类型推断的前提是有待定类型和上下文。
1、由定义推断实现的类型；
2、由赋值推断声明的类型；
3、由实现推断泛型的类型；
Type inference refers to the automatic detection of the data type of an expression in a programming language.
Type inference is the ability to automatically deduce, either partially or fully, the type of an expression at compile time. The compiler is often able to infer the type of a variable or the type signature of a function, without explicit type annotations having been given. In many cases, it is possible to omit type annotations from a program completely if the type inference system is robust enough, or the program or language is simple enough.
https://www.cnblogs.com/feng9exe/p/9151219.html
Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)
http://www.cnblogs.com/feng9exe/p/9102835.html
Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide. Swift 4 uses type inference to work out the appropriate type as follows.
https://www.tutorialspoint.com/swift/swift_data_types.htm
Swift Type Inference Closure Constraints
http://rosslebeau.com/2016/swift-type-inference-closure-constraints
It’s rare that you need to write type annotations in practice. If you provide an initial value for a constant or variable at the point that it’s defined, Swift can almost always infer the type to be used for that constant or variable, as described in Type Safety and Type Inference. In the welcomeMessage example above, no initial value is provided, and so the type of the welcomeMessage variable is specified with a type annotation rather than being inferred from an initial value.
https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html
Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.
Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)
https://docs.swift.org/swift-book/LanguageGuide/TheBasics.html#ID322
Inferring Type From Context
Because the sorting closure is passed as an argument to a method, Swift can infer the types of its parameters and the type of the value it returns. The sorted(by:) method is being called on an array of strings, so its argument must be a function of type (String, String) -> Bool. This means that the (String, String) and Bool types do not need to be written as part of the closure expression’s definition. Because all of the types can be inferred, the return arrow (->) and the parentheses around the names of the parameters can also be omitted:
- reversedNames = names.sorted(by: { s1, s2 in return s1 > s2 } )
It is always possible to infer the parameter types and return type when passing a closure to a function or method as an inline closure expression. As a result, you never need to write an inline closure in its fullest form when the closure is used as a function or method argument.
Swift’s closure expressions have a clean, clear style, with optimizations that encourage brief, clutter-free syntax in common scenarios. These optimizations include:
- Inferring parameter and return value types from context
- Implicit returns from single-expression closures
- Shorthand argument names
- Trailing closure syntax
If you use these shorthand argument names within your closure expression, you can omit the closure’s argument list from its definition, and the number and type of the shorthand argument names will be inferred from the expected function type. The in keyword can also be omitted, because the closure expression is made up entirely of its body:
- reversedNames = names.sorted(by: { $0 > $1 } )
Here, $0 and $1 refer to the closure’s first and second String arguments.
Operator Methods
There’s actually an even shorter way to write the closure expression above. Swift’s String type defines its string-specific implementation of the greater-than operator (>) as a method that has two parameters of type String, and returns a value of type Bool. This exactly matches the method type needed by the sorted(by:) method. Therefore, you can simply pass in the greater-than operator, and Swift will infer that you want to use its string-specific implementation:
- reversedNames = names.sorted(by: >)
https://docs.swift.org/swift-book/LanguageGuide/Closures.html