# Effects

Effects are functions that are ran in response to source updates.
A source and effect is analogous to a signal and connection.

Effects are created using `effect()`.

```luau
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
```

Any source read inside an effect is tracked and will rerun the effect when
that source is updated.

The effect runs its callback once immediately to initially figure out what
sources are being read.

Derived sources are also tracked, it does not matter how deeply nested
inside a function a source is.

```luau
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.

You can also read from a source within an effect without the effect tracking it.

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

local a = source(0)
local b = source(0)

effect(function()
    print(`a: {a()} b: {untrack(b)}`)
end)

a(1) -- prints "a: 1 b: 0"
b(1) -- prints nothing
a(2) -- prints "a: 2 b: 1"
```