Make Logger contravariant over the log type (#17)

src/consoleLogger.ts

@@ -1,16 +1,16 @@
-import {Log, LogDetails, Logger, LogLevel} from './logger';
+import {Log, Logger, LogLevel} from './logger';
 
 /**
  * A logger that writes to the console.
  */
-export class ConsoleLogger<T extends LogDetails = LogDetails> implements Logger<T> {
+export class ConsoleLogger<T extends Log = Log> implements Logger<T> {
     private readonly console: Console;
 
     public constructor(targetConsole: Console = console) {
         this.console = targetConsole;
     }
 
-    public log(log: Log<T>): void {
+    public log(log: T): void {
         const args = log.details === undefined
             ? [log.message]
             : [log.message, log.details];

src/logger.ts

@@ -53,11 +53,11 @@ export type Log<D extends LogDetails = LogDetails> = {
 /**
  * A common interface for loggers.
  */
-export interface Logger<D extends LogDetails = LogDetails> {
+export interface Logger<L extends Log = Log> {
     /**
      * Logs a message.
      *
      * @param log The log message.
      */
-    log(log: Log<D>): void;
+    log(log: L): void;
 }

src/prefixedLogger.ts

@@ -1,9 +1,9 @@
-import {Log, LogDetails, Logger} from './logger';
+import {Log, Logger} from './logger';
 
 /**
  * A logger that prepends a prefix to all log messages.
  */
-export class PrefixedLogger<T extends LogDetails = LogDetails> implements Logger<T> {
+export class PrefixedLogger<T extends Log = Log> implements Logger<T> {
     /**
      * The logger to which messages are forwarded.
      */
@@ -25,7 +25,7 @@ export class PrefixedLogger<T extends LogDetails = LogDetails> implements Logger
         this.prefix = prefix;
     }
 
-    public log({message, ...log}: Log<T>): void {
+    public log({message, ...log}: T): void {
         this.logger.log({
             message: `[${this.prefix}] ${message}`,
             ...log,

src/suppressedLogger.ts

@@ -1,9 +1,9 @@
-import {LogDetails, Logger} from './logger';
+import {Log, Logger} from './logger';
 
 /**
  * A logger that suppresses all logging.
  */
-export class SuppressedLogger<T extends LogDetails = LogDetails> implements Logger<T> {
+export class SuppressedLogger<T extends Log = Log> implements Logger<T> {
     public log(): void {
         // suppress debug logs
     }

test/variance.test-d.ts

@@ -0,0 +1,17 @@
+import {Log, Logger} from '../src';
+
+declare function make<T>(): T;
+
+declare function receive<T>(x: T): void;
+
+type TestLog = Log<{
+    foo: string,
+}>;
+
+// Covariance Log test:
+// A custom Log type must be assignable to the base logger type.
+receive<Log>(make<TestLog>());
+
+// Contravariance Logger test:
+// A base logger must be assignable to a logger with a custom Log type.
+receive<Logger<TestLog>>(make<Logger>());

tsconfig.build.json

@@ -8,6 +8,6 @@
   "exclude": [
     "node_modules",
     "build",
-    "**/*.test.ts"
+    "test/**/*.ts"
   ]
 }