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The type unit does not have null as a proper value
f#

I've just upgraded to the May 2009 CTP and am trying to get my code working again. I'm getting an error claiming that the type 'unit' does not have 'null' as a proper value. The problem as I'm seeing it appears to be coming from the use of a mutalble value initially set to null.

I have an inline function which I use generically to attempt retries of code that might fail. I get the error on the invocation of some other function under control of the retry. The retry fuction looks like:

exception RetryFail
let inline retry (f : 'a) (count : int) (delay : int) (logger : ILog) : 'b =
let mutable remainCount = count
let mutable success = false
let mutable result = null
let getCall = fun () -> nextAvailableCall
let setCall = fun x -> nextAvailableCall <- x
while remainCount > 0 && not success do
if (count <> remainCount) then
Thread.Sleep delay
log.Info "Error Retry Sleep Invoked"
remainCount <- remainCount - 1
try
let sleep = minWaitForWebCall(getCall, setCall) throttleRate
if sleep > zero then
Thread.Sleep sleep //could sleep for shorter interval, but extra delta should not be a problem
result <- f ()
success <- true
with
| :? System.Net.WebException as exc ->
let msg = exc.Message
let stat = exc.Status.value__
logger.Error (sprintf "retry - Web Exception: %d\tMessage: %s" stat msg)
success <- false
| :? System.IO.IOException as exc ->
let msg = exc.Message
logger.Error (sprintf "retry - System IO Exception - Message: %s" msg)
success <- false
if success then
result
else
log.Error (sprintf "Failure after %d retries" count)
raise RetryFail (sprintf "Failure after %d retries" count)

The invocation occurs in a test:

let testfun =
(fun () ->
match !counter with
| 0 -> counter := !counter + 1
| 1 | 2 -> counter := !counter + 1
raise ( System.Net.WebException "Web Except")
| 3 | 4 | 5 | 6 -> counter := !counter + 1
raise (System.IO.IOException "IO Except")
|_ -> Assert.Fail("Unexpected Count when testfun called")
)
retry testfun 4 10 logger

with the error being reported on the 'testfun' reference in the retry.

Insights appreciated.
Thanks,
--Don

.

Hi,
first, of all, let me just slightly modify the type signature (because the first argument is a function that takes unit as the argument and returns a value which is the overall result):

1

let inline retry (f : unit -> 'a) (count : int) (delay : int) (logger : ILog) : 'a = //...

The problem with the code is that you're calling the 'retry' function with a function of type 'unit -> unit' as the argument. This means that when the function f is called inside the retry loop, you'll assign value of type 'unit' to the 'result' mutable variable. However, when initializing the 'result' variable, you assign it the 'null' value, which isn't compatible with the 'unit' type.

To fix this, you can use 'Unchecked.defaultof<'a>' primitive that returns default value of some type (and works with any type):

1
2

 
let mutable result = Unchecked.defaultof<'a>
By on 5/27/2009 5:12 AM ()

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