1 files changed, 97 insertions, 0 deletions
diff --git a/errors.go b/errors.go
new file mode 100644
index 0000000..4d72a93
--- /dev/null
+++ b/errors.go
@@ -0,0 +1,97 @@
+package pflag
+
+import "fmt"
+
+// notExistErrorMessageType specifies which flavor of "flag does not exist"
+// is printed by NotExistError. This allows the related errors to be grouped
+// under a single NotExistError struct without making a breaking change to
+// the error message text.
+type notExistErrorMessageType int
+
+const (
+	flagNotExistMessage notExistErrorMessageType = iota
+	flagNotDefinedMessage
+	flagNoSuchFlagMessage
+	flagUnknownFlagMessage
+	flagUnknownShorthandFlagMessage
+)
+
+// NotExistError is the error returned when trying to access a flag that
+// does not exist in the FlagSet.
+type NotExistError struct {
+	name                string
+	specifiedShorthands string
+	messageType         notExistErrorMessageType
+}
+
+// Error implements error.
+func (e *NotExistError) Error() string {
+	switch e.messageType {
+	case flagNotExistMessage:
+		return fmt.Sprintf("flag %q does not exist", e.name)
+
+	case flagNotDefinedMessage:
+		return fmt.Sprintf("flag accessed but not defined: %s", e.name)
+
+	case flagNoSuchFlagMessage:
+		return fmt.Sprintf("no such flag -%v", e.name)
+
+	case flagUnknownFlagMessage:
+		return fmt.Sprintf("unknown flag: --%s", e.name)
+
+	case flagUnknownShorthandFlagMessage:
+		c := rune(e.name[0])
+		return fmt.Sprintf("unknown shorthand flag: %q in -%s", c, e.specifiedShorthands)
+	}
+
+	panic(fmt.Errorf("unknown flagNotExistErrorMessageType: %v", e.messageType))
+}
+
+// ValueRequiredError is the error returned when a flag needs an argument but
+// no argument was provided.
+type ValueRequiredError struct {
+	flag                *Flag
+	specifiedName       string
+	specifiedShorthands string
+}
+
+// Error implements error.
+func (e *ValueRequiredError) Error() string {
+	if len(e.specifiedShorthands) > 0 {
+		c := rune(e.specifiedName[0])
+		return fmt.Sprintf("flag needs an argument: %q in -%s", c, e.specifiedShorthands)
+	}
+
+	return fmt.Sprintf("flag needs an argument: --%s", e.specifiedName)
+}
+
+// InvalidValueError is the error returned when an invalid value is used
+// for a flag.
+type InvalidValueError struct {
+	flag  *Flag
+	value string
+	cause error
+}
+
+// Error implements error.
+func (e *InvalidValueError) Error() string {
+	flag := e.flag
+	var flagName string
+	if flag.Shorthand != "" && flag.ShorthandDeprecated == "" {
+		flagName = fmt.Sprintf("-%s, --%s", flag.Shorthand, flag.Name)
+	} else {
+		flagName = fmt.Sprintf("--%s", flag.Name)
+	}
+	return fmt.Sprintf("invalid argument %q for %q flag: %v", e.value, flagName, e.cause)
+}
+
+// InvalidSyntaxError is the error returned when a bad flag name is passed on
+// the command line.
+type InvalidSyntaxError struct {
+	specifiedFlag string
+}
+
+// Error implements error.
+func (e *InvalidSyntaxError) Error() string {
+	return fmt.Sprintf("bad flag syntax: %s", e.specifiedFlag)
+}