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Computation Expressions and for loops
f#

I've been having trouble getting a computation expression with a for loop to work. I'm trying to create an afterword monad, where you can bind an expression to be evaluated only after the rest of the computation has been performed. It works, except I can't get any of the looping constructs to function.

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type AfterwordBuilder() =
    member this.Bind(x,rest) = rest();x()
    member this.Delay(f) = f()
    member this.Return(x) = x
    member this.Zero() = ()
    member this.For(e,f) = for x in e do f x
    member this.Combine(a,b) = a();b


 do afterword {
    let counter = ref 0
    for xform in flame.xforms do
        let n = !counter
        xforms.[n] <- new Xform(offset,[||])
        //let decisionTree = buildDecisionTree xform.next
        //xforms.[n].decisionTree <- decisionTree
        offset <- xforms.[n].nextOffset
        do! (fun _ ->   xforms.[n].flameArrayBase <- flameArray
                              xforms.[n].setXform flame.xforms.[n].self |> ignore)
        incr counter
    flameArray <- Array.create(offset) 0.0f
    }

However, this gives the error at for xform in flame.xforms do complaining about expecting a unit -> unit but only getting a unit.

Anyone know what I'm doing wrong?

.

Also, how do I display code on the forum without the darn smilies?? :-P

By on 3/16/2011 6:56 PM ()

The problem is your Combine() method. Check out

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type AfterwordBuilder() =

    member this.Bind(x,rest) = rest();x()

    member this.Delay(f) = f()

    member this.Return(x) = x

    member this.Zero() = ()

    member this.For(e,f) = for x:int in e do f x

    //member this.Combine(a,b) = a();b

 

let afterward = new AfterwordBuilder()

 

do afterward {

    let counter = ref 0

    for xform in [1;2] do

        let n = !counter

        ()

    //()

    }

where it works, but uncommenting both comments causes the failure. See also

[link:msdn.microsoft.com]

and

[link:research.microsoft.com]

(6.3.10 Computation Expressions).

By on 3/16/2011 8:58 PM ()

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