Add new comparators: Exists and NotExists (#33)

internal/testutils/multilines.go

@@ -5,6 +5,60 @@ import (
 	"strings"
 )
 
+// lineMismatchedError is an error indicating unmatched line when comparing
+// string using ExpectMultiLineString.
+type lineMismatchedError struct {
+	LineNumber int
+	Expected   string
+	Received   string
+}
+
+// NewLineMismatchedError is a constructor for lineMismatchedError.
+func NewLineMismatchedError(lineNumber int, expected, received string) error {
+	return lineMismatchedError{
+		LineNumber: lineNumber,
+		Expected:   expected,
+		Received:   received,
+	}
+}
+
+func (err lineMismatchedError) Error() string {
+	return fmt.Sprintf("at line %d\nexpected: %v\nreceived: %v", err.LineNumber,
+		err.Expected, err.Received)
+}
+
+type missingLinesError struct {
+	MissingLines []string
+}
+
+// NewMissingLinesError is a constructor for missingLinesError.
+func NewMissingLinesError(missingLines []string) error {
+	return missingLinesError{
+		MissingLines: missingLines,
+	}
+}
+
+func (err missingLinesError) Error() string {
+	return fmt.Sprintf("missing lines:\n%s",
+		strings.Join(err.MissingLines, "\n"))
+}
+
+type unexpectedLinesError struct {
+	UnexpectedLines []string
+}
+
+// NewUnexpectedLinesError is a constructor for unexpectedLinesError.
+func NewUnexpectedLinesError(unexpectedLines []string) error {
+	return unexpectedLinesError{
+		UnexpectedLines: unexpectedLines,
+	}
+}
+
+func (err unexpectedLinesError) Error() string {
+	return fmt.Sprintf("unexpected lines:\n%s",
+		strings.Join(err.UnexpectedLines, "\n"))
+}
+
 // ExpectMultiLineString compares two multi-line strings and report the
 // difference.
 func ExpectMultiLineString(expected, actual string) error {
@@ -18,14 +72,14 @@ func ExpectMultiLineString(expected, actual string) error {
 
 	for i := 0; i < numberOfComparableLines; i++ {
 		if expectedLines[i] != actualLines[i] {
-			return fmt.Errorf("at line %d\nexpected: %v\nreceived: %v", i+1, expectedLines[i], actualLines[i])
+			return NewLineMismatchedError(i+1, expectedLines[i], actualLines[i])
 		}
 	}
 
 	if len(expectedLines) < len(actualLines) {
-		return fmt.Errorf("unexpected lines:\n%s", strings.Join(actualLines[len(expectedLines):], "\n"))
+		return NewUnexpectedLinesError(actualLines[len(expectedLines):])
 	} else if len(expectedLines) > len(actualLines) {
-		return fmt.Errorf("missing lines:\n%s", strings.Join(expectedLines[len(actualLines):], "\n"))
+		return NewMissingLinesError(expectedLines[len(actualLines):])
 	}
 
 	return nil