F# - Exception Handpng

An exception is a problem that arises during the execution of a program. An F# exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to spanide by zero.

Exceptions provide a way to transfer control from one part of a program to another. F# exception handpng provides the following constructs −

Let us start with the basic syntax of Exception Handpng.

Syntax

Basic syntax for F# exception handpng block is −

exception exception-type of argument-type

Where,

exception-type is the name of a new F# exception type.

argument-type represents the type of an argument that can be suppped when you raise an exception of this type.

Multiple arguments can be specified by using a tuple type for argument-type.

The try...with expression is used for exception handpng in the F# language.

Syntax for the try … with expression is −

try
   expression1
with
   | pattern1 -> expression2
   | pattern2 -> expression3
...

The try...finally expression allows you to execute clean-up code even if a block of code throws an exception.

Syntax for the try … finally expression is −

try
   expression1
finally
   expression2

The raise function is used to indicate that an error or exceptional condition has occurred. It also captures the information about the error in an exception object.

Syntax for the raise function is −

raise (expression)

The failwith function generates an F# exception.

Syntax for the failwith function is −

failwith error-message-string

The invapdArg function generates an argument exception.

invapdArg parameter-name error-message-string

Example of Exception Handpng

Example 1

The following program shows the basic exception handpng with a simple try… with block −

let spanisionprog x y =
   try
      Some (x / y)
   with
      | :? System.DivideByZeroException -> printfn "Division by zero!"; None

let result1 = spanisionprog 100 0

Division by zero!

exception Error1 of string
// Using a tuple type as the argument type.
exception Error2 of string * int

let myfunction x y =
   try
      if x = y then raise (Error1("Equal Number Error"))
      else raise (Error2("Error Not detected", 100))
   with
      | Error1(str) -> printfn "Error1 %s" str
      | Error2(str, i) -> printfn "Error2 %s %d" str i
myfunction 20 10
myfunction 5 5

Error2 Error Not detected 100
Error1 Equal Number Error

exception InnerError of string
exception OuterError of string

let func1 x y =
   try
      try
         if x = y then raise (InnerError("inner error"))
         else raise (OuterError("outer error"))
      with
         | InnerError(str) -> printfn "Error:%s" str
   finally
      printfn "From the finally block."

let func2 x y =
   try
      func1 x y
   with
      | OuterError(str) -> printfn "Error: %s" str

func2 100 150
func2 100 100
func2 100 120

From the finally block.
Error: outer error
Error:inner error
From the finally block.
From the finally block.
Error: outer error

let spanisionFunc x y =
   if (y = 0) then failwith "Divisor cannot be zero."
   else
      x / y

let tryspanisionFunc x y =
   try
      spanisionFunc x y
   with
      | Failure(msg) -> printfn "%s" msg; 0

let result1 = tryspanisionFunc 100 0
let result2 = tryspanisionFunc 100 4
printfn "%A" result1
printfn "%A" result2

Divisor cannot be zero.
0
25

let days = [| "Sunday"; "Monday"; "Tuesday"; "Wednesday"; "Thursday"; "Friday"; "Saturday" |]
let findDay day =
   if (day > 7 || day < 1)
      then invapdArg "day" (sprintf "You have entered %d." day)
   days.[day - 1]

printfn "%s" (findDay 1)
printfn "%s" (findDay 5)
printfn "%s" (findDay 9)

Sunday
Thursday
Unhandled Exception:
System.ArgumentException: You have entered 9.
…