function addCache(servicesHost: typescript.ModuleResolutionHost): {
  moduleResolutionHost: typescript.ModuleResolutionHost,
  clearCache: () => void,
} {
  const clearCacheFunctions: Action[] = [];
  return {
    moduleResolutionHost: {
      ...servicesHost,
      fileExists: createCache(servicesHost.fileExists),
      directoryExists: servicesHost.directoryExists && createCache(servicesHost.directoryExists),
      realpath: servicesHost.realpath && createCache(servicesHost.realpath),
    },
    clearCache: () => clearCacheFunctions.forEach(clear => clear()),
  };

  function createCache<TOut>(originalFunction: (arg: string) => TOut) {
    const cache = new Map<string, TOut>();
    clearCacheFunctions.push(() => cache.clear());
    return function getCached(arg: string) {
      let res = cache.get(arg);
      if (res !== undefined) {
        return res;
      }

      res = originalFunction(arg);
      cache.set(arg, res);
      return res;
    };
  }
}

  let clearCache: null | (() => void) = null;
  let moduleResolutionHost: ModuleResolutionHost = {
    fileExists,
    readFile: readFileWithFallback,
    realpath: compiler.sys.realpath,
    directoryExists: compiler.sys.directoryExists,
    getCurrentDirectory: compiler.sys.getCurrentDirectory,
    getDirectories: compiler.sys.getDirectories
  };

  if (enableFileCaching) {
    const cached = addCache(moduleResolutionHost);
    clearCache = cached.clearCache;
    moduleResolutionHost = cached.moduleResolutionHost;
  }