Send an error when no provider.

module/context.go

@@ -2,6 +2,7 @@ package module
 
 import (
 	"context"
+	"fmt"
 )
 
 type createPanic struct {
@@ -27,7 +28,7 @@ func (c *moduleContext) Value(key any) any {
 
 	provider, ok := c.providers[moduleKey]
 	if !ok {
-		return nil
+		panic(createPanic{key: moduleKey, err: fmt.Errorf("can't find module provideing %q", moduleKey)})
 	}
 
 	instance, err := provider.value(c)

module/example_test.go

@@ -34,9 +34,6 @@ type Cache struct {
 
 func NewCache(ctx context.Context) (*Cache, error) {
 	db := ModuleDB.Value(ctx)
-	if db == nil {
-		return nil, fmt.Errorf("no db as fallback")
-	}
 	return &Cache{
 		fallback: db,
 	}, nil
@@ -190,7 +187,7 @@ func ExampleModule_notExistingProvider() {
 	}
 
 	// Output:
-	// inject error: module *module_test.Cache creates an instance error: no db as fallback
+	// inject error: module module_test.DB creates an instance error: can't find module provideing "module_test.DB"
 }
 
 type FileSystem interface {
@@ -223,13 +220,13 @@ func (f *mockFileSystem) Write() {}
 func ExampleModule_duplicatingProviders() {
 	defer func() {
 		p := recover().(string)
-		fmt.Println("panic:", regexp.MustCompile(`at .*\/go-espresso`).ReplaceAllString(p, "at .../go-espresso"))
+		fmt.Println("panic:", regexp.MustCompile(`at .*`).ReplaceAllString(p, "at <removed file and line>"))
 	}()
 
 	repo := module.NewRepo()
 	repo.AddModule(RealFileSystem)
 	repo.AddModule(MockFileSystem)
 
 	// Output:
-	// panic: already have a provider with type "module_test.FileSystem", added at .../go-espresso/module/example_test.go:230
+	// panic: already have a provider with type "module_test.FileSystem", added at <removed file and line>
 }