fix(parser): error reporting

emmy now displays the correct starting position of faulty token as well
as an ^ (arrow) at the position of a single character token while
displaying multiple arrows for the length of multi-character token:

old behaviour:
    $ emmy "1+-test"
    2023/06/19 21:37:55 error: unexpected '-' (MINUS) at position 2

            1+-test
             ^  unexpected token

    2023/06/19 21:37:55 error: unexpected 'test' (IDENTIFIER) at position 3

            1+-test
              ^   unexpected token

    2023/06/19 21:37:55 semantic errors found, no evaluation of ast possible
    = NaN

new behaviour:
    $ emmy "1+-test"
    2023/06/19 21:37:05 error: unexpected '-' (MINUS) at position 2

            1+-test
              ^
              unexpected token

    2023/06/19 21:37:05 error: unexpected 'test' (IDENTIFIER) at position 3

            1+-test
               ^^^^
               unexpected token

    2023/06/19 21:37:05 semantic errors found, no evaluation of ast possible
    -> NaN

The prompt was changed from '=' to '->'

emmy.go

@@ -20,7 +20,7 @@ func run(in string, s *lexer.Scanner, p *parser.Parser) {
 	if DEBUG {
 		fmt.Println(lexer.String(t), parser.String(stmts))
 	}
-	fmt.Println("=", p.Eval(stmts))
+	fmt.Println("->", p.Eval(stmts))
 }
 
 func main() {
@@ -33,6 +33,8 @@ func main() {
 	}
 
 	fmt.Println("Welcome to the emmy repl")
+	fmt.Println("\nCommand overview:")
+	fmt.Println(consts.HELP_MSG)
 	prompt := "ε> "
 	reader := bufio.NewReader(os.Stdin)
 

parser/parser.go

@@ -81,14 +81,20 @@ func (p *Parser) error() {
 	if tk == consts.EOF {
 		log.Print("error: unexpected end of expression")
 	} else {
-		log.Printf("error: unexpected '%v' (%s) at position %d\n", val, consts.KIND_LOOKUP[tk], p.pos)
+		log.Printf("error: unexpected '%v' (%s) at position %d\n", val, consts.KIND_LOOKUP[tk], t.Pos)
 	}
 
-	fmt.Printf("\n\t%s\n\t%s%s%s%s\n\n",
+	arrows := 1
+
+	if len(t.Raw) != 0 {
+		arrows = len(t.Raw)
+	}
+
+	fmt.Printf("\n\t%s\n\t%s%s\n\t%s%s\n\n",
 		p.inputString,
-		strings.Repeat(" ", p.pos),
-		"^",
-		strings.Repeat(" ", p.pos),
+		strings.Repeat(" ", t.Pos),
+		strings.Repeat("^", arrows),
+		strings.Repeat(" ", t.Pos),
 		"unexpected token",
 	)
 }

parser/parser_test.go

@@ -11,7 +11,6 @@ import (
 func testHelper(t *testing.T, m map[string]any, matcher func(k string, v any) (bool, any)) {
 	for k, v := range m {
 		ok, val := matcher(k, v)
-		// t.Logf("PASSED: [wanted=%v;got=%v;key=%s]", v, val, k)
 		if !ok {
 			t.Errorf("wanted %v: got %v, for %s", v, val, k)
 		}
@@ -83,7 +82,7 @@ func TestParserPemdas(t *testing.T) {
 	// examples taken from https://pemdas.info/
 	tests := map[string]any{
 		"1+2*3": 7.0,
-		// TODO: this is not correct!, emmy returns 7, not 8, somehow +3/3 is not included in the resulting ast
+		// BUG: this is not correct!, emmy returns 7, not 8, somehow +3/3 is not included in the resulting ast
 		// "7-1*0+3/3": 8.0,
 		"1-2*3*4": -23.0,
 		"3+4/2-4": 1.0,