feat: Add parsing configuration decorator

README.md

@@ -103,6 +103,66 @@ func main() {
 }
 ```
 
+### Configuration
+
+Ecdysis provides an automatic way to parse a configuration file, environment variables, and flags using the [`viper`](https://github.com/spf13/viper) library. To use it, you need to implement the `CommandWithConfig` interface.
+
+The order of precedence for configuration values is:
+
+1. Default values (slices and maps are not currently supported)
+2. Configuration file
+3. Environment variables
+4. Flags
+
+```go
+var (
+    _ ecdysis.CommandWithConfiguration = (*RootCommand)(nil)
+)
+
+type ConduitConfig struct {
+    ConduitCfgPath string `long:"config.path" usage:"global conduit configuration file" default:"./conduit.yaml"`
+
+    Connectors struct {
+        Path string `long:"connectors.path" usage:"path to standalone connectors' directory"`
+    }
+
+    // ...
+}
+
+type RootFlags struct {
+    ConduitConfig // you can embed any configuration, and it'll use the proper tags
+}
+
+type RootCommand struct {
+    flags RootFlags
+    cfg   ConduitConfig
+}
+
+func (c *RootCommand) ParseConfig() ecdysis.Config {
+    return ecdysis.Config{
+        EnvPrefix:  "CONDUIT", // prefix for environment variables
+        ParsedCfg:  &c.cfg, // where configuration will be parsed
+        ConfigPath: c.flags.ConduitCfgPath, // where to read the configuration file
+        DefaultCfg: c.cfg, // where to extract default values from
+    }
+}
+
+func (c *RootCommand) Execute(_ context.Context) error {
+    // c.cfg is now populated with the right parsed configuration
+    return nil
+}
+
+func (c *RootCommand) Flags() []ecdysis.Flag {
+    flags := ecdysis.BuildFlags(&c.flags)
+
+    // set a default value for each flag
+    flags.SetDefault("config.path", c.cfg.ConduitCfgPath) 
+    // ...
+
+    return flags
+}
+````
+
 ## Flags
 
 Ecdysis provides a way to define flags using field tags. Flags will be

configuration.go

@@ -0,0 +1,73 @@
+// Copyright © 2024 Meroxa, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ecdysis
+
+import (
+	"reflect"
+
+	"github.com/spf13/viper"
+)
+
+type Config struct {
+	EnvPrefix  string
+	ParsedCfg  any
+	DefaultCfg any
+	ConfigPath string
+}
+
+// setDefaults sets the default values for the configuration. slices and maps are not supported.
+func setDefaults(v *viper.Viper, defaults interface{}) {
+	val := reflect.ValueOf(defaults)
+	typ := reflect.TypeOf(defaults)
+
+	if val.Kind() == reflect.Ptr {
+		if val.IsNil() {
+			return
+		}
+		val = val.Elem()
+		typ = typ.Elem()
+	}
+
+	if val.Kind() != reflect.Struct {
+		return
+	}
+
+	for i := 0; i < val.NumField(); i++ {
+		field := val.Field(i)
+		fieldType := typ.Field(i)
+
+		fieldName := fieldType.Tag.Get("long")
+		if fieldName == "" {
+			fieldName = fieldType.Tag.Get("short")
+		}
+
+		if fieldName == "" {
+			continue
+		}
+
+		switch field.Kind() { //nolint:exhaustive // no need to handle all cases
+		case reflect.Struct:
+			setDefaults(v, field.Interface())
+		case reflect.Ptr:
+			if !field.IsNil() {
+				setDefaults(v, field.Interface())
+			}
+		default:
+			if field.CanInterface() {
+				v.SetDefault(fieldName, field.Interface())
+			}
+		}
+	}
+}

decorators.go

@@ -21,17 +21,23 @@ import (
 	"fmt"
 	"log/slog"
 	"os"
+	"reflect"
 	"strings"
 	"time"
 
 	"github.com/spf13/cobra"
 	"github.com/spf13/pflag"
+	"github.com/spf13/viper"
 )
 
 var DefaultDecorators = []Decorator{
 	CommandWithLoggerDecorator{},
 	CommandWithAliasesDecorator{},
 	CommandWithFlagsDecorator{},
+
+	// CommandWithParsingConfigDecorator needs to be after CommandWithFlagsDecorator to make sure the flags are parsed.
+	CommandWithParsingConfigDecorator{},
+
 	CommandWithDocsDecorator{},
 	CommandWithHiddenDecorator{},
 	CommandWithSubCommandsDecorator{},
@@ -242,6 +248,87 @@ func (CommandWithFlagsDecorator) Decorate(_ *Ecdysis, cmd *cobra.Command, c Comm
 	return nil
 }
 
+// -- PARSING CONFIGURATION --------------------------------------------------------------------
+
+// CommandWithConfiguration can be implemented by a command to parsing configuration.
+type CommandWithConfiguration interface {
+	Command
+
+	ParseConfig() Config
+}
+
+// CommandWithParsingConfigDecorator is a decorator that sets the command flags.
+type CommandWithParsingConfigDecorator struct{}
+
+// Decorate parses the configuration based on flags.
+func (CommandWithParsingConfigDecorator) Decorate(_ *Ecdysis, cmd *cobra.Command, c Command) error {
+	v, ok := c.(CommandWithConfiguration)
+	if !ok {
+		return nil
+	}
+
+	old := cmd.PreRunE
+	cmd.PreRunE = func(cmd *cobra.Command, args []string) error {
+		if old != nil {
+			err := old(cmd, args)
+			if err != nil {
+				return err
+			}
+		}
+
+		usrCfg := v.ParseConfig()
+
+		// Ensure ParsedCfg is a pointer
+		if reflect.ValueOf(usrCfg.ParsedCfg).Kind() != reflect.Ptr {
+			return fmt.Errorf("ParsedCfg must be a pointer")
+		}
+
+		// Ensure both usrCfg.ParsedCfg and usrCfg.DefaultCfg are the same type
+		if reflect.TypeOf(usrCfg.ParsedCfg) != reflect.TypeOf(usrCfg.DefaultCfg) {
+			return fmt.Errorf("ParsedCfg and DefaultCfg must be the same type")
+		}
+
+		viper := viper.New()
+
+		// Set default values
+		setDefaults(viper, usrCfg.DefaultCfg)
+
+		// Handle env variables
+		viper.SetEnvPrefix(usrCfg.EnvPrefix)
+		viper.AutomaticEnv()
+		viper.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
+
+		// Handle config file
+		viper.SetConfigFile(usrCfg.ConfigPath)
+		if err := viper.ReadInConfig(); err != nil {
+			return fmt.Errorf("fatal error config file: %w", err)
+		}
+
+		var errors []error
+
+		// Handle flags
+		cmd.Flags().VisitAll(func(f *pflag.Flag) {
+			if err := viper.BindPFlag(f.Name, f); err != nil {
+				errors = append(errors, err)
+			}
+		})
+
+		if len(errors) > 0 {
+			var errStrs []string
+			for _, err := range errors {
+				errStrs = append(errStrs, err.Error())
+			}
+			return fmt.Errorf("error binding flags: %s", strings.Join(errStrs, "; "))
+		}
+
+		if err := viper.Unmarshal(usrCfg.ParsedCfg); err != nil {
+			return fmt.Errorf("error unmarshalling config: %w", err)
+		}
+		return nil
+	}
+	return nil
+}
+
 // -- DOCS ---------------------------------------------------------------------
 
 // CommandWithDocs can be implemented by a command to provide documentation.

go.mod

@@ -6,7 +6,31 @@ require (
 	github.com/google/go-cmp v0.6.0
 	github.com/spf13/cobra v1.8.1
 	github.com/spf13/pflag v1.0.5
+	github.com/spf13/viper v1.19.0
+	github.com/stretchr/testify v1.9.0
 	go.uber.org/mock v0.5.0
 )
 
-require github.com/inconshreveable/mousetrap v1.1.0 // indirect
+require (
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
+	github.com/fsnotify/fsnotify v1.7.0 // indirect
+	github.com/hashicorp/hcl v1.0.0 // indirect
+	github.com/inconshreveable/mousetrap v1.1.0 // indirect
+	github.com/magiconair/properties v1.8.7 // indirect
+	github.com/mitchellh/mapstructure v1.5.0 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.2 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
+	github.com/sagikazarmark/locafero v0.4.0 // indirect
+	github.com/sagikazarmark/slog-shim v0.1.0 // indirect
+	github.com/sourcegraph/conc v0.3.0 // indirect
+	github.com/spf13/afero v1.11.0 // indirect
+	github.com/spf13/cast v1.6.0 // indirect
+	github.com/subosito/gotenv v1.6.0 // indirect
+	go.uber.org/atomic v1.9.0 // indirect
+	go.uber.org/multierr v1.9.0 // indirect
+	golang.org/x/exp v0.0.0-20230905200255-921286631fa9 // indirect
+	golang.org/x/sys v0.18.0 // indirect
+	golang.org/x/text v0.14.0 // indirect
+	gopkg.in/ini.v1 v1.67.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)

go.sum

@@ -1,14 +1,79 @@
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