Add global DefaultClient (readonly), WithUserAgent option

Signed-off-by: Vartan Benohanian <vartanbeno@gmail.com>

reddit/reddit-options.go

@@ -4,11 +4,23 @@ import (
 	"errors"
 	"net/http"
 	"net/url"
+	"os"
 )
 
 // Opt is a configuration option to initialize a client.
 type Opt func(*Client) error
 
+// WithCredentials sets the credentials used to authenticate with the Reddit API.
+func WithCredentials(id, secret, username, password string) Opt {
+	return func(c *Client) error {
+		c.ID = id
+		c.Secret = secret
+		c.Username = username
+		c.Password = password
+		return nil
+	}
+}
+
 // WithHTTPClient sets the HTTP client which will be used to make requests.
 func WithHTTPClient(httpClient *http.Client) Opt {
 	return func(c *Client) error {
@@ -20,6 +32,16 @@ func WithHTTPClient(httpClient *http.Client) Opt {
 	}
 }
 
+// WithUserAgent sets the User-Agent header for requests made with the client.
+// Reddit recommends the following format for the user agent:
+// <platform>:<app ID>:<version string> (by /u/<reddit username>)
+func WithUserAgent(ua string) Opt {
+	return func(c *Client) error {
+		c.userAgent = ua
+		return nil
+	}
+}
+
 // WithBaseURL sets the base URL for the client to make requests to.
 func WithBaseURL(u string) Opt {
 	return func(c *Client) error {
@@ -43,3 +65,25 @@ func WithTokenURL(u string) Opt {
 		return nil
 	}
 }
+
+// FromEnv configures the client with values from environment variables.
+// Supported environment variables:
+// GO_REDDIT_CLIENT_ID to set the client's id.
+// GO_REDDIT_CLIENT_SECRET to set the client's secret.
+// GO_REDDIT_CLIENT_USERNAME to set the client's username.
+// GO_REDDIT_CLIENT_PASSWORD to set the client's password.
+func FromEnv(c *Client) error {
+	if v := os.Getenv("GO_REDDIT_CLIENT_ID"); v != "" {
+		c.ID = v
+	}
+	if v := os.Getenv("GO_REDDIT_CLIENT_SECRET"); v != "" {
+		c.Secret = v
+	}
+	if v := os.Getenv("GO_REDDIT_CLIENT_USERNAME"); v != "" {
+		c.Username = v
+	}
+	if v := os.Getenv("GO_REDDIT_CLIENT_PASSWORD"); v != "" {
+		c.Password = v
+	}
+	return nil
+}