Merge pull request #985 from SethTisue/type-inference

add new language tour section on type inference

Author: SethTisue

_ba/tour/packages-and-imports.md

@@ -9,6 +9,7 @@ partof: scala-tour
 
 num: 35
 previous-page: named-arguments
+next-page: type-inference
 ---
 
 # Packages and Imports

_ba/tour/type-inference.md

@@ -0,0 +1,15 @@
+---
+layout: tour
+title: Type Inference
+language: ba
+
+discourse: true
+
+partof: scala-tour
+
+num: 36
+previous-page: packages-and-imports
+---
+
+(this section of the tour has not been translated yet. pull request
+with translation welcome!)

_es/tour/packages-and-imports.md

@@ -9,6 +9,7 @@ partof: scala-tour
 
 num: 35
 previous-page: named-arguments
+next-page: type-inference
 ---
 
 # Packages and Imports

_es/tour/type-inference.md

@@ -0,0 +1,15 @@
+---
+layout: tour
+title: Type Inference
+language: es
+
+discourse: true
+
+partof: scala-tour
+
+num: 36
+previous-page: packages-and-imports
+---
+
+(this section of the tour has not been translated yet. pull request
+with translation welcome!)

_ko/tour/packages-and-imports.md

@@ -9,6 +9,7 @@ partof: scala-tour
 
 num: 35
 previous-page: named-arguments
+next-page: type-inference
 ---
 
 # Packages and Imports

_ko/tour/type-inference.md

@@ -0,0 +1,15 @@
+---
+layout: tour
+title: Type Inference
+language: ko
+
+discourse: true
+
+partof: scala-tour
+
+num: 36
+previous-page: packages-and-imports
+---
+
+(this section of the tour has not been translated yet. pull request
+with translation welcome!)

_pl/tour/packages-and-imports.md

@@ -9,6 +9,7 @@ partof: scala-tour
 
 num: 35
 previous-page: named-arguments
+next-page: type-inference
 ---
 
 # Packages and Imports

_pl/tour/type-inference.md

@@ -0,0 +1,15 @@
+---
+layout: tour
+title: Type Inference
+language: pl
+
+discourse: true
+
+partof: scala-tour
+
+num: 36
+previous-page: packages-and-imports
+---
+
+(this section of the tour has not been translated yet. pull request
+with translation welcome!)

_pt-br/tour/packages-and-imports.md

@@ -9,6 +9,7 @@ partof: scala-tour
 
 num: 35
 previous-page: named-arguments
+next-page: type-inference
 ---
 
 # Packages and Imports

_pt-br/tour/type-inference.md

@@ -0,0 +1,15 @@
+---
+layout: tour
+title: Type Inference
+language: pt-br
+
+discourse: true
+
+partof: scala-tour
+
+num: 36
+previous-page: packages-and-imports
+---
+
+(this section of the tour has not been translated yet. pull request
+with translation welcome!)

_tour/packages-and-imports.md

@@ -8,6 +8,7 @@ partof: scala-tour
 
 num: 35
 previous-page: named-arguments
+next-page: type-inference
 ---
 
 # Packages and Imports

_tour/type-inference.md

@@ -0,0 +1,73 @@
+---
+layout: tour
+title: Type Inference
+
+discourse: true
+
+partof: scala-tour
+
+num: 36
+previous-page: packages-and-imports
+---
+
+The Scala compiler can often infer the type of an expression so you don't have to declare it explicitly.
+
+## Omitting the type
+
+```tut
+val businessName = "Montreux Jazz Café"
+```
+The compiler can detect that `businessName` is a String. It works similarly with methods:
+
+```tut
+def squareOf(x: Int) = x * x
+```
+The compiler can infer that the return type is an `Int`, so no explicit return type is required.
+
+For recursive methods, the compiler is not able to infer a result type. Here is a program which will fail the compiler for this reason:
+
+```tut:fail
+def fac(n: Int) = if (n == 0) 1 else n * fac(n - 1)
+```
+
+It is also not compulsory to specify type parameters when [polymorphic methods](polymorphic-methods.html) are called or [generic classes](generic-classes.html) are instantiated. The Scala compiler will infer such missing type parameters from the context and from the types of the actual method/constructor parameters.
+
+Here are two examples:
+
+```tut
+case class MyPair[A, B](x: A, y: B);
+val p = MyPair(1, "scala") // type: MyPair[Int, String]
+
+def id[T](x: T) = x
+val q = id(1)              // type: Int
+```
+
+The compiler uses the types of the arguments of `MyPair` to figure out what type `A` and `B` are. Likewise for the type of `x`.
+
+## Parameters
+
+The compiler never infers method parameter types. However, in certain cases, it can infer anonymous function parameter types when the function is passed as argument.
+
+```tut
+Seq(1, 3, 4).map(x => x * 2)  // List(2, 6, 8)
+```
+
+The parameter for map is `f: A => B`. Because we put integers in the `Seq`, the compiler knows that `A` is `Int` (i.e. that `x` is an integer). Therefore, the compiler can infer from `x * 2` that `B` is type `Int`.
+
+## When _not_ to rely on type inference
+
+It is generally considered more readable to declare the type of members exposed in a public API.  Therefore, we recommended that you make the type explicit for any APIs that will be exposed to users of your code.
+
+Also, type inference can sometimes infer a too-specific type.  Suppose we write:
+
+```tut
+var obj = null
+```
+
+Then we can't then go on and make this reassignment:
+
+```tut:fail
+obj = new AnyRef
+```
+
+It won't compile, because the type inferred for `obj` was `Null`. Since the only value of that type is `null`, it is impossible to assign a different value.