spec: cover indexing expressions

docs/src/SUMMARY.md

@@ -43,6 +43,7 @@
         * [Call expressions](spec/expr_call.md)
         * [Tuple expressions](spec/expr_tuple.md)
         * [List expressions](spec/expr_list.md)
+        * [Index expressions](spec/expr_index.md)
         * [Attribute expressions](spec/expr_attribute.md)
         * [Literal expressions](spec/expr-literal.md)
         * [Arithmetic Operators](spec/arithmetic_operators.md)

docs/src/spec/expr_index.md

@@ -0,0 +1,32 @@
+# Index expressions
+
+> **<sup>Syntax</sup>**\
+> _IndexExpression_ :\
+> &nbsp;&nbsp; [_Expression_] `[` [_Expression_] `]`
+
+[Array] and [Map] types can be indexed by by writing a square-bracket-enclosed expression after them. For arrays, the type of the index key has to be `u256` whereas for [Map] types it has to be equal to the key type of the map.
+
+
+Example:
+
+```python
+contract Foo:
+
+    balances: Map<address, u256>
+
+    pub fn baz(values: u256[10]):
+        # Assign value at slot 5
+        values[5] = 1000
+        # Read value at slot 5
+        let val1: u256 = values[5]
+
+        # Assign value for address zero
+        self.balances[address(0)] = 10000
+
+        # Read balance of address zero
+        let bal: u256 = self.balances[address(0)]
+```
+
+[_Expression_]: expressions.md
+[Array]: array_types.md
+[Map]: hashmap_types.md

docs/src/spec/index.md

@@ -37,6 +37,7 @@
     * [Call expressions](expr_call.md)
     * [Tuple expressions](expr_tuple.md)
     * [List expressions](expr_list.md)
+    * [Index expressions](expr_index.md)
     * [Attribute expressions](expr_attribute.md)
     * [Literal expressions](expr-literal.md)
     * [Arithmetic Operators](arithmetic_operators.md)