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42-533: IIF may refer to: IIf (abbreviation for immediate if) and ?: , the inline-if computing function Indirect immunofluorescence, one of several types of immunofluorescence Institute of International Finance , an association of international financial institutions International Indonesia Forum , an organisation which holds annual interdisciplinary seminars in Indonesia. International Institute of Forecasters ,

84-409: A conflict in the use of x for both integer and floating-point expressions. The correct type-inference algorithm for such a situation has been known since 1958 and has been known to be correct since 1982. It revisits the prior inferences and uses the most general type from the outset: in this case floating-point. This can however have detrimental implications, for instance using a floating-point from

126-454: A floating-point variable, and the addition implicitly converts x to a floating point. This can be correct if the calling contexts never supply a floating point argument. Such a situation shows the difference between type inference , which does not involve type conversion , and implicit type conversion , which forces data to a different data type, often without restrictions. Finally, a significant downside of complex type-inference algorithm

168-415: A friend" is nonsense. At best it might be metaphor; bending type rules is a feature of poetic language. A term's type can also affect the interpretation of operations involving that term. For instance, "a song" is of composable type, so we interpret it as the thing created in the phrase "write a song". On the other hand, "a friend" is of recipient type, so we interpret it as the addressee in the phrase "write

210-436: A friend". In normal language, we would be surprised if "write a song" meant addressing a letter to a song or "write a friend" meant drafting a friend on paper. Terms with different types can even refer to materially the same thing. For example, we would interpret "to hang up the clothes line" as putting it into use, but "to hang up the leash" as putting it away, even though, in context, both "clothes line" and "leash" might refer

252-412: A hypothetical language supporting type inference, the code might be written like this instead: This is identical to how code is written in the language Dart , except that it is subject to some added constraints as described below. It would be possible to infer the types of all the variables at compile time. In the example above, the compiler would infer that result and x have type integer since

294-400: A large number of compiled versions of the same function. Each compiled version can then be optimized for a different set of types. For instance, JIT compilation allows there to be at least two compiled versions of add_one : 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

336-551: A library function. These examples evaluate mathematical expressions and return one of two strings depending on the outcome. Because IIf is a library function, it will always require the overhead of a function call, whereas a conditional operator will more likely produce inline code. Furthermore, the data type of its arguments is Variant . If the function is called with arguments of other types (variables or literals), there will be additional overhead to convert these to Variant . There may also be additional overhead to check

378-408: A nonprofit organization devoted to advancing the science of forecasting Intuit Interchange Format , a file format used by Intuit's Quickbooks software See also [ edit ] IFF (disambiguation) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title IIF . If an internal link led you here, you may wish to change

420-417: A particular expression can take on at run-time . Increasingly, just-in-time compilation blurs the distinction between run time and compile time. However, historically, if the type of a value is known only at run-time, these languages are dynamically typed . In other languages, the type of an expression is known only at compile time ; these languages are statically typed . In most statically typed languages,

462-456: A simple form of type inference; the Hindley–Milner type system can provide more complete type inference. The ability to infer types automatically makes many programming tasks easier, leaving the programmer free to omit type annotations while still permitting type checking. In some programming languages, all values have a data type explicitly declared at compile time , limiting the values

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504-546: A true conditional operator , called simply "If", which also eliminates this problem. Its syntax is similar to the IIf function's syntax: $ iif() is present in mIRC script, with similar syntax. Calling /testiif will print out "testing $ iif: 1 execution(s)". mIRC's $ iif acts more like C's ?: than IIf() in VB since it won't pre-evaluate both. IIF() is a function in dBase and xBase (1992 and older). iif()

546-410: A type constructor like · -> · is invertible in its arguments: Substituting ζ -> ι for ε and β -> [γ] -> [δ] for α , keeping the second constraint around so that we can recover α at the end: And, finally, substituting (ζ -> ι) for β as well as ζ for γ and ι for δ because a type constructor like [ · ] is invertible eliminates all

588-457: A typed language, a term's type determines the ways it can and cannot be used in that language. For example, consider the English language and terms that could fill in the blank in the phrase "sing _." The term "a song" is of singable type, so it could be placed in the blank to form a meaningful phrase: "sing a song." On the other hand, the term "a friend" does not have the singable type, so "sing

630-437: 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. To obtain the information required to infer the type of an expression, the compiler either gathers this information as an aggregate and subsequent reduction of

672-489: Is also a compiler magic function of Oxygene . It is not a real function and is at compile time unrolled to conditional statements. In this example a new strong type string named "someString" is created (using Type inference ) and the iif function will fill it depending on the outcome of the boolean expression. SQL Server 2012 and newer implements the IIF() function ( Transact-SQL ): IIf in C (and its variants) and Perl

714-419: Is also the most general type, since no further constraints apply. As the inferred type of map is parametrically polymorphic , the type of the arguments and results of f are not inferred, but left as type variables, and so map can be applied to functions and lists of various types, as long as the actual types match in each invocation. The algorithms used by programs like compilers are equivalent to

756-527: Is also traditionally called type reconstruction . If a term is well-typed in accordance with Hindley–Milner typing rules, then the rules generate a principal typing for the term. The process of discovering this principal typing is the process of "reconstruction". The origin of this algorithm is the type inference algorithm for the simply typed lambda calculus that was devised by Haskell Curry and Robert Feys in 1958. In 1969 J. Roger Hindley extended this work and proved that their algorithm always inferred

798-500: Is not always possible for the compiler to infer as much, and type annotations are occasionally necessary for disambiguation. For instance, type inference with polymorphic recursion is known to be undecidable. Furthermore, explicit type annotations can be used to optimize code by forcing the compiler to use a more specific (faster/smaller) type than it had inferred. Some methods for type inference are based on constraint satisfaction or satisfiability modulo theories . As an example,

840-403: Is that the resulting type inference resolution is not going to be obvious to humans (notably because of the backtracking), which can be detrimental as code is primarily intended to be comprehensible to humans. The recent emergence of just-in-time compilation allows for hybrid approaches where the type of arguments supplied by the various calling context is known at compile time, and can generate

882-527: Is the ?: conditional operator : IIf in Python : IIf (either) in Red and Rebol : Type inference Type inference , sometimes called type reconstruction , refers to the automatic detection of the type of an expression in a formal language . These include programming languages and mathematical type systems , but also natural languages in some branches of computer science and linguistics . In

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924-439: The map function proceeds as follows. map is a function of two arguments, so its type is constrained to be of the form a -> b -> c . In Haskell, the patterns [] and ( first : rest ) always match lists, so the second argument must be a list type: b = [ d ] for some type d . Its first argument f is applied to the argument first , which must have type d , corresponding with

966-551: The Haskell function map applies a function to each element of a list, and may be defined as: (Recall that : in Haskell denotes cons , structuring a head element and a list tail into a bigger list or destructuring a nonempty list into its head element and its tail. It does not denote "of type" as in mathematics and elsewhere in this article; in Haskell that "of type" operator is written :: instead.) Type inference on

1008-521: The argument types and convert one of them if they do not have the same type. Another issue with IIf arises because it is a library function: unlike the C-derived conditional operator, both truepart and the falsepart will be evaluated regardless of which one is actually returned. In the following code snippet: although TrueFunction is the function intended to be called, IIf will call both TrueFunction and FalseFunction . Similarly, While

1050-434: The computer. In manifestly typed languages, this means that most types have to be declared explicitly. Type inference aims to alleviate this burden, freeing the author from declaring types that the computer should be able to deduce from context. In a typing, an expression E is opposed to a type T, formally written as E : T. Usually a typing only makes sense within some context, which is omitted here. In this setting,

1092-433: The constant 1 is type integer, and hence that add_one is a function int -> int . The variable result2 isn't used in a legal manner, so it wouldn't have a type. In the imaginary language in which the last example is written, the compiler would assume that, in the absence of information to the contrary, + takes two integers and returns one integer. (This is how it works in, for example, OCaml .) From this,

1134-776: The following questions are of particular interest: For the simply typed lambda calculus , all three questions are decidable . The situation is not as comfortable when more expressive types are allowed. Types are a feature present in some strongly statically typed languages. It is often characteristic of functional programming languages in general. Some languages that include type inference include C23 , C++11 , C# (starting with version 3.0), Chapel , Clean , Crystal , D , F# , FreeBASIC , Go , Haskell , Java (starting with version 10), Julia , Kotlin , ML , Nim , OCaml , Opa , Q#, RPython , Rust , Scala , Swift , TypeScript , Vala , Dart , and Visual Basic (starting with version 9.0). The majority of them use

1176-436: The function call with inline code. In Visual Basic , IIf is not the sole way to evaluate and perform actions based on whether an expression is true or false. The following example uses IIf: It could also be written in the following way, using standard conditionals : The above example would also eliminate the problem of IIf evaluating both its truepart and falsepart parameters. Visual Basic 2008 (VB 9.0) introduced

1218-588: The informally structured reasoning above, but a bit more verbose and methodical. The exact details depend on the inference algorithm chosen (see the following section for the best-known algorithm), but the example below gives the general idea. We again begin with the definition of map : (Again, remember that the : here is the Haskell list constructor, not the "of type" operator, which Haskell instead spells :: .) First, we make fresh type variables for each individual term: Then we make fresh type variables for subexpressions built from these terms, constraining

1260-449: The input and output types of functions and local variables ordinarily must be explicitly provided by type annotations. For example, in ANSI C : The signature of this function definition, int add_one(int x) , declares that add_one is a function that takes one argument, an integer , and returns an integer. int result; declares that the local variable result is an integer. In

1302-509: The intent may be to avoid a division by zero, whenever b is zero the error will actually happen. This is because the code in the snippet is executed as if by This issue makes the IIf() call less useful than the conditional operator. To solve this issue, Microsoft developers had considered converting IIf to an intrinsic function; had this happened, the compiler would have been able to perform type inference and short-circuiting by replacing

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1344-483: The link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=IIF&oldid=1119785248 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages IIf The syntax of the IIf function is as follows: All three parameters are required: Many languages have an operator to accomplish

1386-506: The most general type. In 1978 Robin Milner , independently of Hindley's work, provided an equivalent algorithm, Algorithm W . In 1982 Luis Damas finally proved that Milner's algorithm is complete and extended it to support systems with polymorphic references. By design, type inference will infer the most general type appropriate. However, many languages, especially older programming languages, have slightly unsound type systems, where using

1428-472: The outset can introduce precision issues that would have not been there with an integer type. Frequently, however, degenerate type-inference algorithms are used that cannot backtrack and instead generate an error message in such a situation. This behavior may be preferable as type inference may not always be neutral algorithmically, as illustrated by the prior floating-point precision issue. An algorithm of intermediate generality implicitly declares result2 as

1470-408: The same purpose, generally referred to as a conditional operator (or, less precisely, as a ternary operator); the best known is ?: , as used in C, C++, and related languages. Some of the problems with the IIf function, as discussed later, do not exist with a conditional operator, because the language is free to examine the type and delay evaluation of the operands, as opposed to simply passing them to

1512-417: The same rope, just at different times. Typings are often used to prevent an object from being considered too generally. For instance, if the type system treats all numbers as the same, then a programmer who accidentally writes code where 4 is supposed to mean "4 seconds" but is interpreted as "4 meters" would have no warning of their mistake until it caused problems at runtime. By incorporating units into

1554-482: The type annotations given for its subexpressions, or through an implicit understanding of the type of various atomic values (e.g. true : Bool; 42 : Integer; 3.14159 : Real; etc.). It is through recognition of the eventual reduction of expressions to implicitly typed atomic values that the compiler for a type inferring language is able to compile a program completely without type annotations. In complex forms of higher-order programming and polymorphism , it

1596-405: The type in the list argument, so f :: d -> e ( :: means "is of type") for some type e . The return value of map f , finally, is a list of whatever f produces, so [ e ] . Putting the parts together leads to map :: ( d -> e ) -> [ d ] -> [ e ] . Nothing is special about the type variables, so it can be relabeled as It turns out that this

1638-452: The type inferencer can infer that the type of x + 1 is an integer, which means result is an integer and thus the return value of add_one is an integer. Similarly, since + requires both of its arguments be of the same type, x must be an integer, and thus, add_one accepts one integer as an argument. However, in the subsequent line, result2 is calculated by adding a decimal 1.0 with floating-point arithmetic , causing

1680-584: The type of the function being invoked accordingly: We also constrain the left and right sides of each equation to unify with each other: κ ~ [δ] and μ ~ ο . Altogether the system of unifications to solve is: Then we substitute until no further variables can be eliminated. The exact order is immaterial; if the code type-checks, any order will lead to the same final form. Let us begin by substituting ο for μ and [δ] for κ : Substituting ζ for η , [ζ] for θ and λ , ι for ν , and [ι] for ξ and ο , all possible because

1722-469: The type system, these mistakes can be detected much earlier. As another example, Russell's paradox arises when anything can be a set element and any predicate can define a set, but more careful typing gives several ways to resolve the paradox. In fact, Russell's paradox sparked early versions of type theory. There are several ways that a term can get its type: Especially in programming languages, there may not be much shared background knowledge available to

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1764-473: The variables specific to the second constraint: No more substitutions are possible, and relabeling gives us map :: ( a -> b ) -> [ a ] -> [ b ] , the same as we found without going into these details. The algorithm first used to perform type inference is now informally termed the Hindley–Milner algorithm, although the algorithm should properly be attributed to Damas and Milner. It

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