datamatrix.net c# example An Example: Success/Failure Workflows in Visual Basic .NET

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An Example: Success/Failure Workflows
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Perhaps the simplest kind of workflow is one where failure of a computation is made explicit: for example, where each step of the workflow may either succeed, by returning a result Some(v), or fail, by returning the value None. You can model such a workflow using functions of type unit -> 'T option that is, functions that may compute a result or may not. In this section, you can assume that these functions are pure and terminating: they have no side effects, raise no exceptions, and always terminate. Whenever you define a new kind of workflow, it s useful to give a name to the type of values/objects generated by the workflow. In this case, let s call them Attempt objects: type Attempt<'T> = (unit -> 'T option)
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CHAPTER 9 INTRODUCING LANGUAGE-ORIENTED PROGRAMMING
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Of course, you can use regular functional programming to start to build Attempt<'T> objects: let succeed x = (fun () -> Some(x)) : Attempt<'T> let fail = (fun () -> None) : Attempt<'T> let runAttempt (a:Attempt<'T>) = a() These conform to the following types: val succeed : 'T -> Attempt<'T> val fail : Attempt<'T> val runAttempt : Attempt<'T> -> 'T option However, using only normal F# expressions to build Attempt values can be a little tedious and lead to a proliferation of many different functions that stitch together Attempt values in straightforward ways. Luckily, as you ve seen with sequence expressions, F# comes with predefined syntax for building objects such as Attempt values. You can use this syntax with a new type by defining a builder object that helps stitch together the fragments that make up the computation expression. Here s an example of the signature of an object you have to define in order to use workflow syntax with a new type (note that this is a type signature for an object, not actual code we show how to define the AttemptBuilder type and its members later in this section): type AttemptBuilder = member Bind : Attempt<'T> * ('T -> Attempt<'U>) -> Attempt<'U> member Delay : (unit -> Attempt<'T>) -> Attempt<'T> member Let : 'T * ('T -> Attempt<'T>) -> Attempt<'T> member Return : 'T -> Attempt<'T> Typically, there is one global instance of each such builder object. For example: let attempt = new AttemptBuilder() val attempt : AttemptBuilder First, let s see how you can use the F# syntax for workflows to build Attempt objects. You can build Attempt values that always succeed: > let alwaysOne = attempt { return 1 };; val alwaysOne: Attempt<int> > let alwaysPair = attempt { return (1,"two") };; val alwaysPair: Attempt<int * string> > runAttempt alwaysOne;; val it : int option = Some 1 > runAttempt alwaysPair;; val it : (int * string) option = Some(1,"two")
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CHAPTER 9 INTRODUCING LANGUAGE-ORIENTED PROGRAMMING
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Note that Attempt values such as alwaysOne are just functions; to run an Attempt value, you just apply it. These correspond to uses of the succeed function, as you see shortly. You can also build more interesting Attempt values that check a condition and return different Attempt values on each branch, as shown in the next example: > let failIfBig n = attempt { if n > 1000 then return! fail else return n };; val failIfBig: int -> Attempt<int> > runAttempt (failIfBig 999);; val it : int option = Some 999 > runAttempt (failIfBig 1001);; val it : int option = None Here, one branch uses return! to return the result of running another Attempt value, and the other uses return to give a single result. These correspond to yield! and yield in sequence expressions. Next, you can build Attempt values that sequence together two Attempt values by running one, getting its result, binding it to a variable, and running the second. You do this by using the syntax form let! pat = expr, which is unique to computation expressions: > let failIfEitherBig (inp1,inp2) = attempt { let! n1 = failIfBig inp1 let! n2 = failIfBig inp2 return (n1,n2) };; val failIfEitherBig: int * int -> Attempt<int * int> > runAttempt (failIfEitherBig (999,998));; val it : (int * int) option = Some(999,998) > runAttempt (failIfEitherBig (1003,998));; val it : (int * int) option = None > runAttempt (failIfEitherBig (999,1001));; val it : (int * int) option = None Let s look at this more closely. First, what does the first let! do It runs the Attempt value failIfBig inp1; and if this returns None, the whole computation returns None. If the computation on the right delivers a value (that is, returns Some), then it binds the result to the variable n1 and continues. It s important to note the following for the expression let! n1 = failIfBig inp1: The expression on the right (failIfBig inp1) has type Attempt<int>. The variable on the left (n1) is of type int.
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This is somewhat similar to a sequence of normal let binding. However, let! also controls whether the rest of the computation is executed; in the case of the Attempt type, it executes the rest of the computation only when it receives a Some value. Otherwise, it returns None, and the rest of the code is never executed. You can use normal let bindings in computation expressions. For example:
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