gint: add an interactive REPL

1 files changed, 41 insertions, 4 deletions

diff --git a/cmd/gint/main.go b/cmd/gint/main.go
index 3ddddc8..5b2b3a2 100644
--- a/cmd/gint/main.go
+++ b/cmd/gint/main.go
@@ -1,12 +1,16 @@
 package main
 
 import (
+	"bufio"
+	"errors"
 	"fmt"
+	"io"
 	"log"
 	"os"
 
 	"github.com/gnolang/parscan/codegen"
 	"github.com/gnolang/parscan/lang/golang"
+	"github.com/gnolang/parscan/scanner"
 	"github.com/gnolang/parscan/vm0"
 )
 
@@ -16,16 +20,49 @@ type Interpreter interface {
 
 func main() {
 	log.SetFlags(log.Lshortfile)
-	buf, err := os.ReadFile("/dev/stdin")
-	if err != nil {
-		log.Fatal(err)
-	}
 	var interp Interpreter = vm0.New(golang.GoParser)
 	if len(os.Args) > 1 && os.Args[1] == "1" {
 		interp = codegen.NewInterpreter(golang.GoParser)
 		interp.(*codegen.Interpreter).AddSym(fmt.Println, "println", false)
 	}
+	in := os.Stdin
+
+	if isatty(in) {
+		// Provide an interactive line oriented Read Eval Print Loop (REPL).
+		liner := bufio.NewScanner(in)
+		text, prompt := "", "> "
+		fmt.Printf(prompt)
+		for liner.Scan() {
+			text += liner.Text()
+			err := interp.Eval(text + "\n")
+			if err == nil {
+				text, prompt = "", "> "
+			} else if errors.Is(err, scanner.ErrBlock) {
+				prompt = ">> "
+			} else {
+				text, prompt = "", "> "
+				fmt.Println("Error:", err)
+			}
+			fmt.Printf(prompt)
+		}
+		return
+	}
+
+	buf, err := io.ReadAll(in)
+	if err != nil {
+		log.Fatal(err)
+	}
 	if err := interp.Eval(string(buf)); err != nil {
 		log.Fatal(err)
 	}
 }
+
+// isatty returns true if the input stream is a tty (i.e. a character device).
+func isatty(in io.Reader) bool {
+	s, ok := in.(interface{ Stat() (os.FileInfo, error) })
+	if !ok {
+		return false
+	}
+	stat, err := s.Stat()
+	return err == nil && stat.Mode()&os.ModeCharDevice != 0
+}