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Last updated December 13, 2011 21:46, by Kin-man Chung
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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 JDK 8 see jsr 335 edr, though much simplified, since there is no type declarations in EL.

Grammar:

  Lambda := LambdaParameters '->' Expression
  LambdaParameters := identifier | '(' Parameters ')'
  Parameters := (identifier (',' identifier)*)?
  

Example:

  ()->  No parameter, void result
  ()->42  No parameter
  (x) -> x+1  One parameter
  x -> x+1  Parenthesis optional for one parameter
  (x,y) -> x+y  Two parameters

Operations on Lambda Expression

  • Lambda expressions can be nested
  • 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  Note the use of parenthesis to disambiguate
  A lambda expression can also be invoked in Java with javax.el.LambdaExpression.invoke, described below.
  • 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.
 
 
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