Statements

Statements execute program behavior. Top-level code is intentionally limited; loops run inside functions.

Syntax

x = 1
y: num = x + 1
xs = [1, 2, 3]
xs[0] = 10

if y > 1:
  print(y)
else:
  print(0)

Semantics

Assignment binds a value to a name. An optional type annotation checks the value at runtime.

Subscript assignment is pure rebinding syntax for lists and dicts. Normal subscript assignment supports nested indexing. Slice assignment is not implemented.

xs = [1, 2, 3]
xs[1] = 20

counts = {"a": 1}
counts["b"] = 2

matrix = [[1, 2], [3, 4]]
matrix[0][1] = 9

Destructuring assigns values from a list or tuple to multiple names.

name, score = ("nilakan", 10)
(first, rest) = [1, [2, 3]]

The source value must have the same number of items as the target list. Empty destructuring is not supported. Parentheses are allowed around a destructuring target list.

Function call statements are allowed when the return value is intentionally discarded. They are also the only legal way to call an effectful procedure.

def observe(value: num) -> num:
  return value

observe(1)

print(...) is an effectful builtin procedure. It may be called at top level, from a function, or inside a commit loop, but it cannot be used where an expression value is required. A user-defined function that calls print, directly or through another function, is treated as an effectful procedure and is likewise callable only as a statement.

if, elif, and else select a branch using boolean guards. Branches may be indented blocks or single-line blocks. Single-line blocks cannot contain nested block statements.

return ends a function and must return a value.

Constraints And Errors

return is illegal at top level. Top-level while and init are not supported; put commit loops inside functions. Output produced during a commit-loop round is transactional: Nilakan publishes it only if that round commits. See Commit Loops.

Primed destructuring is only legal inside a commit loop and only for state variables introduced by init. A fallible right-hand side still needs an error handler:

def take_first(values: list[num]) -> list[num]:
  init:
    rest = values
    done = False
  while not done:
    item, rest' = rest.pop(0) catch (0, rest)
    done' = True
  return rest

Primed subscript assignment supports one index only. Nested primed subscript assignment is not implemented.

Examples

def choose(flag: bool) -> str:
  result = "yes" if flag else "no"
  return result

print(choose(True))