lexer: introduce Error{}

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/lexer.go

@@ -1,31 +1,17 @@
 // Package lexer provides basic helpers to implement parsers
 package lexer
 
-import (
-	"errors"
-	"io"
-)
-
 // StateFn is a State Function of the parser
 type StateFn func() (StateFn, error)
 
 // Run runs a state machine until the state function either
 // returns nil or an error
 func Run(fn StateFn) error {
-	for fn != nil {
-		var err error
+	var err error
 
+	for fn != nil && err == nil {
 		fn, err = fn()
-		switch {
-		case errors.Is(err, io.EOF):
-			// EOF
-			return nil
-		case err != nil:
-			// failed
-			return err
-		}
 	}
 
-	// ended
-	return nil
+	return err
 }

lexer/position.go

@@ -41,26 +41,28 @@ func (p *Position) Step() {
 	p.Column++
 }
 
-// StepN moves the column N places forward
-func (p *Position) StepN(n int) {
+// Next returns a new Position one rune forward
+// on the line
+func (p Position) Next() Position {
 	if p.Line == 0 {
 		p.Reset()
 	}
 
-	switch {
-	case n > 0:
-		p.Column += n
-	default:
-		panic(fmt.Errorf("invalid %v increment", n))
+	return Position{
+		Line:   p.Line,
+		Column: p.Column + 1,
 	}
 }
 
-// StepLine moves position to the start of the next line
-func (p *Position) StepLine() {
+// NextLine returns a new Position at the begining of the next
+// line.
+func (p Position) NextLine() Position {
 	if p.Line == 0 {
 		p.Reset()
 	}
 
-	p.Line++
-	p.Column = 1
+	return Position{
+		Line:   p.Line + 1,
+		Column: 1,
+	}
 }