[x/tour] go-tour: add Type Inference and Zero Values slides

Also check off a couple of finished things from the TODO list.

LGTM=campoy
R=campoy, golang-codereviews
CC=golang-codereviews
https://golang.org/cl/116090043
X-Tour-Commit: f82b5da64d2d1a0de764ca341592c8ecf83079f1

tour/TODO

@@ -15,8 +15,9 @@ should be added (prefixed by -). It should be kept up-to-date with tour.article.
 * Variables with initializers
 * Short variable declarations
 * Basic types
+* Type inference
 * Type conversions
-- The zero value
+* Zero values
 * Constants
 * Numeric Constants
 * For
@@ -36,7 +37,7 @@ should be added (prefixed by -). It should be kept up-to-date with tour.article.
 * Slices
 * Slicing slices
 * Making slices
-- Append
+* Append
 - Copy
 * Nil slices
 * Range
@@ -90,7 +91,7 @@ should be added (prefixed by -). It should be kept up-to-date with tour.article.
 * Exercise: Equivalent Binary Trees
 * Exercise: Web Crawler
 - More language features
-- Defer
+* Defer
 - Panic and recover
 - init functions
 - Tools

tour/content/basics.article

@@ -142,6 +142,18 @@ as with import statements.
 
 .play basics/basic-types.go
 
+* Zero values
+
+Variables declared without an explicit initial value are given their
+_zero_value_.
+
+The zero value is:
+- `0` for numeric types,
+- `false` the boolean type, and
+- `""` (the empty string) for strings.
+
+.play basics/zero.go
+
 * Type conversions
 
 The expression `T(v)` converts the value `v` to the type `T`.
@@ -164,6 +176,25 @@ Try removing the `float64` or `int` conversions in the example and see what happ
 
 .play basics/type-conversions.go
 
+* Type inference
+
+When declaring a variable without specifying its type (using `var` without a type or the `:=` syntax), the variable's type is _inferred_ from the value on the right hand side.
+
+When the right hand side of the declaration is typed, the new variable is of that same type:
+
+	var i int
+	j := i // j is an int
+
+But when the right hand side contains an untyped numeric constant, the new variable may be an `int`, `float64`, or `complex128` depending on the precision of the constant:
+
+	i := 42           // int
+	f := 3.142        // float64
+	g := 0.867 + 0.5i // complex128
+
+Try changing the initial value of `v` in the example code and observe how its type is affected.
+
+.play basics/type-inference.go
+
 * Constants
 
 Constants are declared like variables, but with the `const` keyword.

tour/content/basics/type-inference.go

@@ -0,0 +1,8 @@
+package main
+
+import "fmt"
+
+func main() {
+	v := 42 // change me!
+	fmt.Printf("v is of type %T\n", v)
+}

tour/content/basics/zero.go

@@ -0,0 +1,11 @@
+package main
+
+import "fmt"
+
+func main() {
+	var i int
+	var f float64
+	var b bool
+	var s string
+	fmt.Printf("%v %v %v %q\n", i, f, b, s)
+}