Iterables

Iterable values introduce a unified concept for processing sequences in various data structures. While the compiler uses an internal type iterable<T>, which cannot be explicitly declared in code, several types can be used as iterable<T>.

Iterable types

1. range: iterable<integer>

   • Generates a sequence of integers.

   • Examples:

     • range(10) produces 0 to 9.

     • range(1, 10) produces 1 to 9.

     • range(0, 10, 2) produces 0, 2, 4, 6, 8.

2. list<T>: iterable<T>

   • Stores an ordered collection of elements of type T.

   • Example:

     • list<integer> = [1, 2, 3, 4, 5]

3. set<T>: iterable<T>

   • Holds a unique collection of elements of type T.

   • Example:

     • set<text> = {"apple", "banana", "cherry"}

4. map<K,V>: iterable<(K,V)>

   • Associates keys of type K with values of type V.

   • Example:

     • map<text, integer> = {"one": 1, "two": 2, "three": 3}

5. Virtual collections

   • virtual<list<T>>, virtual<set<T>>, virtual<map<K,V>>

   • Represent virtual collections that behave like their non-virtual counterparts.

Key applications of iterables

1. for loops

   Easily iterate through elements:

   for (x in range(10)) {
       print(x);
   }

2. Collection-at expressions

   Apply operations to each element:

   val doubled = range(1, 6) @* {} ( $ * 2 );

3. List/Set/Map constructors

   Build new collections from iterables:

   struct Person {
       name: text;
       age: integer;
   }
   
   val people = [
       Person("Alice", 28),
       Person("Bob", 35),
       Person("Charlie", 22),
       Person("David", 41),
       Person("Eve", 19)
   ];
   
   // List constructor: names of people over 30
   val senior_names = list<text>();
   for (p in people) {
       if (p.age > 30) {
           senior_names.add(p.name);
       }
   }
   
   // Set constructor: unique ages
   val unique_ages = set<integer>();
   for (p in people) {
       unique_ages.add(p.age);
   }
   
   // Map constructor: name to age mapping
   val name_to_age = map<text, integer>();
   for (p in people) {
       name_to_age[p.name] = p.age;
   }
   
   // Printing names of seniors
   for (name in senior_names) {
       print("Senior name: " + name);
   }
   
   // Printing unique ages
   for (age in unique_ages) {
       print("Unique age: " + age);
   }
   
   // Printing name to age map
   for (name in name_to_age.keys()) {
       print(name + " is " + name_to_age[name] + " years old.");
   }

Practical examples

Working with ranges

// Generate squares of numbers from 0 to 9
val squares = list<integer>();
for (i in range(10)) {
    squares.add(i * i);
}
print("Squares: " + squares);

// Create a set of natural numbers from 1 to 9
val naturals = set<integer>();
for (i in range(1, 10)) {
    naturals.add(i);
}
print("Naturals: " + naturals);

// Generate even numbers from 0 to 18
val evens = list<integer>();
for (i in range(0, 20, 2)) {
    evens.add(i);
}
print("Evens: " + evens);

Constructing and manipulating maps

// Construct a map from tuples
val tuples = [(1,'A'), (2,'B'), (3,'C')];
val m = map(tuples);
print(m);  // Output: {1=A, 2=B, 3=C}

// Convert a map to a list of tuples
val L = list(m);
print(L);  // Output: [(1,A), (2,B), (3,C)]

// Create a map from two lists
val keys = ["name", "age", "city"];
val values = ["Alice", "30", "New York"];
val details = map<text, text>();
for (i in range(keys.size())) {
    details[keys[i]] = values[i];
}
print(details);  // Output: {name=Alice, age=30, city=New York}

Working with sets

// Create a set from a list with duplicates
val numbers = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4];
val unique_numbers = set(numbers);
print(unique_numbers);  // Output: [1, 2, 3, 4]

Advanced iterable usage

// Filter and transform a list
val numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
val even_numbers = list<integer>();
for (number in numbers) {
    if (number % 2 == 0) {
        even_numbers.add(number);
    }
}
print(even_numbers); // Output: [2, 4, 6, 8, 10]



// Combine multiple iterables
val numbers1 = [1, 2, 3];
val numbers2 = [4, 5, 6];
val combined = list<integer>();
for (number in numbers1) {
    combined.add(number);
}
for (number in numbers2) {
    combined.add(number);
}
print(combined);  // Output: [1, 2, 3, 4, 5, 6]