parser: introduce parsing callbacks

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

parser/lexer.go

@@ -8,6 +8,8 @@ import (
 
 // Run parses the source
 func (p *Parser) Run() error {
+	p.setDefaults()
+
 	p.pos.Reset()
 	return lexer.Run(p.lexStart)
 }

parser/parser.go

@@ -12,6 +12,56 @@ type Parser struct {
 	src *lexer.Reader
 
 	pos lexer.Position
+
+	// OnSection is called after a [section] is parsed.
+	// Returning an error will abort the process.
+	OnSection func(pos lexer.Position, name, subname string, hasSubname bool) error
+
+	// OnField is called after a `key = value` entry is parsed
+	// Returning an error will abort the process.
+	OnField func(pos lexer.Position, key, value string) error
+
+	// OnComment is called after a comment is parsed
+	// Returning an error will abort the process.
+	OnComment func(pos lexer.Position, comment string) error
+
+	// OnError is called after each parsing error, which you are allowed to
+	// override.
+	// OnError is called for EOF as well, but this error isn't returned as such by
+	// Parser.Run(). The caller will receive (nil, nil) instead indicating the
+	// processes terminated correctly.
+	OnError func(pos lexer.Position, content string, err error) error
+}
+
+func defaultOnSection(_ lexer.Position, _, _ string, _ bool) error { return nil }
+func defaultOnField(_ lexer.Position, _, _ string) error           { return nil }
+func defaultOnComment(_ lexer.Position, _ string) error            { return nil }
+
+func defaultOnError(pos lexer.Position, content string, err error) error {
+	return &lexer.Error{
+		Line:    pos.Line,
+		Column:  pos.Column,
+		Content: content,
+		Err:     err,
+	}
+}
+
+func (p *Parser) setDefaults() {
+	if p.OnSection == nil {
+		p.OnSection = defaultOnSection
+	}
+
+	if p.OnField == nil {
+		p.OnField = defaultOnField
+	}
+
+	if p.OnComment == nil {
+		p.OnComment = defaultOnComment
+	}
+
+	if p.OnError == nil {
+		p.OnError = defaultOnError
+	}
 }
 
 // NewParser creates a dosini-style parser using