docs/tut/crash-course/5-derived-state.md

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+# [Derived State](./index.md)
+
+You can create new state from existing states. This is known as *deriving
+state*.
+
+```lua
+local count = source(0)
+
+local function text()
+    return "count: " .. count()
+end
+
+create "TextLabel" {
+    Text = text
+}
+```
+
+Assigning a non-event property a function will bind that property to that
+function, anytime a state being read from inside that function is changed, the
+function will be re-ran and the property value updated.
+
+Sometimes when using expensive computations to derive state, you only want to
+recalculate it when a source state has changed.
+
+```lua
+local derive = vide.derive
+```
+
+```lua
+local count = source(0)
+
+local factorial = derive(function()
+    local n = 1
+    for i = 2, count() do
+        n *= i
+    end
+    return n
+end)
+```
+
+`derive()` will cache and return the same value until a source state has
+changed, where it will recompute and cache a new value.
+
+```lua
+create "TextLabel" {
+    Text = function()
+        return "factorial: " .. factorial()
+    end
+}
+
+count(3) -- displays "factorial: 6"
+count(4) -- displays "factorial: 24"
+```
+
+--------------------------------------------------------------------------------
+
+### [← State](./4-State.md) | [Table State →](./6-table-state.md)