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failwith: Exception Handling
This page was last reviewed on Mar 1, 2023.
Dot Net Perls
Failwith. Special operators, like failwith and invalidArg, are available in the F# language. These create exceptions that enter into an alternate control flow.
Understanding failure. In F# we do not have the same try, catch and throw syntax. Instead, we can use failwith to "throw" an exception.
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.
Start We call validLength with an argument of 2. We get a result of "true" and use printfn to display this.
Next We use an argument of 200 and failwith is reached. This terminates the program with an unhandled exception.
// 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
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.
Info The raise statement creates a NotImplementedException with a custom message. The try-block is stopped.
Note The with statement matches the NotImplementedException type. In response it prints a special message. The program does not terminate.
open System try // Raise a special exception. raise (NotImplementedException "Not ready") with | :? NotImplementedException -> printfn "Not implemented, ignoring"
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.
try // An error occurs. failwith "Not valid" finally // A finally block always runs, so we can try to recover. printfn "Can recover here"
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.
Dot Net Perls is a collection of tested code examples. Pages are continually updated to stay current, with code correctness a top priority.
Sam Allen is passionate about computer languages. In the past, his work has been recommended by Apple and Microsoft and he has studied computers at a selective university in the United States.
This page was last updated on Mar 1, 2023 (edit).
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