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Lambda Expressions

A lambda expression is basically a piece of code, which is wrapped in an object to pass it around. As a Java developer it is best to think of a lambda expression as an anonymous class with a single method.

1. Basic Definition

A lambda expression is code surrounded by square brackets (inspired from Smalltalk). Like a method declaration, a lambda expression may declare parameters.

[ e : ActionEvent |
    textField.text = "Something happened!" + e.toString
]

The lambda above has one parameter called e which is of type ActionEvent. The code after the | operator is the internal code of the lambda expression.

2. Inferred Parameter Type

You do not have to specify the type explicitly because it can be inferred from the context. For example, when using inferred type, the code in the previous section becomes:

[ e |
    textField.text = "Something happened!" + e.toString
]

The type of e is inferred according to its usage.

3. Implicit Parameters: it

As lambdas with one parameter are a common case, there a special short hand notation for these parameters, which is to leave the declaration including the vertical bar out.

The name of the single parameter becomes it.

[
    textField.text = "Something happened!" + it.toString
]

4. Empty List of Parameters

A lambda expression with zero arguments is written with a bar after the opening bracket:

val runnable : Runnable = [ |
    println("Hello I'm executed!")
]

5. Short notation of a Lambda Expression

When the lamda expression is passed to a single-parameter function, the brackets that are enclosing the lambda declaration could be missed. It specific syntax is called the short notation of a lambda expression.

Consider the following definition of a function with lambda expression as a single parameter:

def aFunction(aParameter : (Object) => Object) {
}

The call to the previously defined function with the short notation is:

aFunction(a | new Object)

This call is equivalent to the following one, with the standard notation:

aFunction([a | new Object])

As for the standard notation, the parameter and the bar could be missed when their is no formal parameter.

6. Lambda as the Last Parameter of a Method

When the last argument of a method call is a lambda, it can be passed right after the argument list.

For instance if you want to sort some strings by their length, you could use either of the following two examples. The first example uses the notation with the lambda expression inside the argument list.

var t : List<String>
// Lambda expression is written outside the parenthesis
Collections.sort(t, [ a, b | a.length - b.length ])

The second example uses the notation with the lambda expression outside the argument list.

var t : List<String>
// Lambda expression is written outside the parenthesis
Collections.sort(t) [ a, b | a.length - b.length ]

7. Type of a Lambda Expression

7.1. Pure SARL Notation

Because SARL is a strongly typed programming language, each lambda expression has a type. The syntax for specifying the type of a lambda is: (parameter types) => return type

The following example defines a variable f, which is a lambda taking one parameter of String, and returning a value of Boolean.

var f : (String) => Boolean

If the lambda is a procedure, i.e. it has no return type, then the return type to specify must be void.

7.2. Java-like Notation

The SARL lambda expressions are mapped to the Java types defined in Functions or Procedures.

These two Java interfaces contains the definitions of inner interfaces for function/procedure with different numbers of parameters.

For example, the SARL erasure (String) => Boolean may be written with the Java notation:

// Same type of function.
var f2 : Functions.Function1<? super String,? extends Boolean>

The Function1 interface represents a function with a single formal parameter (with the type equals to the first generic parameter), and returning a value of type equals to the second generic parameter. Depending of the number of parameters, you may use Function0 to Function5, or Procedure0 to Procedure6.

8. Acknowledgements

This documentation is inspired by the documentations from the Xtext and Xtend projects.

Copyright © 2014-2018 the original authors or authors.

Licensed under the Apache License, Version 2.0; you may not use this file except in compliance with the License. You may obtain a copy of the License.

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