refactor: add custom error types for better error handling

pagination.go

@@ -2,28 +2,38 @@ package pagination
 
 import (
 	"encoding/base64"
-	"fmt"
+	"errors"
 	"slices"
 )
 
+var (
+	ErrFirstAndLastProvided   = errors.New("only one of first and last can be provided")
+	ErrFirstNegative          = errors.New("first must be greater than 0")
+	ErrLastNegative           = errors.New("last must be greater than 0")
+	ErrAfterAndBeforeProvided = errors.New("only one of after and before can be provided")
+	ErrInvalidAfterCursor     = errors.New("after is not a valid cursor")
+	ErrInvalidBeforeCursor    = errors.New("before is not a valid cursor")
+	ErrInvalidCursor          = errors.New("invalid cursor")
+)
+
 func Validate(first *int, after *string, last *int, before *string) error {
 	if first != nil && last != nil {
-		return fmt.Errorf("only one of first and last can be provided")
+		return ErrFirstAndLastProvided
 	}
 	if first != nil && *first < 0 {
-		return fmt.Errorf("first must be greater than 0")
+		return ErrFirstNegative
 	}
 	if last != nil && *last < 0 {
-		return fmt.Errorf("last must be greater than 0")
+		return ErrLastNegative
 	}
 	if after != nil && len(*after) > 0 && before != nil && len(*before) > 0 {
-		return fmt.Errorf("only one of after and before can be provided")
+		return ErrAfterAndBeforeProvided
 	}
 	if ValidateCursor(after) != nil {
-		return fmt.Errorf("after is not a valid cursor")
+		return ErrInvalidAfterCursor
 	}
 	if ValidateCursor(before) != nil {
-		return fmt.Errorf("before is not a valid cursor")
+		return ErrInvalidBeforeCursor
 	}
 	return nil
 }
@@ -42,7 +52,7 @@ func DecodeCursor(cursor *string) (string, error) {
 	}
 	b64, err := base64.StdEncoding.DecodeString(*cursor)
 	if err != nil {
-		return "", err
+		return "", ErrInvalidCursor
 	}
 	return string(b64), nil
 }