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  1. Why the language is like this

Why values instead of references

Every type is a value and the binding decides about mutation, which removes every mutable-and-immutable type pair at the price of one local, lintable trap.

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Most languages answer two questions about a value: can the binding be changed, and can the thing be changed. TorbScript answers one. This page is the argument for that, and for the one cost it has.

The decision#

Every type is a value, and the binding decides whether it can be changed. This is mutable value semantics, as in Swift's structs and in Hylo.

  • Assigning, passing and capturing a value is a copy. Two bindings never refer to the same value.
  • const is deep: through a const binding nothing changes.
  • Mutation needs a var path - a var binding, var parameter or a var fn receiver, then var fields all the way down.
  • References are second-class: they exist only as a var parameter or a var fn receiver, for the duration of one call. They cannot be stored in a field, returned, or captured by a closure that escapes.
  • Identity is the marked exception: a shared type is not copied when it is assigned.

Why#

Because the alternative tripled every concept. The design started with an immutable type plus a mutable reference var type. That produced List, MutableList and ListView; builders next to the things they build; snapshots with toList(); and "is this a type or a var type?" as the first question about every type anybody wrote. With one rule - no var path, no mutation - all of that disappears. There is one Point and one List, and a const binding is the immutable one.

Because a change then happens exactly where it is written. The hardest class of bug in a language with references is a change that arrives through a name you were not looking at. That bug cannot be written here: nothing is aliased, so nothing changes behind your back. The compiler does not need a borrow checker for that, and you do not need defensive copies.

Because it makes concurrency uninteresting. A task gets copies, so there is nothing to race for. Data races are impossible by construction rather than by discipline, and the only things that cross a task boundary and are not copied are Channel and Task. A value that contains a shared object is confined to the task that created it, and the compiler derives that without an annotation.

Because references can stay second-class. Since a reference only exists for the duration of a call, there are no lifetimes, no borrow checker, no 'a, and nothing can dangle. A mutable slice - the thing Rust needs &mut [T] and Go needs slices for - is a var path to a range: samples[0..100].sort { _ }.

Because the cost is not what it looks like. "Always a copy" is what the model says; what the implementation does is something else and is not observable. A small value like a Point is really copied. A List, a Map or a String shares its storage until somebody writes to it, and then the writer copies - copy on write, implemented once, in the native collections. A type built from those gets it for free. var parameters and var paths are passed as references, and their "copy in, copy out" meaning is never executed literally.

What was rejected#

  • Immutable types plus a mutable reference type (var type). Rejected for the reason above: three names for every concept, and a question to answer about every declaration.
  • Lifetimes and a borrow checker, as in Rust. They buy first-class references, which the language does not need once mutation needs a var path and every value is copied. They cost a concept that every reader and every error message has to carry.
  • A weak reference, and a cycle collector. Values cannot form cycles, and a closure that captures a var binding may not escape its scope, so only shared type objects can. There is no cycle collector for them either: trees and graphs hold handles instead of references, a stored callback takes its owner as a receiver, and a leaked cycle is reported by type at the end of a test (the destructors design record, section 9).
  • Destructors that run on any value. A destructor exists, and only a shared type may have one: close(), run exactly once by the last release (planned, the destructors design record). A value is copied on assignment and has no identity for a destructor to belong to.

Consequences#

The copy trap is the price. Taking an element out of a collection takes a copy, so the program below prints 1 0: the copy changed and the list did not.

type Counter {
  var count: Int = 0

  var fn increment() {
    count = count + 1
  }
}

var counters = [Counter(), Counter()]
var first = counters[0]
first.increment()
print "{first.count} {counters[0].count}"
// prints 1 0

The rule that catches this is general: a change that cannot have an effect is a compile error. With value semantics such a change is always a mistake and never a defensive line, so it is not a lint. A copy that is changed and then read is not reported, because the change has an effect - on the copy. The fix is to reach through the path:

type Counter {
  var count: Int = 0

  var fn increment() {
    count = count + 1
  }
}

var counters = [Counter(), Counter()]
counters[0].increment()
print counters[0].count

A method that changes and a method that returns a copy are different words. sort and sorted, append and appended, remove and removed. The verb is a var fn; the participle reads its receiver and answers a new value. The participles are default members of the collection traits, so an implementation writes only the verbs.

There are no getters and no defensive copies. protected var hands an outsider a const path to a field, and const is deep, so config.routes can be read and iterated from outside while config.routes.append(...) is an error. What somebody takes out of it is a copy anyway.

Exclusivity replaces the borrow checker, and it is conservative. While a var access to a path is running, the same path cannot be accessed another way. The access of a call begins once all of its arguments have been evaluated, so items.removeAt(items.length() - 1) is ordinary code; what is left is what really overlaps - two var accesses of the same call, and a closure argument that reaches the path the call is changing. What the compiler cannot prove is an error, and there is no check at runtime.

Nothing escapes, not even a captured var binding. A closure that captures a var binding shares it with its scope - the only sharing of a variable in the language - so it follows the rule of a reference: it may be the argument of a call that just calls it, and it is never bound, stored or returned. Otherwise two copies of a value could share a variable through a closure they hold, which is exactly the aliasing values exist to rule out. The binding is exempt from the dead-change rule, because the read can be anywhere the call runs the closure.

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