Documentation.

docs/contracts/functions.rst

@@ -6,6 +6,34 @@
 Functions
 *********
 
+Functions can be defined inside and outside of contracts.
+
+Functions outside of a contract, also called "free functions", always have implicit ``internal``
+:ref:`visibility<visibility-and-getters>`. Their code is included in all contracts
+that call them, similar to internal library functions.
+
+::
+
+    // SPDX-License-Identifier: GPL-3.0
+    pragma solidity >0.7.0 <0.8.0;
+
+    function sum(uint[] memory _arr) pure returns (uint s) {
+        for (uint i = 0; i < _arr.length; i++)
+            s += _arr[i];
+    }
+
+    contract ArrayExample {
+        bool found;
+        function f(uint[] memory _arr) public {
+            // This calls the free function internally.
+            // The compiler will add its code to the contract.
+            uint s = sum(_arr);
+            require(s >= 10);
+            found = true;
+        }
+    }
+
+
 .. _function-parameters-return-variables:
 
 Function Parameters and Return Variables