assemble: clean up error handling, add src/dest args to main function

main.go

@@ -5,25 +5,47 @@ import (
 	"flag"
 	"fmt"
 	"io"
-	"io/ioutil"
 	"os"
 )
 
 //
 
 func main() {
-	src := flag.String("Src", "", "Input file")
+	srcName := flag.String("Src", "-", "Input file (default: stdin)")
+	destName := flag.String("Dest", "-", "Output file (default: stdout)")
 	flag.Parse()
 
-	fh, err := os.OpenFile(*src, os.O_RDONLY, 0644)
+	var (
+		err   error
+		srch  io.Reader
+		desth io.Writer
+	)
+
+	if *srcName == "-" {
+		srch = os.Stdin
+	} else {
+		srch, err = os.OpenFile(*srcName, os.O_RDONLY, 0644)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	if *destName == "-" {
+		desth = os.Stdout
+	} else {
+		desth, err = os.OpenFile(*destName, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0644)
+		if err != nil {
+			panic(err)
+		}
+	}
+
+	err = assemble(srch, desth)
 	if err != nil {
 		panic(err)
 	}
-
-	assemble(fh, ioutil.Discard)
 }
 
-func assemble(src io.Reader, dest io.Writer) {
+func assemble(src io.Reader, dest io.Writer) error {
 	lx := NewLexer(src)
 	cc := NewCompiler()
 
@@ -38,20 +60,23 @@ mainloop:
 			}
 
 			// Real error
-			panic(err)
+			return fmt.Errorf("lexer: %w", err)
 		}
 
 		fmt.Printf("[line %d] %#v\n", lx.lineno, tok)
 
 		err = cc.Compile(tok)
 		if err != nil {
-			panic(err)
+			return fmt.Errorf("compile: %w", err)
 		}
 
 	}
 
 	err := cc.Finalize(dest)
 	if err != nil {
-		panic(err)
+		return fmt.Errorf("finalize: %w", err)
 	}
+
+	// Success
+	return nil
 }

main_test.go

@@ -52,6 +52,9 @@ global _start:         ;tell linker entry point
 		panic(err)
 	}
 
-	assemble(strings.NewReader(src), fh) // ioutil.Discard)
+	err = assemble(strings.NewReader(src), fh) // ioutil.Discard)
+	if err != nil {
+		panic(err)
+	}
 
 }