Flow-sensitive typing

In programming language theory, flow-sensitive typing (also called flow typing or occurrence typing) is a type system where the type of an expression depends on its position in the control flow.

Type systems
General concepts
Major categories
Minor categories
See also

In statically typed languages, a type of an expression is determined by the types of the sub-expressions that compose it. However, in flow-sensitive typing, an expression's type may be updated to a more specific type if it follows a statement that validates its type. The type is determined by using type inference and type information is carried using algebraic data types.

Example

Ceylon

See the following example in Ceylon which illustrates the concept:

// Object? means the variable "name" is of type Object or else null
void hello(Object? name) {
    if (is String name) {
        // "name" now has type String in this block
        print("Hello, ``name``!");
        // and it is possible to call String methods on the variable
        print(" String.size is ``name.size``");
    }
    else if (exists name) {
        // "name" now has type Object in this block
        print("Hello, object ``name``!");
    }
    else {
        print("Hello, world!");
    }
}

          
hello(null);
hello(1);
hello("John Doe");

Which outputs:

Hello, world!
Hello, object 1!
Hello, John Doe!
 String.size is 8

Kotlin

See this example in Kotlin:

fun hello(obj: Any) {
    // A type cast fails if `obj` is not a String
    obj as String

    // Since the type cast did not fail, `obj` must be a String!
    val l = obj.length

    println("'$obj' is a string of length $l")
}
          
hello("Mooooo")

Benefits

This technique coupled with type inference reduces the need for writing type annotations for all variables or to do type casting, like is seen with dynamic languages that use duck typing. It reduces verbosity and makes up for terser code, easier to read and modify.

It can also help language implementers provide implementations that execute dynamic languages faster by predicting the type of objects statically.[1]

Finally, it increases type safety and can prevent problems due to null pointers, labeled by C.A.R. Hoare—the null reference inventor—as "the billion dollar mistake"[2]

Implementations

Typed Scheme, a type system for Scheme, was the first type system with this feature.[3] Shortly thereafter, David J. Pearce independently reinvented it in Whiley, the first language to have this feature built-in.[4][5]

The successor to Typed Scheme, Typed Racket (a dialect of Racket), still makes extensive use of occurrence typing.[6] It has since appeared in Ceylon,[7] TypeScript[8] and Facebook Flow.[9]

There are also a few languages that don't have union types but do have nullable types, that have a limited form of this feature that only applies to nullable types, such as C#,[10] Kotlin,[11][12] and Lobster.[13]

External references
  1. Lukas Eder (11 December 2014). "The Inconvenient Truth About Dynamic vs. Static Typing". blog.jooq.org. Retrieved 11 March 2016.
  2. Tony Hoare (2009-08-25). "Null References: The Billion Dollar Mistake". InfoQ.com. I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.
  3. "The Design and Implementation of Typed Scheme | Lambda the Ultimate, 2008". lambda-the-ultimate.org.
  4. David J. Pearce (22 September 2010). "On Flow-Sensitive Types in Whiley". whiley.org. Retrieved 11 March 2016.
  5. David J. Pearce (8 April 2012). "Whiley - Flow Typing". whiley.org. Retrieved 11 March 2016.
  6. "5 Occurrence Typing". docs.racket-lang.org.
  7. "Ceylon - Quick introduction - Typesafe null and flow-sensitive typing". ceylon-lang.org. Retrieved 11 March 2016.
  8. Ryan Cavanaugh (18 November 2014). "TypeScript 1.4 sneak peek: union types, type guards, and more". blogs.msdn.microsoft.com. Retrieved 11 March 2016.
  9. Avik Chaudhuri, Basil Hosmer, Gabriel Levi (18 November 2014). "Flow, a new static type checker for JavaScript". code.facebook.com. Retrieved 11 March 2016.CS1 maint: uses authors parameter (link)
  10. "Design with nullable reference types". docs.microsoft.com.
  11. "Null Safety". kotlinlang.org. Retrieved 11 March 2016.
  12. "Type Checks and Casts". kotlinlang.org. Retrieved 11 March 2016.
  13. "The Lobster Type System". aardappel.github.io.
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