StringEvaluator

Functional Interface:

This is a functional interface and can therefore be used as the assignment target for a lambda expression or method reference.
```
@FunctionalInterface
public interface StringEvaluator
```
A StringEvaluator is a function that typically encapsulates the logic for evaluating an expression that returns a string.

- Method Summary
  
  All Methods Instance Methods Abstract Methods
  Modifier and Type Method Description
  
  java.lang.String eval(XPathContext context)
  Evaluate the encapsulated expression

- Method Detail
  - eval
```
java.lang.String eval(XPathContext context)
               throws XPathException
```
    Evaluate the encapsulated expression
    
    Parameters:
    
    context - the evaluation context
    
    Returns:
    
    the result, as a string; or null to represent an empty sequence
    
    Throws:
    
    XPathException - if a dynamic error occurs