From 97a695752040ccf96335c5e593bd9856a61a5fb6 Mon Sep 17 00:00:00 2001
From: Jeff Carr <jcarr@wit.com>
Date: Tue, 14 Oct 2025 18:04:39 -0500
Subject: thoughts on errors

---
 errors.go | 110 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 109 insertions(+), 1 deletion(-)

(limited to 'errors.go')

diff --git a/errors.go b/errors.go
index 807386d..7f318af 100644
--- a/errors.go
+++ b/errors.go
@@ -1,5 +1,113 @@
 package config
 
-import "errors"
+import (
+	"errors"
+	"fmt"
+)
 
 var NotInitialized error = errors.New("your application config not initialized")
+
+// Finally, declare a package-level variable of your new type.
+// This is the "sentinel" error that users will compare against.
+var ErrNotInitialized = NotInitializedError{}
+
+func ErrdKeyFalse(varname string) error {
+	return errors.New("config key '" + varname + "' was not 'true'")
+}
+
+func ErrdKeyNotSet(varname string) error {
+	return errors.New("config key '" + varname + "' was not set")
+}
+
+// thanks gemini for this example custom error example
+
+// --- 1. A Custom Error Type for "Not Initialized" ---
+
+// First, define a new, named type for your error. A struct is common.
+type NotInitializedError struct{}
+
+// Now, attach the Error() method to your new type. This makes it
+// satisfy the built-in 'error' interface.
+func (e NotInitializedError) Error() string {
+	return "your application config not initialized"
+}
+
+// (Your Goal) Attach a custom String() method for pretty printing.
+// This will be used by fmt.Println, for example.
+func (e NotInitializedError) String() string {
+	return "func main() didn't initialize this"
+}
+
+// --- 2. A Custom Error Type with an Is() Method ---
+
+// Let's make a more complex error for key-not-set issues.
+type KeyNotSetError struct {
+	KeyName string // It can hold data!
+}
+
+// Make it satisfy the 'error' interface.
+func (e KeyNotSetError) Error() string {
+	return fmt.Sprintf("config key '%s' was not set", e.KeyName)
+}
+
+// (Your Goal) Define a custom Is() method.
+// errors.Is(someError, ErrKeyNotSet) will call this method.
+func (e KeyNotSetError) Is(target error) bool {
+	// We can check if the target is of the same type.
+	// This allows errors.Is(err, KeyNotSetError{}) to work,
+	// checking for the general *type* of error.
+	_, ok := target.(KeyNotSetError)
+	return ok
+}
+
+// A helper function to create this error.
+func NewKeyNotSetError(varname string) error {
+	return KeyNotSetError{KeyName: varname}
+}
+
+// A generic sentinel error for this category.
+var ErrKeyNotSet = KeyNotSetError{}
+
+/*
+// --- Example Usage ---
+
+func checkConfig() error {
+	// Pretend the config is not initialized.
+	return ErrNotInitialized
+}
+
+func checkKey(key string) error {
+	// Pretend a specific key is not set.
+	return NewKeyNotSetError(key)
+}
+
+func main() {
+	// --- Test Case 1: The NotInitializedError ---
+	err1 := checkConfig()
+
+	// errors.Is() works perfectly because it compares the variable directly.
+	if errors.Is(err1, ErrNotInitialized) {
+		fmt.Println("Check 1: errors.Is correctly identified ErrNotInitialized.")
+	}
+
+	// fmt.Println will use your custom String() method!
+	fmt.Println("Check 2: Printing the error uses the custom String() method:")
+	fmt.Println(err1)
+	fmt.Println()
+
+	// --- Test Case 2: The KeyNotSetError ---
+	err2 := checkKey("database_url")
+
+	// This works because of your custom Is() method!
+	// We are checking if err2 is of the *type* KeyNotSetError.
+	if errors.Is(err2, ErrKeyNotSet) {
+		fmt.Println("Check 3: errors.Is correctly identified the general KeyNotSetError type.")
+	}
+
+	// You can also get the underlying error to inspect its data.
+	var keyErr KeyNotSetError
+	if errors.As(err2, &keyErr) {
+		fmt.Printf("Check 4: errors.As extracted the error data. Key not set: %s\n", keyErr.KeyName)
+	}
+}
+*/
-- 
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