# Sources

*Sources* in Vide are special objects that store a single value. They are the
core of reactivity in Vide. Each source represents a source of data, and they
can be composed and derived to create new sources of data.

A source in Vide can be created using `source()`.

```lua
local source = vide.source

local count = source(0)
```

The value passed to `source()` is the initial value of the source.

The value of a source can be set by calling it with an argument, and can be read
by calling it with no arguments.

```lua
count(count() + 1) -- increment count by 1
```

Sources can be *derived* by wrapping them in functions. A wrapped source
effectively becomes a new source.

```lua
local count = source(0)

local text = function()
    return "count: " .. tostring(count())
end

print(text()) -- "count: 0"
count(1)
print(text()) -- "count: 1"
```

Derived sources should be pure functions. This is where the same output is
always produced for the same input. As well as making source updates more
predictable, knowing that updates are pure allows Vide to use optimizations such
as caching, to avoid updating derived sources if their inputs are the same.