Dart lambda/shortland function confusion

Question

I'm still pretty new to Dart and the syntax of => (fat arrow) still confuses me (I come from C# background).

So in C# fat arrow ( => ) says: goes to so for example:

Action<string> action1 = (str) => { System.Diagnostic.Debug.WriteLine("Parameter received: " + str.ToString()); }

action1("Some parameter");

means: whatever send as parameter to action1 (if it could be casted to string) goes to inner scope (in our case it just printed in Debug.WriteLine()

but in Dart it's something different.... (?)

for example in Future.then

ClassWithFutures myClass = new ClassWithFutures();
myClass.loadedFuture.then( 
   (str) => { print("Class was loaded with info: $str"),
   onError: (exp) => { print("Error occurred in class loading. Error is: $exp"); }
);

Dart editor warn me that the first and second print is: Expected string literal for map entry key. I think in C# way that str it just name for parameter that will be filled by internal callback that Future.then uses to call onValue or onError

What I'm doing wrong ?

there is a missing brace in you example: ```(str) => { print("Class was loaded with info: $str") },``` — M . Franklin, Apr 16 '21 at 02:14

score 118 · Accepted Answer · edited Dec 24 '22 at 02:24

You need to choose either block syntax or single expression syntax, but not both.

You can't combine => with {}.

Your two options are as follows using your example:

ClassWithFutures myClass = new ClassWithFutures();
myClass.loadedFuture.then( 
  (str) => print("Class was loaded with info: $str"),
  onErrro: (exp) => print("Error occurred in class loading. Error is: $exp")
);

or

ClassWithFutures myClass = new ClassWithFutures();
myClass.loadedFuture.then( 
  (str) { print("Class was loaded with info: $str"); },
  onErrro: (exp) { print("Error occurred in class loading. Error is: $exp"); }
);

In both cases, it is just a way to express an anonymous function.

Normally if you want to just run a single expression, you use the => syntax for cleaner and more to the point code. Example:

someFunction.then( (String str) => print(str) );

or you can use a block syntax with curly braces to do more work, or a single expression.

someFunction.then( (String str) {
  str = str + "Hello World";
  print(str);
});

but you can't combine them since then you are making 2 function creation syntaxes and it breaks.

You can use => with {} when you have multiple expression. (value) => { condition = false, print('periodic timer!') } — Davoud, Jan 07 '21 at 09:02
@DB no, it's wrong! () => {} means "single line expression, returning a Set literal" — Sergey Molchanovsky, Jan 08 '23 at 08:38

score 61 · Answer 2 · answered Apr 04 '13 at 06:47

In Dart => xxx is just a syntaxic sugar to avoid { return xxx; }. Thus the two following functions are equivalent :

var a = (String s) => s;
var b = (String s) { return s; } ;

You can also use => on method definitions :

String myFunc(String s) => s;
String myFunc(String s) {
  return s;
}

score 4 · Answer 3 · answered Mar 26 '19 at 08:05

That syntax works well in a language like javascript and also c# where it supports (param1, param2, …, paramN) => { statements } with statement being separated with semi colon. In dart, the fat arrow only supports expression which is shorthand for { return expr; }.
That explains your Error. Your code with the curly brace (exp) => { print("Error occurred in class loading. Error is: $exp"); } means you are returning a map, so it expects to see something like (param) => {"key": "value"} where key is a string literal.

Dart lambda/shortland function confusion

3 Answers3

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