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F# Failwith: Exception Handling

Handle exceptions and use the failwith and raise operators. Use failwith inside match.
Failwith. A siren rings in the night sky. The air is cold. Things seem ominous. With F# we deal with errors in a rational way: we fail with them.
Special operators, like failwith and invalidArg, are available in this language. These create exceptions that enter into an alternate control flow.
First example. Here we introduce a function called validLength. This function returns true if the argument is 1 or 2, but uses failwith in all other cases.

Match: First we call validLength with an argument of 2. We get a result of "true" and use printfn to display this.

Exception: Next we use an argument of 200 and failwith is reached. This terminates the program with an unhandled exception.

F# program that uses failwith // This function tests its argument. // ... If 1 or 2, it returns true. // Otherwise it uses a failwith command. let validLength v = match v with | 1 | 2 -> true | _ -> failwith "Length not valid" // This prints true. let result1 = validLength 2 printfn "%A" result1 // This fails. let result2 = validLength 200 printfn "%A" result2 Output true Unhandled Exception: System.Exception: Length not valid at Microsoft.FSharp.Core.Operators.FailWith[T](String message) at Program.validLength(Int32 v)... at <StartupCode$ConsoleApplication3>.$Program.main@()...
Try, raise. Here we use the try and raise keywords. With try, we enter into a protected region of code—exceptions may be thrown, but we can recover.

Raise: This statement creates a NotImplementedException with a custom message. The try-block is stopped.

With: The with statement matches the NotImplementedException type. In response it prints a special message. The program does not terminate.

F# program that uses try with, raise open System try // Raise a special exception. raise (NotImplementedException "Not ready") with | :? NotImplementedException -> printfn "Not implemented, ignoring" Output Not implemented, ignoring
Finally. This block always runs. We can place some recovery logic in a finally block. If an exception is thrown, we will still enter the finally afterwards.
F# program that uses finally try // An error occurs. failwith "Not valid" finally // A finally block always runs, so we can try to recover. printfn "Can recover here" Output Unhandled Exception: System.Exception: Not valid at Microsoft.FSharp.Core.Operators.FailWith[T](String message)... Can recover here
A review. Exception handling is powerful. With it we access a separate control flow, one that can trap and fix known problems. But some things we cannot recover from.
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