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Exceptions: raise and reraise functions

  • Article
  • The raise function is used to indicate that an error or exceptional condition has occurred. Information about the error is captured in an exception object.
  • The reraise function is used to propagate a handled exception up the call chain.

Syntax

raise (expression)

Remarks

The raise function generates an exception object and initiates a stack unwinding process. The stack unwinding process is managed by the common language runtime (CLR), so the behavior of this process is the same as it is in any other .NET language. The stack unwinding process is a search for an exception handler that matches the generated exception. The search starts in the current try...with expression, if there is one. Each pattern in the with block is checked, in order. When a matching exception handler is found, the exception is considered handled; otherwise, the stack is unwound and with blocks up the call chain are checked until a matching handler is found. Any finally blocks that are encountered in the call chain are also executed in sequence as the stack unwinds.

The raise function is the equivalent of throw in C# or C++.

The following code examples illustrate the use of the raise function to generate an exception.

exception InnerError of string
exception OuterError of string

let function1 x y =
   try
     try
        if x = y then raise (InnerError("inner"))
        else raise (OuterError("outer"))
     with
      | InnerError(str) -> printfn "Error1 %s" str
   finally
      printfn "Always print this."


let function2 x y =
  try
     function1 x y
  with
     | OuterError(str) -> printfn "Error2 %s" str

function2 100 100
function2 100 10

The raise function can also be used to raise .NET exceptions, as shown in the following example.

let divide x y =
  if (y = 0) then raise (System.ArgumentException("Divisor cannot be zero!"))
  else
     x / y

Reraising an exception

The reraise function can be used in a with block to propagate a handled exception up the call chain. reraise does not take an exception operand. It's most useful when a method passes on an argument from a caller to some other library method, and the library method raises an exception that must be passed on to the caller.

The reraise function may not be used on the with block of try/with constructs in computed lists, arrays, sequences, or computation expressions including task { .. } or async { .. }.

open System

let getFirstCharacter(value: string) =
    try
        value[0]
    with :? IndexOutOfRangeException as e ->
        reraise()

let s = getFirstCharacter("")
Console.WriteLine($"The first character is {s}")

// The example displays the following output:
//   System.IndexOutOfRangeException: Index was outside the bounds of the array.
//      at System.String.get_Chars(Int32 index)
//      at getFirstCharacter(String value)
//      at <StartupCode>.main@()

See also