Chapter 14

What Rill deliberately does not have

• Mutation. No mutable bindings, no mutable fields. State is threaded through arguments or channels. The exceptions are the two kinds of storage: a buffer element, b[i] = v (and +=, -=, *=, /=), and a map entry, m[k] = v. Neither is a binding that changes — what changes is the storage the name has always pointed at.
• Exceptions. Errors are Result values, and ? carries one out without the carrying being written.
• Null. Absence is Option.
• Implicit conversions. Numeric types never mix silently.
• Field assignment. A field is read with p.x and never written: a record is a value, and a changed one is a new one built from the old.
• Inheritance or subtyping. Composition and traits instead.
• A garbage collector. Reference counting, decided at compile time.