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This revision made October 28, 2011 20:13, by Kin-man Chung

Lambda Expression in EL

To support query expressions on collections, it is necessary to pass EL expressions as argument to methods. These expressions will be evaluated in the methods. So essentially we need closures in EL. Fortunately, EL already has the concept of deferred expressions, so we just need to put the pieces together to add lambda expressions to EL.

An EL lambda expression is a deferred expression with parameters.

Syntax

The proposed syntax follows that of the one used in Lambda in JDK 8 http://cr.openjdk.java.net/~briangoetz/lambda/lambda-state-3.html

Of course the syntax in JDK is not final until it FCSed, but it is comforting to see the familiar syntax #{...}. :-)

Grammar:

  Lambda := '#{'  (ArgList '->')? Body '}'
  ArgList := Identifier (',' Identifier)*
  Body := Expression
  

Example:

  #{ 42 }
  ${x -> x+1}
  #{x,y -> x+y}

Operations on Lambda Expression

  • It can be assigned to a variable:
    fn = #{x -> x+1}
  • It can be invoked:
    fn(100) yields 101
    #{x, y -> x+y}(3,4) yields 7
  • It can be passed to a method:
    employees.where(#{p->p.name.first == 'Larry'})

API

  • A couple of methods in ELContext for managing arguments for lambda expressions, making it easy to set up arguments for calling the lambda expression, and to remove the arguments from ELContext after the call. See here
  • A new class LambdaExpression to encapsulate the EL Lambda expression, and to call it with arguments.


Additional Considerations

  • Current EL insists that functions be bound and mapped at parse time. This will not work well with lambda, which may be evaluated another JVM or even another machine. We'll need to consider lazy and late function binding at evaluation time.
Difference compared to previous revision
<span> employees.where(#{p->p.name.first == 'Larry'}) ==API== *A couple of methods in ELContext for managing arguments for lambda expressions, making it easy to set up arguments for <span style="text-decoration:line-through;color:red">the </span>calling the lambda expression, and to remove the arguments from ELContext after the call. See [http://java.net/projects/el-spec/sources/source-code/content/trunk/api/src/main/java/javax/el/ELContext.java here] *A new class [http://java.net/projects/el-spec/sources/source-code/content/trunk/api/src/main/java/javax/el/LambdaExpression.java LambdaExpression] to encapsulate the EL Lambda expression, and to call it with arguments. <span>
 
 
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