978:). 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.
216:, 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 an operation that validates its type. Validating operations can include type predicates, imperative updates, and control flow.
661:
and making up for terser code, easier to read and modify. It achieves this is in a different way, it allows to match the type of a structure, extract data out of it at the same time by declaring new variable. As such, it reduces the ceremony around type casting and value extraction. Pattern matching
590:
From a
Programming Languages perspective, it's reasonable to say that flow-sensitive typing is the feature that finally made it possible to build usable type-safe programming languages with union types and without rampant dynamic checking. Until this point, attempts to add this feature to languages
925:
In a statically typed language, the advantage of pattern matching over flow-sensitive typing is that the type of a variable always stays the same: it does not change depending on control flow. When writing down the pattern to be matched, a new variable is declared that will have the new type.
614:
Typed JavaScript observed that in "scripting" languages, flow-typing depends on more than conditional predicates; it also depends on state and control flow. This style has since been adopted in languages like
974:
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 (
591:
such as Scheme generally resulted in intractably large type representations. One example of a system with limited support for union types is Wright and
Cartwright's "Soft Scheme."
1206:
851:// try to type cast "Node" into "MulNode", and create the variables "left" and "right" of type "Node".
788:// try to type cast "Node" into "AddNode", and create the variables "left" and "right" of type "Node".
1161:
572:
It can also help language implementers provide implementations that execute dynamic languages faster by predicting the type of objects statically.
1064:
1450:
1821:
1455:
1184:
1138:
1445:
1440:
168:
746:// try to type cast "Node" into "NegNode", and create the variable "n" of type "Node".
713:// try to type cast "Node" into "IntNode", and create the variable "i" of type "int".
58:
1428:
1329:
969:
607:), is also based on occurrence typing. Shortly after Typed Scheme, David J. Pearce independently reinvented flow-typing in
1091:
943:
1606:
1579:
1117:
1696:
1501:
1433:
1395:
854:// If that works, then return the multiplication of evaluating the "left" and "right" nodes
639:
643:
616:
608:
604:
600:
432:
230:
202:
1268:
1596:
1526:
1374:
1851:
1486:
791:// If that works, then return the addition of evaluating the "left" and "right" nodes
182:
1474:
161:
1882:
1784:
1736:
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1626:
1621:
1549:
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132:
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914:// no "default" because the compiler knows all the possible cases have been enumerated
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38:
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8:
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1794:
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1315:
1224:
603:, was the first type system with this feature. Its successor, Typed Racket (a dialect of
72:
638:, that have a limited form of this feature that only applies to nullable types, such as
1799:
1779:
1731:
1706:
1491:
1460:
1246:
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137:
127:
63:
666:
because all the cases can be enumerated and statically checked by the compiler.
1789:
1611:
1601:
1509:
749:// If that works, then return the negation of evaluating the "n" node
635:
558:
550:
112:
102:
67:
1095:
1866:
1711:
238:// Object? means the variable "name" is of type Object or else null
213:
122:
97:
1668:
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631:
584:
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206:
142:
1007:
1846:
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1465:
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562:
198:
81:
33:
19:
549:
This technique coupled with type inference reduces the need for writing
1751:
1746:
1663:
1631:
1536:
1479:
1139:"Ceylon - Quick introduction - Typesafe null and flow-sensitive typing"
965:
620:
1182:
1826:
1804:
1761:
1756:
1423:
1379:
1338:
658:
566:
1741:
624:
975:
657:
reaches the same goals as flow-sensitive typing, namely reducing
423:
Hello, world! Hello, object 1! Hello, John Doe! String.size is 8
1287:
1183:
Avik
Chaudhuri; Basil Hosmer; Gabriel Levi (18 November 2014).
1162:"TypeScript 1.4 sneak peek: union types, type guards, and more"
1307:
1285:
587:—the null reference inventor—as "the billion dollar mistake"
1028:"The Design and Implementation of Typed Scheme | POPL, 2008"
476:// Since the type cast did not fail, `obj` must be a String!
301:// and it is possible to call String methods on the variable
1359:
1354:
944:"The Inconvenient Truth About Dynamic vs. Static Typing"
716:// If that works, then return the value of "i"
569:
and makes for terser code, easier to read and modify.
996:
340:// "name" now has type Object in this block
286:// "name" now has type String in this block
1118:"Typing Local Control and State Using Flow Analysis"
1286:Gavin Bierman and Brian Goetz (19 September 2023).
990:Wright, Andrew; Cartwright, Robert (1 Jan 1997).
1864:
1185:"Flow, a new static type checker for JavaScript"
989:
1089:
1062:
630:There are also a few languages that don't have
1159:
1323:
970:"Null References: The Billion Dollar Mistake"
464:// A type cast fails if `obj` is not a String
162:
1330:
1316:
964:
941:
169:
155:
992:"A practical soft type system for scheme"
662:works best when used in conjunction with
310:" String.size is ``name.size``"
1865:
1288:"JEP 441: Pattern Matching for switch"
1207:"Design with nullable reference types"
1311:
1063:David J. Pearce (22 September 2010).
1160:Ryan Cavanaugh (18 November 2014).
1065:"On Flow-Sensitive Types in Whiley"
349:"Hello, object ``name``!"
13:
594:
14:
1899:
1090:David J. Pearce (8 April 2012).
669:See this example mock for Java:
599:Typed Scheme, a type system for
579:and can prevent problems due to
1279:
1261:
1239:
1217:
1199:
1176:
1153:
942:Lukas Eder (11 December 2014).
649:
233:which illustrates the concept:
205:depends on its position in the
1337:
1131:
1110:
1083:
1056:
1038:
1020:
983:
958:
935:
1:
1396:Arbitrary-precision or bignum
929:
229:See the following example in
1094:. whiley.org. Archived from
1067:. whiley.org. Archived from
512:' is a string of length
295:"Hello, ``name``!"
7:
553:for all variables or to do
544:
219:
183:programming language theory
10:
1904:
1164:. blogs.msdn.microsoft.com
214:statically typed languages
1770:
1737:Strongly typed identifier
1679:
1565:
1535:
1500:
1388:
1345:
1269:"The Lobster Type System"
426:
370:"Hello, world!"
224:
671:
437:
235:
1812:Parametric polymorphism
1247:"Type Checks and Casts"
1092:"Whiley - Flow Typing"
575:Finally, it increases
39:Strong vs. weak typing
1046:"5 Occurrence Typing"
1008:10.1145/239912.239917
201:where the type of an
187:flow-sensitive typing
1050:docs.racket-lang.org
664:algebraic data types
557:, like is seen with
431:See this example in
412:"John Doe"
1817:Primitive data type
1722:Recursive data type
1575:Algebraic data type
1451:Quadruple precision
1273:aardappel.github.io
1187:. code.facebook.com
420:and which outputs:
1780:Abstract data type
1461:Extended precision
1420:Reduced precision
1211:docs.microsoft.com
536:"Mooooo"
1860:
1859:
1592:Associative array
1456:Octuple precision
1141:. ceylon-lang.org
559:dynamic languages
195:occurrence typing
179:
178:
1895:
1883:Program analysis
1832:Type constructor
1717:Opaque data type
1649:Record or Struct
1446:Double precision
1441:Single precision
1332:
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1249:. kotlinlang.org
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1227:. kotlinlang.org
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1098:on 11 March 2016
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1071:on 11 March 2016
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90:Minor categories
47:Major categories
26:General concepts
16:
15:
1903:
1902:
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1837:Type conversion
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1766:
1702:Enumerated type
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1555:null-terminated
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949:
947:
946:. blog.jooq.org
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646:, and Lobster.
597:
595:Implementations
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1429:Half precision
1426:
1416:Floating point
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1002:(1): 87--152.
982:
968:(2009-08-25).
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651:
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636:nullable types
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1226:
1225:"Null Safety"
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972:. InfoQ.com.
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583:, labeled by
582:
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565:. It reduces
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189:(also called
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133:Substructural
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1873:Type systems
1617:Intersection
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650:Alternatives
634:but do have
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585:C.A.R. Hoare
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118:Intersection
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20:Type systems
1888:Type theory
1847:Type theory
1842:Type system
1692:Bottom type
1639:Option type
1580:generalized
1466:Long double
1411:Fixed point
1297:14 November
1292:openjdk.org
1123:14 November
632:union types
577:type safety
563:duck typing
503:"'
199:type system
191:flow typing
82:Duck typing
34:Type safety
1878:Data types
1867:Categories
1752:Empty type
1747:Type class
1697:Collection
1654:Refinement
1632:metaobject
1480:signedness
1339:Data types
1032:dl.acm.org
1013:2024-05-04
966:Tony Hoare
930:References
621:TypeScript
203:expression
128:Refinement
77:structural
1827:Subtyping
1822:Interface
1805:metaclass
1757:Unit type
1727:Semaphore
1707:Exception
1612:Inductive
1602:Dependent
1567:Composite
1545:Character
1527:Reference
1424:Minifloat
1380:Bit array
659:verbosity
567:verbosity
561:that use
103:Dependent
1852:Variable
1742:Top type
1607:Equality
1515:physical
1492:Rational
1487:Interval
1434:bfloat16
1253:11 March
1231:11 March
1191:11 March
1168:11 March
1145:11 March
1102:11 March
1075:11 March
950:11 March
625:Facebook
545:Benefits
220:Examples
98:Abstract
68:inferred
64:Manifest
1795:Generic
1771:Related
1687:Boolean
1644:Product
1520:virtual
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1475:Integer
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1401:Complex
1389:Numeric
976:ALGOL W
860:MulNode
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722:IntNode
497:println
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143:Session
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1669:tagged
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698:switch
695:return
644:Kotlin
627:Flow.
617:Ceylon
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473:String
433:Kotlin
427:Kotlin
328:exists
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225:Ceylon
138:Unique
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1664:Union
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1370:Tryte
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902:eval
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