.net data matrix barcode Defining Discriminated Unions in VB.NET

Creator Data Matrix in VB.NET Defining Discriminated Unions

Defining Discriminated Unions
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The second kind of concrete type definition discussed in this section is a discriminated union. Here is a very simple example: type type type type Route = int Make = string Model = string Transport = | Car of Make * Model | Bicycle | Bus of Route
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Each alternative of a discriminated union is called a discriminator. You can build values by using the discriminator much as if it were a function: > let nick = Car("BMW","360");; val nick : Transport > let don = [ Bicycle; Bus 8 ];; val don : Transport list > let james = [ Car ("Ford","Fiesta"); Bicycle ];; val james : Transport list
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CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
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You can also use discriminators in pattern matching: let averageSpeed (tr: Transport) = match tr with | Car _ -> 35 | Bicycle -> 16 | Bus _ -> 24 Several of the types you ve already met are defined as discriminated unions. For example, the 'T option type is defined as follows: type 'T option = | None | Some of 'T Discriminated unions can include recursive references (the same is true of records and other type definitions). This is frequently used when representing structured languages via discriminated unions, a topic covered in depth in 9: type Proposition = | True | And of Proposition * Proposition | Or of Proposition * Proposition | Not of Proposition Recursive functions can be used to traverse such a type. For example: let rec eval (p: Proposition) = match p with | True -> true | And(p1,p2) -> eval p1 && eval p2 | Or (p1,p2) -> eval p1 || eval p2 | Not(p1) -> not (eval p1) The F# type of immutable lists is defined in this way: type 'T list = | ([]) | (::) of 'T * 'T list A broad range of tree-like data structures are conveniently represented as discriminated unions. For example: type Tree<'T> = | Tree of 'T * Tree<'T> * Tree<'T> | Tip of 'T You can use recursive functions to compute properties of trees: let rec size tree = match tree with | Tree(_,l,r) -> 1 + size l + size r | Tip _ -> 1
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CHAPTER 3 CREATING YOUR FIRST F# PROGRAM
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Here is the inferred type of size: val size : Tree<'T> -> int Here is an example of a constructed tree term and the use of the size function: > let small = Tree("1",Tree("2",Tip("a"),Tip("b")),Tip("c"));; val small : Tree<string> > small;; val it : Tree<string> = Tree ("1",Tree ("2",Tip("a"),Tip("b")),Tip("c")) > size small;; val it : int = 5 s 8, 9, and 12 discuss symbolic manipulation based on trees.
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Note Discriminated unions are a powerful and important construct and are useful when modeling a finite,
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sealed set of choices. This makes them a perfect fit for many constructs that arise in applications and symbolic analysis libraries. They are, by design, nonextensible: subsequent modules can t add new cases to a discriminated union. This is deliberate: you get strong and useful guarantees by placing a limit on the range of possible values for a type. Extensible types can be defined through the use of records of functions and object interface types, discussed in s 5 and 6.
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Using Discriminated Unions as Records
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Discriminated union types with only one data tag are an effective way to implement record-like types: type Point3D let origin = let unitX = let unitY = let unitZ = = Vector3D of float * float * float Vector3D(0.,0.,0.) Vector3D(1.,0.,0.) Vector3D(0.,1.,0.) Vector3D(0.,0.,1.)
These are particularly effective because they can be decomposed using succinct patterns in the same way as tuple arguments: let length (Vector3D(dx,dy,dz)) = sqrt(dx*dx+dy*dy+dz*dz) This technique is most useful for record-like values where there is some natural order on the constituent elements of the value (as shown earlier) or where the elements have different types.
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