Dart async/await encapsulation -

i want return fractional of decimal, if function takes time, function must give up. tried this, doesn't work... did wrong. show me mistake?

string decimaltofractional(double d) async {   var df = 1.0;   var top = 1;   var bot = 1;    var future = new future<string>(() {     while (df != d) {       if (df < d) {         top += 1;       } else {         bot += 1;         top = (d * bot).toint();       }       df = top / bot;     }     return new future.value('$top/$bot');   });   future.timeout(new duration(seconds: 2), ontimeout: () => new future.value(d.tostring()));   return await future; } 

there several problems code. if want return result of async operation return type has future<...>

future<string> decimaltofractional(double d) async { 

you can consume result

main() async {   print(await decimaltofractional(123456789.123456789)); } 

if want timeout take effect need return event loop. long sync execution running timeout doesn't fire. see https://stackoverflow.com/a/22473556/217408