Collections

Type[] is an ordered array and Type{} is a string-keyed map.
Normal index access returns an optional value because a position or key may be missing.
Collection methods have fixed receiver-specific contracts and typed it bindings.

Array And Map Types

code

1Expense[] expenses = []2number{} balances = {}3number[][] monthly_shares = []4Expense{} expenses_by_id = {}

Type[] stores ordered values of one element type.
Type{} stores values of one type under string keys.
Modifiers can nest, as in number[][] or Expense{}[].
An empty literal needs an explicit collection type because it has no member to infer from.

Index Results

code

1Expense? first = expenses[0]2number? alex_balance = balances["alex"]34with expenses[0] as expense {5  selected = expense6}

Normal indexing is optional because an array position or map key may not exist. Prove presence with with, use ?? for a scalar fallback, or attach a declaration-level container fallback in full/backend Nenu.

Array Selection And Projection

code

1large = expenses.filter(it.amount >= 100)2first_open = expenses.first(it.status != ExpenseStatus.Settled)3last_open = expenses.last(it.status != ExpenseStatus.Settled)4titles = expenses.collect(it.title)5has_large = expenses.any(it.amount >= 100)6all_positive = expenses.all(it.amount > 0)

Method	Result
`filter(predicate)`	All matching elements as `T[]`.
`first(predicate)`	The first match as `T?`.
`last(predicate)`	The last match as `T?`.
`collect(expression)`	Projected values as `U[]`.
`any(predicate)`	`bool`: at least one match.
`all(predicate)`	`bool`: every element matches.

Changing Arrays

code

1expenses.push(new_expense)2expenses = expenses + [new_expense]3expenses = expenses + imported_expenses

push mutates an array and returns no value, so use it as its own statement. Array + returns a new array and requires the same element type on both sides.

Map Methods

code

1member_ids = balances.keys()2amounts = balances.values()3non_zero = balances.filterValues(it != 0)4public_balances = balances.filterKeys(it != "private")56number total = 07loop balances as member_id, amount {8  total = total + amount9}

filterKeys binds it to a string key. filterValues binds it to the map value type. Iterate a map directly with two bindings; loop balances.entries() as key, value is equivalent supported sugar.

Common Mistakes

Wrong	Correct	Reason
`expenses = []` as a first binding	`Expense[] expenses = []`	Empty literals cannot infer an element type.
`expenses.push new_expense`	`expenses.push(new_expense)`	Method calls require parentheses.
Use `push` as a value	Use it as a statement.	Push returns no value.
Treat `expenses[0]` as required	Use `Expense?` or a presence guard.	The index may be missing.

Continue Learning

Loops Iterate arrays and maps with one or two bindings.Defaults And Fallbacks Make missing collection indexes return a declaration-owned fallback.