Dart: How to initialize function-type field in initializer list?-CodePudding

I have a class which accept a function as a constructor parameter. I want to define a secondary constructor where the user can provide a static value instead. I did something like this:

import 'package:dartz/dartz.dart';

class UpdateCheckNotWorking {
  final Function0<Future<String>> versionProvider;

  UpdateCheckNotWorking(this.versionProvider);

  UpdateCheckNotWorking.withStaticVersion(String version)
      : versionProvider = () => Future.value(version);
}

This code does not compile because of 2 errors:

A workaround is to use a late final field and initialize it in the constructor body:

class UpdateCheckOk {
  late final Function0<Future<String>> versionProvider;

  UpdateCheckOk(this.versionProvider);

  UpdateCheckOk.withStaticVersion(String version) {
    versionProvider = () => Future.value(version);
  }
}

But I am wondering, why is the first implementation not valid?

CodePudding user response：

It seems to work if you add some parenthesis:

import 'package:dartz/dartz.dart';

class UpdateCheckOk {
  final Function0<Future<String>> versionProvider;

  UpdateCheckOk(this.versionProvider);

  UpdateCheckOk.withStaticVersion(String version)
      : versionProvider = (() => Future.value(version));
}