Array functions like map/reduce

[patrickgalbraith]

I noticed that the new version supports arrays and structs. Just wondering if there is a way to implement array functions that can take a function/expression as an argument.

Something along the lines of:

products.map(product => product.total * product.quantity)

Or even something like this which I guess should be do-able as long as we can set the this variable of the sub-expression for each item in the loop:

MAP(products, this.total * this.quantity)

[stevenylai]

Something like below should work

@FunctionParameters({
  @FunctionParameter(name = "array"),
  @FunctionParameter(name = "placeholder", isLazy = true),
  @FunctionParameter(name = "mapper", isLazy = true)
})
public class MapFunction extends AbstractFunction {
  @Override
  public EvaluationValue evaluate(
      Expression expression, Token functionToken, EvaluationValue... parameterValues)
      throws EvaluationException {
    List<EvaluationValue> array = parameterValues[0].getArrayValue();
    String placeHolder = parameterValues[1].getExpressionNode().getToken().getValue();
    ASTNode mapper = parameterValues[2].getExpressionNode();

    List<EvaluationValue> mapped = new ArrayList<>();
    for (EvaluationValue value : array) {
      mapped.add(expression.withValues(Map.of(placeHolder, value)).evaluateSubtree(mapper));
    }
    return EvaluationValue.arrayValue(mapped);
  }
}

Once added as a custom function, you should be able to write sth like:

new Expression("MAP(products, quantity, total * quantity)")
  .withValues(Map.<String, Object>of(
    "products": List.of(1, 2, 3),
    "total": 2
  ))
  .evaluate();