DART: syntax of future then

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I don´t understand the syntax of the then() clause.

1. myFuture(6).then( (erg) => print(erg) )

What´s (erg) => expr syntactically?

I thougt it could be a function, but

then( callHandler2(erg)

doesn´t work, Error:

"Multiple markers at this line
- The argument type 'void' cannot be assigned to the parameter type '(String) -> 
 dynamic'
- Undefined name 'erg'
- Expected to find ')'"

2. myFuture(5).then( (erg) { callHandler(erg);}, onError: (e) => print (e) 

What´s `onError: (e) => expr"` syntactically?

3. Is there a difference between the onError: and the .catchError(e) variants?

dart-async dart
2 Answers

I will attempt to elaborate more on Matt's answer, hopefully to give more insights.

What then() requires is a function (callback), whose signature matches the future's type.

For example, given a Future<String> myFuture and doSomething being any function that accepts a String input, you can call myFuture.then(doSomething). Now, there are several ways to define a function that takes a String in Dart:

Function(String) doSomething1 = (str) => /* do something with str */  // only one command
Function(String) doSomething2 = (str) { /* do something with str */ } // several commands
Function(String) doSomething3 = myFunction;
myFunction(String) { // Dart will auto imply return type here
  /* do something with str */ // several commands
}

Any of those 3 function definitions (the right hand side of =) could go inside then(). The first two definitions are called lambda functions, they are created at runtime and cannot be reused unless you manually copy the code. Lambda functions can potentially yield language-like expressions, i.e. (connection) => connection.connect(). The third approach allows the function to be reused. Lambda functions are common in many languages, you can read more about it here: https://medium.com/@chineketobenna/lambda-expressions-vs-anonymous-functions-in-javascript-3aa760c958ae.

The reason why you can't put callHandler2(erg) inside then() is because callHandler2(erg) uses an undefined variable erg. Using the lambda function, you will be able to tell then() that the erg in callHandler2(erg) is the output of the future, so it knows where to get erg value.

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