mini spec: Iota

doc/spec-mini.md

@@ -1848,7 +1848,44 @@ const (
 
 ### Iota
 
-TODO
+Within a [constant declaration](#constant-declarations), the predeclared identifier iota represents successive untyped integer [constants](#constants). Its value is the index of the respective [ConstSpec]() in that constant declaration, starting at zero. It can be used to construct a set of related constants:
+
+```go
+const (
+	c0 = iota  // c0 == 0
+	c1 = iota  // c1 == 1
+	c2 = iota  // c2 == 2
+)
+
+const (
+	a = 1 << iota  // a == 1  (iota == 0)
+	b = 1 << iota  // b == 2  (iota == 1)
+	c = 3          // c == 3  (iota == 2, unused)
+	d = 1 << iota  // d == 8  (iota == 3)
+)
+
+const (
+	u         = iota * 42  // u == 0     (untyped integer constant)
+	v float64 = iota * 42  // v == 42.0  (float64 constant)
+	w         = iota * 42  // w == 84    (untyped integer constant)
+)
+
+const x = iota  // x == 0
+const y = iota  // y == 0
+```
+
+By definition, multiple uses of iota in the same ConstSpec all have the same value:
+
+```go
+const (
+	bit0, mask0 = 1 << iota, 1<<iota - 1  // bit0 == 1, mask0 == 0  (iota == 0)
+	bit1, mask1                           // bit1 == 2, mask1 == 1  (iota == 1)
+	_, _                                  //                        (iota == 2, unused)
+	bit3, mask3                           // bit3 == 8, mask3 == 7  (iota == 3)
+)
+```
+
+This last example exploits the [implicit repetition](#constant-declarations) of the last non-empty expression list.
 
 ### Type declarations