How to Filter Values from a Nilable Array Using Query Expressions

I want to select specific fields from a record with the record type Student[]? in Ballerina.

In the code below, classes[className] has the type Student[]?.

Student[] is iterable but Student[]? is not.

Why is that and how does it figure that out?

import ballerina/io;

type Student record {|
    string firstName;
    string lastName;
    int age;
    string address;
    string className;
|};

function getStudentNames(map<Student[]> classes) returns map<string[]> {
    map<string[]> studentNames = {};

    foreach string className in classes.keys() {
        string[] names = [];
        foreach Student student in classes[className] {
            names.push(student.firstName + " " + student.lastName);
        }
        studentNames[className] = names;
    }

    return studentNames;
}

Student[]? is a union type which means the value can be either an array of Student or nil. You can iterate classes[className] in the foreach (and similarly query expression) only if it is a Student[] and is not nil. So you need to narrow the type down/handle the nil scenario.

For example,

function getStudentNames(map<Student[]> classes) returns map<string[]> {
    map<string[]> studentNames = {};

    foreach string className in classes.keys() {
        string[] names = [];
        if classes[className] is Student[] {
            Student[] students = <Student[]> classes[className];
            foreach Student student in students {
                names.push(student.firstName + " " + student.lastName);
            }
        }
        
        studentNames[className] = names;
    }

    return studentNames;
}

Simply you can do as follows by using the map function in an array.

function getStudentNames(map<Student[]> classes) returns map<string[]> {
    map<string[]> studentNames = {};

    foreach string className in classes.keys() {
        string[] names = classes[className] is Student[] ? (<Student[]> classes[className]).map(student => student.firstName + " " + student.lastName) : [];
        
        studentNames[className] = names;
    }

    return studentNames;
}