From c8a13bbf493b011784d68d948edbe17d3a3fdead Mon Sep 17 00:00:00 2001
From: Samuel Berthe <dev@samuel-berthe.fr>
Date: Sun, 3 Jul 2022 20:59:32 +0200
Subject: [PATCH] doc: explain custom panic message for lo.Must helpers

---
 README.md | 18 ++++++++++++++----
 errors.go | 10 +++++++---
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/README.md b/README.md
index c78ac0f..f838d90 100644
--- a/README.md
+++ b/README.md
@@ -1574,10 +1574,6 @@ val := lo.Must(time.Parse("2006-01-02", "2022-01-15"))
 
 val := lo.Must(time.Parse("2006-01-02", "bad-value"))
 // panics
-
-val := lo.Must(lo.Find(myString, func(i string) bool {
-    return i == requiredChar
-}), "'%s' must always contain '%s'", myString, requiredChar)
 ```
 
 ### Must{0->6}
@@ -1612,6 +1608,20 @@ lo.Must0(math.Signbit(v))
 before, after := lo.Must2(bytes.Cut(s, sep))
 ```
 
+You can give context to the panic message by adding some printf-like arguments.
+
+```go
+val := lo.Must(lo.Find(myString, func(i string) bool {
+    return i == requiredChar
+}), "'%s' must always contain '%s'", myString, requiredChar)
+
+// MustX
+lo.Must0(example0(), "'%s' must always contain '%s'", myString, requiredChar)
+val1 := lo.Must1(example1(), "'%s' must always contain '%s'", myString, requiredChar)
+val1, val2 := lo.Must2(example2(), "'%s' must always contain '%s'", myString, requiredChar)
+...
+```
+
 ## Try
 
 Calls the function and return false in case of error and on panic.
diff --git a/errors.go b/errors.go
index dfe2795..e9f9c8e 100644
--- a/errors.go
+++ b/errors.go
@@ -7,7 +7,10 @@ import (
 
 func messageFromMsgAndArgs(msgAndArgs ...interface{}) string {
 	if len(msgAndArgs) == 1 {
-		return fmt.Sprintf(msgAndArgs[0].(string))
+		if msgAsStr, ok := msgAndArgs[0].(string); ok {
+			return msgAsStr
+		}
+		return fmt.Sprintf("%+v", msgAndArgs[0])
 	}
 	if len(msgAndArgs) > 1 {
 		return fmt.Sprintf(msgAndArgs[0].(string), msgAndArgs[1:]...)
@@ -21,18 +24,19 @@ func must(err any, messageArgs ...interface{}) {
 		return
 	}
 
-	message := messageFromMsgAndArgs(messageArgs...)
-
 	switch e := err.(type) {
 	case bool:
 		if !e {
+			message := messageFromMsgAndArgs(messageArgs...)
 			if message == "" {
 				message = "not ok"
 			}
+
 			panic(message)
 		}
 
 	case error:
+		message := messageFromMsgAndArgs(messageArgs...)
 		if message != "" {
 			panic(message + ": " + e.Error())
 		} else {