I'm implementing an exception monad in F#. First I took an ML approach, which works:
module ExceptionMonad =
type exception_ = string
type 'a t = Raise of exception_ | Return of 'a
let return' t = Return t
let raise' e = Raise e
let (>>=) m k =
match m with
| Raise e -> Raise e
| Return a -> k a
module TestExceptionMonad =
open ExceptionMonad
let test_exception_monad =
begin
return' 1 >>= fun a ->
return' 0 >>= fun b ->
if b = 0
then raise' "divide by zero"
else return' (a / b)
end (* => raise' "divide by zero" *)
Now I'm trying to use F# computation expression instead:
type exception_ = string
type 'a exception_monad = Raise of exception_ | Return of 'a
type ExceptionMonad() =
member x.Bind(m, k) =
match m with
| Raise e -> Raise e
| Return a -> k a
member x.Return(t) = Return t
member x.Zero() = Return 0
module TestExceptionMonad =
let monad = new ExceptionMonad()
let test_exception_monad =
monad {
let! a = Return 1 in
let! b = Return 0 in
if b = 0 then
Raise "divide by zero"
else
return (a / b)
} (* => Raise "divide by zero" *)
But F# is complaining that the expression Raise "divide by zero" is of type unit. That is not true, but I guess there's some code generation going on behind the scenes, and the error refers to that. I'm also not sure why x.Zero() is needed, but it was required by the compiler.
In F# computation expressions, when you have a "monadic value" that you want to return from a computation, you need to use the return! construct:
if b = 0 then return! Raise "divide by zero"
else return (a / b)
To support this, you also need to add ReturnFrom to your computation builder:
member x.ReturnFrom(c) = c
That said, implementing computation builder for exceptions is a good exercise if you're trying to understand how computation builders work in F#. But unless you have very good reasons for actually wanting it, I would not recommend this in practice. F# already has direct support for exceptions that you can use without any special syntax which works well in most of the situations, so there is really no need to replace it with more complicated way of doing the same thing.
If you're looking for input validation (as opposed to ordinary exception handling), then there is a nice F# project that implements a computation for that Chessie, but again - this is more of an input validation than exception handling.
You need to use one of the banged operations to execute monadic ops. Namely return!, let! or do!. In this case you could write:
if b = 0 then
return! Raise "divide by zero"
else
return (a / b)
Related
using the lib 'FsToolkit.ErrorHandling'
and the following code:
let f x =
if x % 2 = 0 then Ok $"even {x}" else Error $"odd {x}"
let xx =
validation {
let! a = f 1
and! b = f 2
and! c = f 3
return $"{a} {b} {c}"
}
printfn $"{xx.GetType()}"
The output is a
Result<string, string list>
Or, more specifically:
Microsoft.FSharp.Core.FSharpResult2[System.String,Microsoft.FSharp.Collections.FSharpList1[System.String]]
But the IDE (Rider) sees it differently:
Is this an expected behavior for some reason? or could it be a bug?
Validation<'a, 'err> is a type alias for Result<'a, 'err list>:
https://github.com/demystifyfp/FsToolkit.ErrorHandling/blob/f5019f10c4418426a2e182377be06beecd09876b/src/FsToolkit.ErrorHandling/Validation.fs#L3
This doesn't create a new type but creates a new way to refer to an existing type, which means that they can be used interchangeably.
I have a computation expression builder that builds up a value as you go, and has many custom operations. However, it does not allow for standard F# language constructs, and I'm having a lot of trouble figuring out how to add this support.
To give a stand-alone example, here's a dead-simple and fairly pointless computation expression that builds F# lists:
type Items<'a> = Items of 'a list
type ListBuilder() =
member x.Yield(()) = Items []
[<CustomOperation("add")>]
member x.Add(Items current, item:'a) =
Items [ yield! current; yield item ]
[<CustomOperation("addMany")>]
member x.AddMany(Items current, items: seq<'a>) =
Items [ yield! current; yield! items ]
let listBuilder = ListBuilder()
let build (Items items) = items
I can use this to build lists just fine:
let stuff =
listBuilder {
add 1
add 5
add 7
addMany [ 1..10 ]
add 42
}
|> build
However, this is a compiler error:
listBuilder {
let x = 5 * 39
add x
}
// This expression was expected to have type unit, but
// here has type int.
And so is this:
listBuilder {
for x = 1 to 50 do
add x
}
// This control construct may only be used if the computation expression builder
// defines a For method.
I've read all the documentation and examples I can find, but there's something I'm just not getting. Every .Bind() or .For() method signature I try just leads to more and more confusing compiler errors. Most of the examples I can find either build up a value as you go along, or allow for regular F# language constructs, but I haven't been able to find one that does both.
If someone could point me in the right direction by showing me how to take this example and add support in the builder for let bindings and for loops (at minimum - using, while and try/catch would be great, but I can probably figure those out if someone gets me started) then I'll be able to gratefully apply the lesson to my actual problem.
The best place to look is the spec. For example,
b {
let x = e
op x
}
gets translated to
T(let x = e in op x, [], fun v -> v, true)
=> T(op x, {x}, fun v -> let x = e in v, true)
=> [| op x, let x = e in b.Yield(x) |]{x}
=> b.Op(let x = e in in b.Yield(x), x)
So this shows where things have gone wrong, though it doesn't present an obvious solution. Clearly, Yield needs to be generalized since it needs to take arbitrary tuples (based on how many variables are in scope). Perhaps more subtly, it also shows that x is not in scope in the call to add (see that unbound x as the second argument to b.Op?). To allow your custom operators to use bound variables, their arguments need to have the [<ProjectionParameter>] attribute (and take functions from arbitrary variables as arguments), and you'll also need to set MaintainsVariableSpace to true if you want bound variables to be available to later operators. This will change the final translation to:
b.Op(let x = e in b.Yield(x), fun x -> x)
Building up from this, it seems that there's no way to avoid passing the set of bound values along to and from each operation (though I'd love to be proven wrong) - this will require you to add a Run method to strip those values back off at the end. Putting it all together, you'll get a builder which looks like this:
type ListBuilder() =
member x.Yield(vars) = Items [],vars
[<CustomOperation("add",MaintainsVariableSpace=true)>]
member x.Add((Items current,vars), [<ProjectionParameter>]f) =
Items (current # [f vars]),vars
[<CustomOperation("addMany",MaintainsVariableSpace=true)>]
member x.AddMany((Items current, vars), [<ProjectionParameter>]f) =
Items (current # f vars),vars
member x.Run(l,_) = l
The most complete examples I've seen are in §6.3.10 of the spec, especially this one:
/// Computations that can cooperatively yield by returning a continuation
type Eventually<'T> =
| Done of 'T
| NotYetDone of (unit -> Eventually<'T>)
[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
module Eventually =
/// The bind for the computations. Stitch 'k' on to the end of the computation.
/// Note combinators like this are usually written in the reverse way,
/// for example,
/// e |> bind k
let rec bind k e =
match e with
| Done x -> NotYetDone (fun () -> k x)
| NotYetDone work -> NotYetDone (fun () -> bind k (work()))
/// The return for the computations.
let result x = Done x
type OkOrException<'T> =
| Ok of 'T
| Exception of System.Exception
/// The catch for the computations. Stitch try/with throughout
/// the computation and return the overall result as an OkOrException.
let rec catch e =
match e with
| Done x -> result (Ok x)
| NotYetDone work ->
NotYetDone (fun () ->
let res = try Ok(work()) with | e -> Exception e
match res with
| Ok cont -> catch cont // note, a tailcall
| Exception e -> result (Exception e))
/// The delay operator.
let delay f = NotYetDone (fun () -> f())
/// The stepping action for the computations.
let step c =
match c with
| Done _ -> c
| NotYetDone f -> f ()
// The rest of the operations are boilerplate.
/// The tryFinally operator.
/// This is boilerplate in terms of "result", "catch" and "bind".
let tryFinally e compensation =
catch (e)
|> bind (fun res -> compensation();
match res with
| Ok v -> result v
| Exception e -> raise e)
/// The tryWith operator.
/// This is boilerplate in terms of "result", "catch" and "bind".
let tryWith e handler =
catch e
|> bind (function Ok v -> result v | Exception e -> handler e)
/// The whileLoop operator.
/// This is boilerplate in terms of "result" and "bind".
let rec whileLoop gd body =
if gd() then body |> bind (fun v -> whileLoop gd body)
else result ()
/// The sequential composition operator
/// This is boilerplate in terms of "result" and "bind".
let combine e1 e2 =
e1 |> bind (fun () -> e2)
/// The using operator.
let using (resource: #System.IDisposable) f =
tryFinally (f resource) (fun () -> resource.Dispose())
/// The forLoop operator.
/// This is boilerplate in terms of "catch", "result" and "bind".
let forLoop (e:seq<_>) f =
let ie = e.GetEnumerator()
tryFinally (whileLoop (fun () -> ie.MoveNext())
(delay (fun () -> let v = ie.Current in f v)))
(fun () -> ie.Dispose())
// Give the mapping for F# computation expressions.
type EventuallyBuilder() =
member x.Bind(e,k) = Eventually.bind k e
member x.Return(v) = Eventually.result v
member x.ReturnFrom(v) = v
member x.Combine(e1,e2) = Eventually.combine e1 e2
member x.Delay(f) = Eventually.delay f
member x.Zero() = Eventually.result ()
member x.TryWith(e,handler) = Eventually.tryWith e handler
member x.TryFinally(e,compensation) = Eventually.tryFinally e compensation
member x.For(e:seq<_>,f) = Eventually.forLoop e f
member x.Using(resource,e) = Eventually.using resource e
The tutorial at "F# for fun and profit" is first class in this regard.
http://fsharpforfunandprofit.com/posts/computation-expressions-intro/
Following a similar struggle to Joel's (and not finding §6.3.10 of the spec that helpful) my issue with getting the For construct to generate a list came down to getting types to line up properly (no special attributes required). In particular I was slow to realise that For would build a list of lists, and therefore need flattening, despite the best efforts of the compiler to put me right. Examples that I found on the web were always wrappers around seq{}, using the yield keyword, repeated use of which invokes a call to Combine, which does the flattening. In case a concrete example helps, the following excerpt uses for to build a list of integers - my ultimate aim being to create lists of components for rendering in a GUI (with some additional laziness thrown in). Also In depth talk on CE here which elaborates on kvb's points above.
module scratch
type Dispatcher = unit -> unit
type viewElement = int
type lazyViews = Lazy<list<viewElement>>
type ViewElementsBuilder() =
member x.Return(views: lazyViews) : list<viewElement> = views.Value
member x.Yield(v: viewElement) : list<viewElement> = [v]
member x.ReturnFrom(viewElements: list<viewElement>) = viewElements
member x.Zero() = list<viewElement>.Empty
member x.Combine(listA:list<viewElement>, listB: list<viewElement>) = List.concat [listA; listB]
member x.Delay(f) = f()
member x.For(coll:seq<'a>, forBody: 'a -> list<viewElement>) : list<viewElement> =
// seq {for v in coll do yield! f v} |> List.ofSeq
Seq.map forBody coll |> Seq.collect id |> List.ofSeq
let ve = new ViewElementsBuilder()
let makeComponent(m: int, dispatch: Dispatcher) : viewElement = m
let makeComponents() : list<viewElement> = [77; 33]
let makeViewElements() : list<viewElement> =
let model = {| Scores = [33;23;22;43;] |> Seq.ofList; Trainer = "John" |}
let d:Dispatcher = fun() -> () // Does nothing here, but will be used to raise messages from UI
ve {
for score in model.Scores do
yield makeComponent (score, d)
yield makeComponent (score * 100 / 50 , d)
if model.Trainer = "John" then
return lazy
[ makeComponent (12, d)
makeComponent (13, d)
]
else
return lazy
[ makeComponent (14, d)
makeComponent (15, d)
]
yield makeComponent (33, d)
return! makeComponents()
}
I want to be able to write a computation expression in F# that will be able to retry an operation if it throws an exception. Right now my code looks like:
let x = retry (fun() -> GetResourceX())
let y = retry (fun() -> GetResourceY())
let z = retry (fun() -> DoThis(x, y))
etc. (this is obviously an astract representation of the actual code)
I need to be able to retry each of the functions a set number of times, which I have defined elswhere.
I was thinking a computation expression could help me here, but I don't see how it could help me remove explicitly wrapping each right hand side to a Retryable<'T>
I could see the computation expression looking something like:
let! x = Retryable( fun() -> GetResourceX())
etc.
I understand that Monads, in a crude fashion, are wrapper types, but I was hoping a way around this. I know I can overload an operator and have a very succinct syntax for converting an operation into a Retryable<'T>, but to me that's just making the repetition/wrapping more succinct; it's still there. I could wrap each function to be a Retryable<'T>, but once again, I don't see the value over doing what's done at the top of the post (calling retry on each operation. At least it's very explicit).
Maybe computation expressions are the wrong abstraction here, I'm not sure. Any ideas on what could be done here?
Computation expressions have a few extensions (in addition to the standard monadic features), that give you a nice way to do this.
As you said, the monads are essentially wrappers (creating e.g. Retryable<'T>) that have some additional behavior. However, F# computation expression can also define Run member which automatically unwraps the value, so the result of retry { return 1 } can have just a type int.
Here is an example (the builder is below):
let rnd = new System.Random()
// The right-hand side evaluates to 'int' and automatically
// retries the specified number of times
let n = retry {
let n = rnd.Next(10)
printfn "got %d" n
if n < 5 then failwith "!" // Throw exception in some cases
else return n }
// Your original examples would look like this:
let x = retry { return GetResourceX() }
let y = retry { return GetResourceY() }
let z = retry { return DoThis(x, y) }
Here is the definition of the retry builder. It is not really a monad, because it doesn't define let! (when you use computation created using retry in another retry block, it will just retry the inner one X-times and the outer one Y-times as needed).
type RetryBuilder(max) =
member x.Return(a) = a // Enable 'return'
member x.Delay(f) = f // Gets wrapped body and returns it (as it is)
// so that the body is passed to 'Run'
member x.Zero() = failwith "Zero" // Support if .. then
member x.Run(f) = // Gets function created by 'Delay'
let rec loop(n) =
if n = 0 then failwith "Failed" // Number of retries exceeded
else try f() with _ -> loop(n-1)
loop max
let retry = RetryBuilder(4)
A simple function could work.
let rec retry times fn =
if times > 1 then
try
fn()
with
| _ -> retry (times - 1) fn
else
fn()
Test code.
let rnd = System.Random()
let GetResourceX() =
if rnd.Next 40 > 1 then
"x greater than 1"
else
failwith "x never greater than 1"
let GetResourceY() =
if rnd.Next 40 > 1 then
"y greater than 1"
else
failwith "y never greater than 1"
let DoThis(x, y) =
if rnd.Next 40 > 1 then
x + y
else
failwith "DoThis fails"
let x = retry 3 (fun() -> GetResourceX())
let y = retry 4 (fun() -> GetResourceY())
let z = retry 1 (fun() -> DoThis(x, y))
Here is a first try at doing this in a single computation expression. But beware that it's only a first try; I have not thoroughly tested it. Also, it's a little bit ugly when re-setting the number of tries within the computation expression. I think the syntax could be cleaned-up a good bit within this basic framework.
let rand = System.Random()
let tryIt tag =
printfn "Trying: %s" tag
match rand.Next(2)>rand.Next(2) with
| true -> failwith tag
| _ -> printfn "Success: %s" tag
type Tries = Tries of int
type Retry (tries) =
let rec tryLoop n f =
match n<=0 with
| true ->
printfn "Epic fail."
false
| _ ->
try f()
with | _ -> tryLoop (n-1) f
member this.Bind (_:unit,f) = tryLoop tries f
member this.Bind (Tries(t):Tries,f) = tryLoop t f
member this.Return (_) = true
let result = Retry(1) {
do! Tries 8
do! tryIt "A"
do! Tries 5
do! tryIt "B"
do! tryIt "C" // Implied: do! Tries 1
do! Tries 2
do! tryIt "D"
do! Tries 2
do! tryIt "E"
}
printfn "Your breakpoint here."
p.s. But I like both Tomas's and gradbot's versions better. I just wanted to see what this type of solution might look like.
F# allows to use checked arithmetics by opening Checked module, which redefines standard operators to be checked operators, for example:
open Checked
let x = 1 + System.Int32.MaxValue // overflow
will result arithmetic overflow exception.
But what if I want to use checked arithmetics in some small scope, like C# allows with keyword checked:
int x = 1 + int.MaxValue; // ok
int y = checked { 1 + int.MaxValue }; // overflow
How can I control the scope of operators redefinition by opening Checked module or make it smaller as possible?
You can always define a separate operator, or use shadowing, or use parens to create an inner scope for temporary shadowing:
let f() =
// define a separate operator
let (+.) x y = Checked.(+) x y
try
let x = 1 +. System.Int32.MaxValue
printfn "ran ok"
with e ->
printfn "exception"
try
let x = 1 + System.Int32.MaxValue
printfn "ran ok"
with e ->
printfn "exception"
// shadow (+)
let (+) x y = Checked.(+) x y
try
let x = 1 + System.Int32.MaxValue
printfn "ran ok"
with e ->
printfn "exception"
// shadow it back again
let (+) x y = Operators.(+) x y
try
let x = 1 + System.Int32.MaxValue
printfn "ran ok"
with e ->
printfn "exception"
// use parens to create a scope
(
// shadow inside
let (+) x y = Checked.(+) x y
try
let x = 1 + System.Int32.MaxValue
printfn "ran ok"
with e ->
printfn "exception"
)
// shadowing scope expires
try
let x = 1 + System.Int32.MaxValue
printfn "ran ok"
with e ->
printfn "exception"
f()
// output:
// exception
// ran ok
// exception
// ran ok
// exception
// ran ok
Finally, see also the --checked+ compiler option:
http://msdn.microsoft.com/en-us/library/dd233171(VS.100).aspx
Here is a complicated (but maybe interesting) alternative. If you're writing something serious then you should probably use one of the Brians suggestions, but just out of curiosity, I was wondering if it was possible to write F# computation expression to do this. You can declare a type that represents int which should be used only with checked operations:
type CheckedInt = Ch of int with
static member (+) (Ch a, Ch b) = Checked.(+) a b
static member (*) (Ch a, Ch b) = Checked.(*) a b
static member (+) (Ch a, b) = Checked.(+) a b
static member (*) (Ch a, b) = Checked.(*) a b
Then you can define a computation expression builder (this isn't really a monad at all, because the types of operations are completely non-standard):
type CheckedBuilder() =
member x.Bind(v, f) = f (Ch v)
member x.Return(Ch v) = v
let checked = new CheckedBuilder()
When you call 'bind' it will automatically wrap the given integer value into an integer that should be used with checked operations, so the rest of the code will use checked + and * operators declared as members. You end up with something like this:
checked { let! a = 10000
let! b = a * 10000
let! c = b * 21
let! d = c + 47483648 // !
return d }
This throws an exception because it overflows on the marked line. If you change the number, it will return an int value (because the Return member unwraps the numeric value from the Checked type). This is a bit crazy technique :-) but I thought it may be interesting!
(Note checked is a keyword reserved for future use, so you may prefer choosing another name)
I'd like to create a builder that builds expressions that returns something like a continuation after each step.
Something like this:
module TwoSteps =
let x = stepwise {
let! y = "foo"
printfn "got: %A" y
let! z = y + "bar"
printfn "got: %A" z
return z
}
printfn "two steps"
let a = x()
printfn "something inbetween"
let b = a()
Where the 'let a' line returns something containing the rest of the expressions to be evaluated later on.
Doing this with a separate type for each number of steps is straightforward but of course not particularly useful:
type Stepwise() =
let bnd (v: 'a) rest = fun () -> rest v
let rtn v = fun () -> Some v
member x.Bind(v, rest) =
bnd v rest
member x.Return v = rtn v
let stepwise = Stepwise()
module TwoSteps =
let x = stepwise {
let! y = "foo"
printfn "got: %A" y
let! z = y + "bar"
printfn "got: %A" z
return z
}
printfn "two steps"
let a = x()
printfn "something inbetween"
let b = a()
module ThreeSteps =
let x = stepwise {
let! y = "foo"
printfn "got: %A" y
let! z = y + "bar"
printfn "got: %A" z
let! z' = z + "third"
printfn "got: %A" z'
return z
}
printfn "three steps"
let a = x()
printfn "something inbetween"
let b = a()
printfn "something inbetween"
let c = b()
And the results are what I'm looking for:
two steps
got: "foo"
something inbetween
got: "foobar"
three steps
got: "foo"
something inbetween
got: "foobar"
something inbetween
got: "foobarthird"
But I can't figure out what the general case of this would be.
What I'd like is to be able to feed events into this workflow, so you could write something like:
let someHandler = Stepwise<someMergedEventStream>() {
let! touchLocation = swallowEverythingUntilYouGetATouch()
startSomeSound()
let! nextTouchLocation = swallowEverythingUntilYouGetATouch()
stopSomeSound()
}
And have events trigger a move to the next step in the workflow. (In particular, I want to play with this sort of thing in MonoTouch - F# on the iPhone. Passing around objc selectors drives me insane.)
the problem with your implementation is that it returns "unit -> 'a" for each call to Bind, so you'll get a different type of result for different number of steps (in general, this is a suspicious definition of monad/computation expression).
A correct solution should be to use some other type, which can represent a computation with arbitrary number of steps. You'll also need to distinguish between two types of steps - some steps just evaluate next step of the computation and some steps return a result (via the return keyword). I'll use a type seq<option<'a>>. This is a lazy sequence, so reading the next element will evaluate the next step of the computation. The sequence will contain None values with the exception of the last value, which will be Some(value), representing the result returned using return.
Another suspicious thing in your implementation is a non-standard type of Bind member. The fact that your bind takes a value as the first parameter means that your code looks a bit simpler (you can write let! a = 1) however, you cannot compose stepwise computation. You may want to be able to write:
let foo() = stepwise {
return 1; }
let bar() = stepwise {
let! a = foo()
return a + 10 }
The type I described above will allow you to write this as well. Once you have the type, you just need to follow the type signature of Bind and Return in the implementation and you'll get this:
type Stepwise() =
member x.Bind(v:seq<option<_>>, rest:(_ -> seq<option<_>>)) = seq {
let en = v.GetEnumerator()
let nextVal() =
if en.MoveNext() then en.Current
else failwith "Unexpected end!"
let last = ref (nextVal())
while Option.isNone !last do
// yield None for each step of the source 'stepwise' computation
yield None
last := next()
// yield one more None for this step
yield None
// run the rest of the computation
yield! rest (Option.get !last) }
member x.Return v = seq {
// single-step computation that yields the result
yield Some(v) }
let stepwise = Stepwise()
// simple function for creating single-step computations
let one v = stepwise.Return(v)
Now, let's look at using the type:
let oneStep = stepwise {
// NOTE: we need to explicitly create single-step
// computations when we call the let! binder
let! y = one( "foo" )
printfn "got: %A" y
return y + "bar" }
let threeSteps = stepwise {
let! x = oneStep // compose computations :-)
printfn "got: %A" x
let! y = one( x + "third" )
printfn "got: %A" y
return "returning " + y }
If you want to run the computation step-by-step, you can simply iterate over the returned sequence, for example using the F# for keyword. The following also prints the index of the step:
for step, idx in Seq.zip threeSteps [ 1 .. 10] do
printf "STEP %d: " idx
match step with
| None _ -> ()
| Some(v) -> printfn "Final result: %s" v
Hope this helps!
PS: I found this problem very interesting! Would you mind if I addapted my answer into a blog post for my blog (http://tomasp.net/blog)? Thanks!
Monads and computation builders confuse the hell out of me, but I've adapted something I've made in an earlier SO post. Maybe some bits and pieces can be of use.
The code below contains an action queue, and a form where the Click event listens to the next action available in the action queue. The code below is an example with 4 actions in succession. Execute it in FSI and start clicking the form.
open System.Collections.Generic
open System.Windows.Forms
type ActionQueue(actions: (System.EventArgs -> unit) list) =
let actions = new Queue<System.EventArgs -> unit>(actions) //'a contains event properties
with
member hq.Add(action: System.EventArgs -> unit) =
actions.Enqueue(action)
member hq.NextAction =
if actions.Count=0
then fun _ -> ()
else actions.Dequeue()
//test code
let frm = new System.Windows.Forms.Form()
let myActions = [
fun (e:System.EventArgs) -> printfn "You clicked with %A" (e :?> MouseEventArgs).Button
fun _ -> printfn "Stop clicking me!!"
fun _ -> printfn "I mean it!"
fun _ -> printfn "I'll stop talking to you now."
]
let aq = new ActionQueue(myActions)
frm.Click.Add(fun e -> aq.NextAction e)
//frm.Click now executes the 4 actions in myActions in order and then does nothing on further clicks
frm.Show()
You can click the form 4 times and then nothing happens with further clicks.
Now execute the following code, and the form will respond two more times:
let moreActions = [
fun _ -> printfn "Ok, I'll talk to you again. Just don't click anymore, ever!"
fun _ -> printfn "That's it. I'm done with you."
]
moreActions |> List.iter (aq.Add)