

gnovm/pkg/repl/repl.go

@@ -161,7 +161,7 @@ func (r *Repl) Process(input string) (out string, err error) {
 		return r.handleExpression(exp)
 	}
 
-	return "", fmt.Errorf("error parsing code:\n\t- as expression (error: %q)\n\t- as declarations (error: %q)", expErr.Error(), declErr.Error())
+	return "", fmt.Errorf("error parsing code:\n\t- as expression: %w\n\t- as declarations: %w", expErr, declErr)
 }
 
 func (r *Repl) handleExpression(e *ast.File) (string, error) {