From d976902768ee320e323bf216897872c935c00a2f Mon Sep 17 00:00:00 2001
From: Christian Rocha <christian@rocha.is>
Date: Sat, 18 Jan 2020 10:13:44 -0500
Subject: [PATCH] Okay, I think that's how error handling should work

---
 examples/input/main.go | 64 ++++++++++++++++++++++++++----------------
 tea.go                 | 18 +++++++++++-
 2 files changed, 57 insertions(+), 25 deletions(-)

diff --git a/examples/input/main.go b/examples/input/main.go
index 69023cc..f9d9806 100644
--- a/examples/input/main.go
+++ b/examples/input/main.go
@@ -3,6 +3,7 @@ package main
 // A simple program that counts down from 5 and then exits.
 
 import (
+	"errors"
 	"fmt"
 	"log"
 	"tea"
@@ -10,8 +11,9 @@ import (
 	"tea/input"
 )
 
-type model struct {
-	input input.Model
+type Model struct {
+	Input input.Model
+	Error error
 }
 
 type tickMsg struct{}
@@ -20,19 +22,21 @@ func main() {
 	tea.UseSysLog("tea")
 
 	p := tea.NewProgram(
-		model{
-			input: input.DefaultModel(),
+		Model{
+			Input: input.DefaultModel(),
+			Error: nil,
 		},
 		update,
 		view,
 		[]tea.Sub{
-			// Just hand off the subscription to the input component
-			func(m tea.Model) tea.Msg {
-				if m, ok := m.(model); ok {
-					return input.Blink(m.input)
+			// We just hand off the subscription to the input component, giving
+			// it the model it expects.
+			func(model tea.Model) tea.Msg {
+				m, ok := model.(Model)
+				if !ok {
+					return tea.NewErrMsg("could not perform assertion on model")
 				}
-				// TODO: return error
-				return nil
+				return input.Blink(m.Input)
 			},
 		},
 	)
@@ -42,9 +46,16 @@ func main() {
 	}
 }
 
-func update(msg tea.Msg, mdl tea.Model) (tea.Model, tea.Cmd) {
+func update(msg tea.Msg, model tea.Model) (tea.Model, tea.Cmd) {
 	var cmd tea.Cmd
-	m, _ := mdl.(model)
+	m, ok := model.(Model)
+	if !ok {
+		// When we encounter errors in Update we simply add the error to the
+		// model so we can handle it in the view. We could also return a command
+		// that does something else with the error, like logs it via IO.
+		m.Error = errors.New("could not perform assertion on model")
+		return m, nil
+	}
 
 	switch msg := msg.(type) {
 	case tea.KeyMsg:
@@ -54,22 +65,27 @@ func update(msg tea.Msg, mdl tea.Model) (tea.Model, tea.Cmd) {
 		case "esc":
 			return m, tea.Quit
 		}
+
+	// We handle errors just like any other message
+	case tea.ErrMsg:
+		m.Error = msg
+		return m, nil
 	}
 
-	m.input, cmd = input.Update(msg, m.input)
+	m.Input, cmd = input.Update(msg, m.Input)
 	return m, cmd
 }
 
-func view(m tea.Model) string {
-	if m, ok := m.(model); ok {
-		help := "(esc to exit)"
-
-		return fmt.Sprintf(
-			"What’s your favorite Pokémon?\n\n%s\n\n%s",
-			input.View(m.input),
-			help,
-		)
+func view(model tea.Model) string {
+	m, ok := model.(Model)
+	if !ok {
+		return "Oh no: could not perform assertion on model."
+	} else if m.Error != nil {
+		return fmt.Sprintf("Uh oh: %s", m.Error)
 	}
-	// TODO: return error
-	return ""
+	return fmt.Sprintf(
+		"What’s your favorite Pokémon?\n\n%s\n\n%s",
+		input.View(m.Input),
+		"(esc to quit)",
+	)
 }
diff --git a/tea.go b/tea.go
index 326c5a7..9ca665e 100644
--- a/tea.go
+++ b/tea.go
@@ -1,6 +1,7 @@
 package tea
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"log"
@@ -44,6 +45,22 @@ type Program struct {
 	rw            io.ReadWriter
 }
 
+// ErrMsg is just a regular message containing an error. We handle it in Update
+// just like a regular message by case switching. Of course, the developer
+// could also define her own errors as well.
+type ErrMsg struct {
+	error
+}
+
+func (e ErrMsg) String() string {
+	return e.Error()
+}
+
+// NewErrMsg is a convenience function for creating a generic ErrMsg
+func NewErrMsg(s string) ErrMsg {
+	return ErrMsg{errors.New(s)}
+}
+
 // Quit is a command that tells the program to exit
 func Quit() Msg {
 	return quitMsg{}
@@ -63,7 +80,6 @@ func NewProgram(model Model, update Update, view View, subs []Sub) *Program {
 }
 
 // Start initializes the program
-// TODO: error channel
 func (p *Program) Start() error {
 	var (
 		model = p.model