Make config load failures explicit and preserve defaults for missing config

cmd/root.go

@@ -95,7 +95,10 @@ func runDiff(cmd *cobra.Command, args []string) error {
 		}
 	}
 
-	cfg := config.Load()
+cfg, err := config.Load()
+if err != nil {
+	return fmt.Errorf("config: %w", err)
+}
 	t := resolveTheme(cfg)
 	styles := ui.NewStyles(t)
 
@@ -149,7 +152,10 @@ func runCommit(cmd *cobra.Command, args []string) error {
 		return nil
 	}
 
-	cfg := config.Load()
+cfg, err := config.Load()
+if err != nil {
+	return fmt.Errorf("config: %w", err)
+}
 	t := resolveTheme(cfg)
 	styles := ui.NewStyles(t)
 
@@ -170,7 +176,10 @@ func runLog(cmd *cobra.Command, args []string) error {
 		return nil
 	}
 
-	cfg := config.Load()
+cfg, err := config.Load()
+if err != nil {
+	return fmt.Errorf("config: %w", err)
+}
 	t := resolveTheme(cfg)
 	styles := ui.NewStyles(t)
 

internal/config/config.go

@@ -2,6 +2,8 @@ package config
 
 import (
 	"encoding/json"
+	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 )
@@ -25,24 +27,30 @@ func Default() Config {
 }
 
 // Load reads config from ~/.config/differ/config.json.
-// Returns defaults if file doesn't exist.
-func Load() Config {
+// Returns defaults and a nil error if the file doesn't exist; otherwise returns defaults plus an error on failures.
+func Load() (Config, error) {
 	path, err := configPath()
 	if err != nil {
-		return Default()
+		return Default(), fmt.Errorf("could not resolve config path: %w", err)
 	}
 	return LoadFrom(path)
 }
 
-// LoadFrom reads config from the given path. Returns defaults on error.
-func LoadFrom(path string) Config {
+// LoadFrom reads config from the given path.
+// Returns defaults and a nil error if the file doesn't exist; otherwise returns defaults plus an error on failures (e.g., read/parse errors).
+func LoadFrom(path string) (Config, error) {
 	cfg := Default()
 	data, err := os.ReadFile(path)
 	if err != nil {
-		return cfg
+		if errors.Is(err, os.ErrNotExist) {
+			return cfg, nil
+		}
+		return cfg, fmt.Errorf("could not read config file %q: %w", path, err)
 	}
-	_ = json.Unmarshal(data, &cfg)
-	return cfg
+	if err := json.Unmarshal(data, &cfg); err != nil {
+		return cfg, fmt.Errorf("invalid config file %q: %w; fix the JSON or remove the file to use defaults", path, err)
+	}
+	return cfg, nil
 }
 
 // Save writes config to ~/.config/differ/config.json.

internal/config/config_test.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -38,7 +39,10 @@ func TestSaveAndLoad(t *testing.T) {
 		t.Fatalf("SaveTo: %v", err)
 	}
 
-	got := LoadFrom(path)
+	got, err := LoadFrom(path)
+	if err != nil {
+		t.Fatalf("LoadFrom: %v", err)
+	}
 	if got.Theme != "light" {
 		t.Errorf("Theme=%q, want light", got.Theme)
 	}
@@ -56,7 +60,10 @@ func TestSaveAndLoad(t *testing.T) {
 func TestLoad_NoFile(t *testing.T) {
 	t.Parallel()
 	path := filepath.Join(t.TempDir(), "nonexistent.json")
-	cfg := LoadFrom(path)
+	cfg, err := LoadFrom(path)
+	if err != nil {
+		t.Fatalf("LoadFrom missing file: %v", err)
+	}
 	if cfg.Theme != "dark" || cfg.TabWidth != 4 {
 		t.Error("missing file should return defaults")
 	}
@@ -69,7 +76,13 @@ func TestLoad_InvalidJSON(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	cfg := LoadFrom(path)
+	cfg, err := LoadFrom(path)
+	if err == nil {
+		t.Fatal("LoadFrom should return an error for invalid JSON")
+	}
+	if !strings.HasPrefix(err.Error(), "invalid config file") {
+		t.Fatalf("LoadFrom error = %q, want invalid config prefix", err.Error())
+	}
 	if cfg.Theme != "dark" || cfg.TabWidth != 4 {
 		t.Error("invalid JSON should return defaults")
 	}