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Workflow Builder doesn't use Let?
f#

I have a problem that's well suited (I think) to using workflows. Specifically, I want to have a workflow that terminates based on some external variable _after every expression_, not just Bind. Going from earlier definitions (such as on dsyme's blog or in Expert F#), "let" in a computation expression is:

let pat = expr in cexpr = b.Let(expr,(fun pat -> «cexpr»))

Which should allow me to easily add a check in and everything will work fine. Unfortunately, this doesn't seem to be the case anymore. The F# spec says:

{| let binds in cexpr |}C = let binds in {| cexpr |}C)

I've used the Attempt example from here: [link:blogs.msdn.com]

And then played with Let (say, adding a print or a failwith: member b.Let(p,rest) : Attempt<'a> = failwith "test"; rest p) and nothing different happens. Removing Let also works just fine.

What am I missing?

Thanks,
Michael

.

Hi Michael,

I bumped into this issue too. Essentially the Let member has been removed from the workflow builder compilation. See the thread here. Would using let! instead of let be problematic in your case?

regards,

Danny

By on 10/8/2008 2:07 AM ()

I suspected, although the rest of the spec and the implementation of Let in the other builders threw me off.

Well the problem is that I want this auto-checking after every expression, for example:

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let a = attempt { 
            do printf "Input:"
            let x = Console.ReadLine()
            return! failIfBig 10 }

I'd want it to fail depending on an external variable at each line, not just when calling another attempt.

I can actually get this behaviour by changing the signature for Bind to be the same as Let. But then I can't use let! with other attempts, which ends up being somewhat annoying.

Although, perhaps I should shift my problem around a bit to get a better overall solution...

By on 10/8/2008 6:40 AM ()

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