I am trying to understand Closure in F#. Based on the question and answers here my understanding is that, it is the process of carrying context around. One of the answers had a very simple solution like the following
let addCounter =
let counter = ref 0
(fun () -> incr counter; !counter)
[<EntryPoint>]
let main argv =
let firstCounter = addCounter()
printfn "%A" firstCounter
printfn "%A" firstCounter
let k = Console.ReadKey()
I created a high order function called firstCounter and I was expecting the first output to be 1 and the second output to be 2. But when I ran, I get both the answers as 1. Is it something I am doing wrong here?
Try to run this:
let addCounter =
let counter = ref 0
(fun () -> incr counter; !counter)
[<EntryPoint>]
let main argv =
printfn "%d" (addCounter ())
printfn "%d" (addCounter ())
let k = Console.ReadKey()
Your version does this: it calls addCounter once and assigns the result-value 1 to firstCounter - so of course you get the same answer if you evaluate firstCounter (which is really just an integer) multiple times.
The part with the closure is that counter is captured by the function fun () -> incr counter; !counter and it's a rather common thing to use in F# (who needs classes to capture state if you can use a closure - indeed classes (and their methods) and closures have a lot in common!
your additional question
if you want to create counters on the fly you can do it like this:
let createCounter() =
let counter = ref 0
(fun () -> incr counter; !counter)
[<EntryPoint>]
let main argv =
let c1 = createCounter()
let c2 = createCounter()
printfn "%d" (c1 ()) // -> "1"
printfn "%d" (c2 ()) // -> "1"
printfn "%d" (c1 ()) // -> "2"
printfn "%d" (c1 ()) // -> "3"
printfn "%d" (c2 ()) // -> "2"
what is going on here
now everytime you call createCounter a new ref-cell is created and a function to increase and return this is returned - so with every call you get a new counter with context and you just can use those as you used addCounter before
Related
The following example is based on a snippet that produces functions that allow enumerating sequence values one by one.
Here printAreEqual () gives true, print2 () gives 12345678910, but print1 () gives 0000000000.
Why cannot the function returned by enumerate return the values of the sequence generated using yield?
open System.Linq
let enumerate (xs: seq<_>) =
use en = xs.GetEnumerator()
fun () ->
en.MoveNext() |> ignore
en.Current
let s1 = seq { for i in 1 .. 10 do yield i }
let s2 = seq { 1 .. 10 }
let f1 = s1 |> enumerate
let f2 = s2 |> enumerate
let printAreEqual () = Enumerable.SequenceEqual (s1, s2) |> printf "%b" // true
let print1 () = for i in 1 .. 10 do f1() |> printf "%i" // 0000000000
let print2 () = for i in 1 .. 10 do f2() |> printf "%i" // 12345678910
The use en = ... in the enumerate function is effectively doing this:
let enumerate (xs: seq<_>) =
let en = xs.GetEnumerator()
let f =
fun () ->
en.MoveNext() |> ignore
en.Current
en.Dispose()
f
You're always disposing of the enumerator before you start using it, so the behaviour is probably undefined in this situation and it doesn't matter why you get different results for two sequences with different implementations.
Fine-grained control of sequence enumeration is always tricky and it's hard to make helper functions for because of the mutable state.
So far, I have pretty much things set but that dumb printfn is still not working.
open System
[<EntryPoint>]
let main argv =
let n = Console.ReadLine() |> int
let nums = seq { for i in 1..n -> Console.ReadLine() |> int }
printfn "%d" (Seq.sum nums)
0
The answer from Joseph explains what is wrong with your code.
If you wanted to do this in a more F# way, then you'd probably want to eliminate the mutation altogether. One reasonably nice way of doing this would be to use sequence expressions to construct a sequence of all the numbers that you're reading from the console and then use Seq.sum to calculate the sum:
[<EntryPoint>]
let main argv =
let n = Console.ReadLine() |> int
let nums = seq { for i in 1..n -> Console.ReadLine() |> int }
printfn "%d" (Seq.sum nums)
0
Two things I noticed, you need to end the program with 0, that is the exit code.
Second thing is the equals sign in FSharp isn't used for updating values, F# uses the <- operator instead. Here is your program with the updated changes.
open System
[<EntryPoint>]
let main argv =
let mutable sum = 0
let n = Console.ReadLine() |> int
for i in 1..n do
let mutable r = Console.ReadLine() |> int
sum <- sum + r;
printfn "%d" sum
0
trying to work around a problem in outside library - is there a way to try-catch the generator itself item by item (probably not, but just to be sure...)?
let myTest() =
let mySeq = seq { for i in -3 .. 3 -> 1 / i }
// how to keep the line above intact, but modify the code below to try-catch-ignore the bad one?
mySeq |> Seq.iter (fun i -> printfn "%d" i)
()
You can't.
Once the exception happens, the state of the source enumerator is screwed up. If you can't get into the source enumerator to "fix" its state, you can't make it keep producing values.
You can, however, make the whole process "stop" after the exception, but you'll have to go a level below and work with IEnumerator<T>:
let takeUntilError (sq: seq<_>) = seq {
use enm = sq.GetEnumerator()
let next () = try enm.MoveNext() with _ -> false
let cur () = try Some enm.Current with _ -> None
while next() do
match cur() with
| Some c -> yield c
| None -> ()
}
mySeq |> takeUntilError |> Seq.iter (printf "%d")
I want to time my functions, some of them use up to three parameters. Right now I'm using the same code below with some variations for the three.
let GetTime f (args : string) =
let sw = Stopwatch.StartNew()
f (args)
printfn "%s : %A" sw.Elapsed
I want to replace the three functions with this one.
let GetTime f ( args : 'T[]) =
let sW = Stopwatch.StartNew()
match args.Length with
| 1 -> f args.[0]
| 2 -> f (args.[0] args.[1])
printfn "%A" sW.Elapsed
()
But I'm getting an error of type mismatch, if I use the three functions it works. Is it possible to send the function as a parameter and use it like this?
Why not just do something like this?
let getTime f =
let sw = Stopwatch.StartNew()
let result = f ()
printfn "%A" sw.Elapsed
result
Assuming that f1, f2, and f3 are three functions that take respectively 1, 2, and 3 arguments, you can use the getTime function like this:
getTime (fun () -> f1 "foo")
getTime (fun () -> f2 "foo" "bar")
getTime (fun () -> f3 "foo" "bar" "baz")
However, if you just need to time some functions in FSI, this feature is already built-in: just type
> #time;;
and timing will be turned on.
It isn't possible for the compiler to know how many arguments will be passed at runtime, so the function f must satisfy both 'T -> unit and 'T -> 'T -> unit. This form also requires all arguments to be of the same type.
The following approach delays the function execution and may be suitable for your needs.
let printTime f =
let sw = Stopwatch.StartNew()
f() |> ignore
printfn "%A" sw.Elapsed
let f1 s = String.length s
let f2 s c = String.concat c s
printTime (fun () -> f1 "Test")
printTime (fun () -> f2 [| "Test1"; "Test2" |] ",")
You're probably thinking of passing a method group as an argument to GetTime, and then having the compiler decide which overload of the method group to call. That's not possible with any .NET compiler. Method groups are used for code analysis by compilers and tools such as ReSharper, but they are not something that actually exists at runtime.
If your functions take their arguments in tupled form, like these:
let f1 (s: string, b: bool) =
System.Threading.Thread.Sleep 1000
s
let f2 (n: int, s:string, dt: System.DateTime) =
System.Threading.Thread.Sleep 1000
n+1
then the implementation becomes trivial:
let Timed f args =
let sw = System.Diagnostics.Stopwatch.StartNew()
let ret = f args
printfn "Called with arguments %A, elapsed %A" args sw.Elapsed
ret
Usage:
f1
|> Timed // note, at this time we haven't yet applied any arguments
<| ("foo", true)
|> printfn "f1 done, returned %A"
f2
|> Timed
<| (42, "bar", DateTime.Now)
|> printfn "f2 done, returned %A"
However, if the functions take their arguments in curried form, like this:
let f1Curried (s: string) (b: bool) =
System.Threading.Thread.Sleep 1000
s
let f2Curried (n: int) (s:string) (dt: System.DateTime) =
System.Threading.Thread.Sleep 1000
n+1
it becomes a bit tricky. The idea is using standard operators (<|), (<||), and (<|||) that are intended to uncurry the arguments.
let Timed2 op f args =
let sw = System.Diagnostics.Stopwatch.StartNew()
let ret = op f args
printfn "Called with arguments %A, elapsed %A" args sw.Elapsed
ret
f1Curried
|> Timed2 (<||) // again, no arguments are passed yet
<| ("foo", true)
|> printfn "f1Curried done, returned %A"
f2Curried
|> Timed2 (<|||)
<| (42, "bar", DateTime.Now)
|> printfn "f2Curried done, returned %A"
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)