# Effects

Effects are functions that are ran in response to source updates. They are
called effects because they cause *side-effects* when reacting to source
updates.

Effects are created using `effect()`.

```lua
local source = vide.source
local effect = vide.effect

local count = source(0)

effect(function()
    print("count: " .. count())
end)

-- "count: 0" printed
count(1)
-- "count: 1" printed
```

The callback given to `effect()` is initially ran immediately in a
*reactive scope*. Any source read from inside a reactive scope will be tracked,
so that if any of those sources update, the effect will be reran too.

Effects also work with derived sources, it doesn't matter how deeply nested
inside a function a source is.

```lua
local source = vide.source
local effect = vide.effect

local count = source(1)

local doubled = function()
    return count() * 2
end

effect(function()
    print("doubled count: " .. doubled())
end)

-- "doubled count: 2" printed
count(2)
-- "doubled count: 4" printed
```

If a source is updated with the same value it already had, it will not rerun
effects depending on it.