Can I use both optional positional parameters and named parameters in the same function with Dart?

Question

I assume that Dart doesn't allow for both named and optional positional parameters. Am I correct in assuming this? Is there a way that I can create this function without having to use a named parameter for the second string?

Here is my function:

void log(Object? s,{ int priority = 1,LogType logType = LogType.misc,bool on = true}){
  if (priority >= lowestDisplayedPriority && (activeLogs.contains(logType)) && on){
    print('='+s.toString());
  }
}

I want to turn the function into the following:

void log(Object? s,[Object? s2 = ''],{ int priority = 1,LogType logType = LogType.misc,bool on = true}){
  if (priority >= lowestDisplayedPriority && (activeLogs.contains(logType)) && on){
    print('='+s.toString());
    if(s2 != ''){
      print('='+s2.toString());
    }  
  }
}

I get an error which says "Expected to find ')'."

My desired function's usage would be something like this:

log(A,B, priority : 2) or log(A, priority : 2)

If you called log(A,B), how is it to know if B is an s2 or a priority? — Scott Hunter
– Scott Hunter, Commented Mar 22, 2023 at 13:57
An example would be log(A,B, priority : 2) or log(A, priority : 2) — Tim
– Tim, Commented Mar 22, 2023 at 14:08
It's not currently possible to have both named and optional positional parameters. See: github.com/dart-lang/language/issues/1076 — jamesdlin
– jamesdlin, Commented Mar 22, 2023 at 15:06
@ScottHunter There would be no ambiguity. For log(A, B), B is positional and not named and therefore would be s2. Unlike Python, parameters are positional or named, never both. Arguments for named parameters must always be named. — jamesdlin
– jamesdlin, Commented Mar 22, 2023 at 15:15

passsy · Accepted Answer · 2023-03-24 19:00:19Z

4

That's not yet possible. Please keep upvoting https://github.com/dart-lang/language/issues/1076

Two alternatives came to mind you could use today:

Use a list as first parameter

This way you always have to provide a list, which sucks for single arguments. But it works quite well.

void main() {
  log([1, 2], priority: Priority.low);
  log(['a', 'b', 'c'], priority: Priority.high);
  log(['hello']);
  // low: 1, 2
  // high: a, b, c
  // low: hello
}


void log(List<Object?> args, {Priority priority = Priority.low}) {
  final text = args.join(', ');
  print('${priority.name}: $text');
}

enum Priority {
  low,
  high,
}

Filter special types from args as options

This doesn't give you named parameters, but it might be easier to call for single arguments.

import 'package:collection/collection.dart';

void main() {
  log(1, 2, Priority.low);
  log('a', 'b', 'c', Priority.high);
  log('hello');
  // low: 1, 2
  // high: a, b, c
  // low: hello
}

void log([
  Object a0 = _default,
  Object a1 = _default,
  Object a2 = _default,
  Object a3 = _default,
]) {
  final allArgs = [a0, a1, a2, a3].where((it) => it != _default);
  final priority = allArgs.whereType<Priority>().firstOrNull ?? Priority.low;
  final text = allArgs.where((it) => it is! Priority).join(', ');

  print('${priority.name}: $text');
}

enum Priority {
  low,
  high,
}

const Object _default = Object();

answered Mar 24, 2023 at 19:00

passsy

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1 Comment

Tim Over a year ago

This will definitely achieve my intended functionality. Thank you.

Tim · Accepted Answer · 2023-03-22 19:41:18Z

0

@jamerdlin Answered my question. It is not currently possible (2023/03/22).

My solution is to create another function to let take in a List<Object?> where I can print multiple things when needed.

answered Mar 22, 2023 at 19:41

Tim

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Can I use both optional positional parameters and named parameters in the same function with Dart?

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Use a list as first parameter

Filter special types from args as options

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2 Answers 2

Use a list as first parameter

Filter special types from args as options

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