Explain mappings example, and add link to further details

Updates from review

Changes from review

Nested example

Update docs/types/mapping-types.rst

Co-Authored-By: chriseth <chris@ethereum.org>
Changes from review

Bring example inline with ERC20

Clarify what maps where

Use OZ contract example

Update docs/types/mapping-types.rst

Co-Authored-By: chriseth <chris@ethereum.org>

update code example

docs/types/mapping-types.rst

@@ -4,7 +4,8 @@
 Mapping Types
 =============
 
-You declare mapping types with the syntax ``mapping(_KeyType => _ValueType)``.
+Mapping types use the syntax ``mapping(_KeyType => _ValueType)`` and variables
+are declared as a mapping type using the syntax ``mapping (_KeyType => _ValueType) _VariableModifiers _VariableName``.
 The ``_KeyType`` can be any elementary type. This means it can be any of
 the built-in value types plus ``bytes`` and ``string``. User-defined
 or complex types like contract types, enums, mappings, structs and any array type
@@ -27,11 +28,16 @@ They cannot be used as parameters or return parameters
 of contract functions that are publicly visible.
 
 You can mark state variables of mapping type as ``public`` and Solidity creates a
-:ref:`getter <visibility-and-getters>` for you. The ``_KeyType`` becomes a
+:ref:`getter <visibility-and-getters>` for you. The ``_KeyType`` becomes a parameter for the getter.
-parameter for the getter. If ``_ValueType`` is a value type or a struct,
+If ``_ValueType`` is a value type or a struct, the getter returns ``_ValueType``.
-the getter returns ``_ValueType``.
 If ``_ValueType`` is an array or a mapping, the getter has one parameter for
-each ``_KeyType``, recursively. For example with a mapping:
+each ``_KeyType``, recursively.
+
+In the example below, the ``MappingExample`` contract defines a public ``balances``
+mapping, with the key type an ``address``, and a value type a ``uint``, mapping
+an Ethereum address to an unsigned integer value. As ``uint`` is a value type, the getter
+returns a value that matches the type, which you can see in the ``MappingUser``
+contract that returns the value at the specified address.
 
 ::
 
@@ -53,6 +59,52 @@ each ``_KeyType``, recursively. For example with a mapping:
         }
     }
 
+The example below is a simplified version of an `ERC20 token <https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC20/ERC20.sol>`_.
+``_allowances`` is an example of a mapping type inside another mapping type.
+The example below uses ``_allowances`` to record the amount someone else is allowed to withdraw from your account.
+
+::
+
+    pragma solidity >=0.4.0 <0.7.0;
+
+    contract MappingExample {
+
+        mapping (address => uint256) private _balances;
+        mapping (address => mapping (address => uint256)) private _allowances;
+
+        event Transfer(address indexed from, address indexed to, uint256 value);
+        event Approval(address indexed owner, address indexed spender, uint256 value);
+
+        function allowance(address owner, address spender) public view returns (uint256) {
+            return _allowances[owner][spender];
+        }
+
+        function transferFrom(address sender, address recipient, uint256 amount) public returns (bool) {
+            _transfer(sender, recipient, amount);
+            approve(sender, msg.sender, amount);
+            return true;
+        }
+
+        function approve(address owner, address spender, uint256 amount) public returns (bool) {
+            require(owner != address(0), "ERC20: approve from the zero address");
+            require(spender != address(0), "ERC20: approve to the zero address");
+
+            _allowances[owner][spender] = amount;
+            emit Approval(owner, spender, amount);
+            return true;
+        }
+
+        function _transfer(address sender, address recipient, uint256 amount) internal {
+            require(sender != address(0), "ERC20: transfer from the zero address");
+            require(recipient != address(0), "ERC20: transfer to the zero address");
+
+            _balances[sender] -= amount;
+            _balances[recipient] += amount;
+            emit Transfer(sender, recipient, amount);
+        }
+    }
+
+
 .. index:: !iterable mappings
 .. _iterable-mappings:
 

docs/types/operators.rst

@@ -17,7 +17,7 @@ equivalent to ``a = 0``, but it can also be used on arrays, where it assigns a d
 array of length zero or a static array of the same length with all elements set to their
 initial value. ``delete a[x]`` deletes the item at index ``x`` of the array and leaves
 all other elements and the length of the array untouched. This especially means that it leaves
-a gap in the array. If you plan to remove items, a mapping is probably a better choice.
+a gap in the array. If you plan to remove items, a :ref:`mapping <mapping-types>` is probably a better choice.
 
 For structs, it assigns a struct with all members reset. In other words, the value of ``a`` after ``delete a`` is the same as if ``a`` would be declared without assignment, with the following caveat: