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

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+# Derived Source
+
+You can create new sources from existing sources. This is known as *deriving
+sources*.
+
+A function that wraps a source effectively becomes a new source. If a source
+used inside a function is updated, the whole function can be re-ran to recompute
+its value.
+
+```lua
+local count = source(0)
+
+local function text()
+    return "count: " .. count()
+end
+
+create "TextLabel" {
+    Text = text
+}
+```
+
+Sometimes when using expensive computations to derive state, you only want to
+recalculate it once when a source state has changed
+
+If you wrap a source with a regular function, its value will be recomputed
+every time you call that function.
+[`derive()`](../../api/reactivity-core.md#derive) accepts a functions whose
+return value will be cached, so that subsequent calls of this derived source
+will return the same cached value until one of its input sources have 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)
+```
+
+This can improve performance in cases where a source is read from multiple times
+between recalculations, like in the example below:
+
+```lua
+create "TextLabel" {
+    Text = function()
+        return "factorial squared: " .. factorial() * factorial()
+    end
+}
+
+count(3) -- displays "factorial squared: 36"
+count(4) -- displays "factorial squared: 576"
+```