v/doc/upcoming.md

V Work In Progress

This document describes features that are not implemented, yet. Please refer to docs.md for the current state of V

Table of Contents

• Concurrency
  • Variable Declarations
  • Strengths
  • Weaknesses
  • Compatibility
  • Automatic Lock
  • Channels

Concurrency

Variable Declarations

Objects that are supposed to be used to exchange data between coroutines have to be declared with special care. Exactly one of the following 4 kinds of declaration has to be chosen:

a := ...
mut b := ...
shared c := ...
atomic d := ...

• a is declared as constant that can be passed to other coroutines and read without limitations. However it cannot be changed.

• b can be accessed reading and writing but only from one coroutine. That coroutine owns the object. A mut variable can be passed to another coroutine (as receiver or function argument in the go statement or via a channel) but then ownership is passed, too, and only the other coroutine can access the object.¹

• c can be passed to coroutines an accessed concurrently.² In order to avoid data races it has to be locked before access can occur and unlocked to allow access to other coroutines. This is done by one the following block structures:

  lock c {
      // read, modify, write c
      ...
  }
  

  rlock c {
      // read c
      ...
  }
  

  Several variables may be specified: lock x, y, z { ... }. They are unlocked in the opposite order.

• d can be passed to coroutines and accessed concurrently, too.³ No lock is needed in this case, however atomic variables can only be 32/64 bit integers (or pointers) and access is limited to a small set of predefined idioms that have native hardware support.

To help making the correct decision the following table summarizes the different capabilities:

	default	mut	shared	atomic
write access		+	+	+
concurrent access	+		+	+
performance	++	++		+
sophisticated operations	+	+	+
structured data types	+	+	+

Strengths

default

• very fast
• unlimited access from different coroutines
• easy to handle

mut

• very fast
• easy to handle

shared

• concurrent access from different coroutines
• data type may be complex structure
• sophisticated access possible (several statements within one lock block)

atomic

• concurrent access from different coroutines
• reasonably fast

Weaknesses

default

• read only

mut

• access only from one coroutine at a time

shared

• lock/unlock are slow
• moderately difficult to handle (needs lock block)

atomic

• limited to single (max. 64 bit) integers (and pointers)
• only a small set of predefined operations possible
• very difficult to handle correctly

¹ The owning coroutine will also free the memory space used for the object when it is no longer needed.
² For shared objects the compiler adds code for reference counting. Once the counter reaches 0 the object is automatically freed.
³ Since an atomic variable is only a few bytes in size allocation would be an unnecessary overhead. Instead the compiler creates a global.

Compatibility

Outside of lock/rlock blocks function arguments must in general match - with the familiar exception that objects declared mut can be used to call functions expecting immutable arguments:

fn f(x St) {...}
fn g(mut x St) {...}
fn h(shared x St) {...}
fn i(atomic x u64) {...}

a := St{...}
f(a)

mut b := St{...}
f(b)
go g(mut b)
// `b` should not be accessed here anymore

shared c := St{...}
h(shared c)

atomic d &u64
i(atomic d)

Inside a lock c {...} block c behaves like a mut, inside an rlock c {...} block like an immutable:

shared c := St{...}
lock c {
    g(mut c)
    f(c)
    // call to h() not allowed inside `lock` block
    // since h() will lock `c` itself
}
rlock c {
    f(c)
    // call to g() or h() not allowed
}

Automatic Lock

In general the compiler will generate an error message when a shared object is accessed outside of any corresponding lock/rlock block. However in simple and obvious cases the necessary lock/unlock can be generated automatically for array/map operations:

shared a := []int{cap: 5}
go h2(shared a)
a << 3
// keep in mind that `h2()` could change `a` between these statements
a << 4
x := a[1] // not necessarily `4`

shared b := map[string]int{}
go h3(shared b)
b['apple'] = 3
c['plume'] = 7
y := b['apple'] // not necessarily `3`

// iteration over elements
for k, v in b {
    // concurrently changed k/v pairs may or may not be included
}

This is handy, but since other coroutines might access the array/map concurrently between the automatically locked statements, the results are sometimes surprising. Each statement should be seen as a single transaction that is unrelated to the previous or following statement. Therefore - but also for performance reasons - it's often better to group consecutive coherent statements in an explicit lock block.

Channels