Using an example from Chris Smith's Programming F# 3.0:
let invalidUseOfMutable() =
let mutable x = 0
let incrementX() = x <- x + 1
incrementX()
x;;
This fails as expected:
error FS0407: The mutable variable 'x' is used in an invalid way.
Mutable variables cannot be captured by closures.
Now cut and paste the body of the function into FSharp Interactive:
let mutable x = 0
let incrementX() = x <- x + 1
incrementX()
x;;
And it works!
val it : int = 1
Why?
Edit: the following answer is correct for F# up to 3.x. Starting with F# 4.0, local mutables are automatically converted into refs if needed, so OP's code will actually successfully compile in all cases.
Short answer: it's not because of fsi, it's because the mutable is global.
Long answer:
For a normal (non-mutable) capture, implementation-wise the captured value is copied into the function object, so that if you return this function and use it outside of the scope in which it has been defined, everything works fine.
let pureAddOne() =
let x = 1
let f y = x + y // the value 1 is copied into the function object
f
let g = pureAddOne()
g 3 // x is now out of scope, but its value has been copied and can be used
On the other hand, in order to capture a mutable, the capture needs to be done by reference, else you wouldn't be able to modify it. But this is impossible, because in the previously mentioned case where the closure is returned and used outside of its definition scope, the mutable is also out of scope and potentially deallocated. This is the reason for the initial limitation.
let mutableAddOne() =
let mutable x = 1
let f y = x <- x + y // x would be referenced, not copied
f
let g = mutableAddOne()
g 3 // x is now out of scope, so the reference is invalid!
// mutableAddOne doesn't compile, because if it did, then this would fail.
However, if the mutable is global, then there is no such scope issue, and the compiler accepts it. It's not just fsi; if you try to compile the following program with fsc, it works:
module Working
let mutable x = 1 // x is global, so it never goes out of scope
let mutableAddOne() =
let f y = x <- x + y // referencing a global. No problem!
f
let g = mutableAddOne()
g 3 // works as expected!
In conclusion, as kwingho said, if you want to have a closure that captures a local mutable value, use a ref. They are heap-allocated (as opposed to the stack-allocated local mutable) so as long as the closure holds a reference to it, it won't be deallocated.
Related
take
let memoization f =
// The dictionary is used to store values for every parameter that has been seen
let cache = Dictionary<_,_>()
fun c ->
let exist, value = cache.TryGetValue (c)
match exist with
| true ->
// Return the cached result directly, no method call
printfn "%O -> In cache" c
value
| _ ->
// Function call is required first followed by caching the result for next call with the same parameters
printfn "%O -> Not in cache, calling function..." c
let value = f c
cache.Add (c, value)
value
then
let f (x:array<_>) = x.Length
then
let g = memoization f
let a = g [|1|]
let b = g [|1|]
I (obviously!) want b to be the retrieved memoized value already calculated, but it recalculated it.
ok, fair enough, with a C# head on, that makes sense, we're back to nasty objects, so how do I memoize a function that takes an array of values?
I notice that lists works nicely
So whats so special about arrays?
The issue is that, by default, Dictionary uses reference equality to check whether an object is in the dictionary. This means that it will only work if you pass it the same array instance. The following gets the value from the cache:
let g = memoization f
let arr = [|1|]
let a = g arr
let b = g arr
If you want to memoize results based on the values in the array, you can use structural equality comparison instead. To do this, all you need to do is to pass HashIdentity.Structural as an argument to Dictionary. This uses an F#-library defined structural comparison that returns the same hash for arrays containing the same values:
let cache = Dictionary<_,_>(HashIdentity.Structural)
With this change, your original example will work as you wanted.
Sorry for my question but I did not understand the answers that was related to this question so I hope someone can enlighten me further.
I am a new data science student and we are going to learn how to program in the functional language F#. We are learning about algorithms and I wanted to write the algorithms as F# functions to check if my calculations on paper were correct.
I get the following error saying:
"This value is not mutable. Consider using the mutable keyword let mutable n = expression"
My code looks like this:
let loop5( n ) =
let mutable x = 0
while n > 0 do
x <- x + 1
n <- n + 1
printfn "loop5(): x=%i for n=%i" x n
loop5(4)
I'm trying to write a function looking like this (pseudocode):
loop5(n)
x = 0
while n > 0
x = x + 1
n = n + 1
return x
Hope I made a clear question and someone can help me out here :-) Have a nice weekend
You're trying to mutate the loop's parameter n. The parameter is not mutable, so the compiler doesn't let you. That's exactly what the error tells you.
Now, normally, to make the error go away, you'd make the variable mutable. However, you can't make a function parameter mutable, so that's not an option.
Here you want to think what the meaning of your program should be. Does the loop function need to pass the updated value of n back to its caller, or is the whole mutation its internal business? If it's the former, please see #AnyMoose's answer, but from your example and explanation, I suspect it's the latter. If that is the case, simply make a mutable copy of the parameter and work with it:
let loop n' =
let mutable x = 0
let mutable n = n'
...
Separately, I want to point out that your program, as written, would actually loop indefinitely (or until it wraps around the max int value anyway), because instead of decreasing n at each step you're increasing it. If you want your program to actually finish before the next Ice Age, you need to make n decrease with each iteration:
n <- n - 1
Ref cells
Ref cells get around some of the limitations of mutables. In fact, ref cells are very simple datatypes which wrap up a mutable field in a record type. Ref cells are defined by F# as follows:
type 'a ref = { mutable contents : 'a }
The F# library contains several built-in functions and operators for working with ref cells:
let ref v = { contents = v } (* val ref : 'a -> 'a ref *)
let (!) r = r.contents (* val (!) : 'a ref -> 'a *)
let (:=) r v = r.contents <- v (* val (:=) : 'a ref -> 'a -> unit *)
The ref function is used to create a ref cell, the ! operator is used to read the contents of a ref cell, and the := operator is used to assign a ref cell a new value. Here is a sample in fsi:
let x = ref "hello";;
val x : string ref
x;; (* returns ref instance *)
val it : string ref = {contents = "hello";}
!x;; (* returns x.contents *)
val it : string = "hello"
x := "world";; (* updates x.contents with a new value *)
val it : unit = ()
!x;; (* returns x.contents *)
val it : string = "world"
Since ref cells are allocated on the heap, they can be shared across multiple functions:
open System
let withSideEffects x =
x := "assigned from withSideEffects function"
let refTest() =
let msg = ref "hello"
printfn "%s" !msg
let setMsg() =
msg := "world"
setMsg()
printfn "%s" !msg
withSideEffects msg
printfn "%s" !msg
let main() =
refTest()
Console.ReadKey(true) |> ignore
main()
The withSideEffects function has the type val withSideEffects : string ref -> unit.
This program outputs the following:
hello
world
Assigned from withSideEffects function
The withSideEffects function is named as such because it has a side-effect, meaning it can change the state of a variable in other functions. Ref Cells should be treated like fire. Use it cautiously when it is absolutely necessary but avoid it in general. If you find yourself using Ref Cells while translating code from C/C++, then ignore efficiency for a while and see if you can get away without Ref Cells or at worst using mutable. You would often stumble upon a more elegant and more maintanable algorithm
Aliasing Ref Cells
Note: While imperative programming uses aliasing extensively, this practice has a number of problems. In particular it makes programs hard to follow since the state of any variable can be modified at any point elsewhere in an application. Additionally, multithreaded applications sharing mutable state are difficult to reason about since one thread can potentially change the state of a variable in another thread, which can result in a number of subtle errors related to race conditions and dead locks.
A ref cell is very similar to a C or C++ pointer. Its possible to point to two or more ref cells to the same memory address; changes at that memory address will change the state of all ref cells pointing to it. Conceptually, this process looks like this:
Let's say we have 3 ref cells looking at the same address in memory:
Three references to an integer with value 7
cell1, cell2, and cell3 are all pointing to the same address in memory. The .contents property of each cell is 7. Let's say, at some point in our program, we execute the code cell1 := 10, this changes the value in memory to the following:
Three references to an integer with value 10
By assigning cell1.contents a new value, the variables cell2 and cell3 were changed as well. This can be demonstrated using fsi as follows:
let cell1 = ref 7;;
val cell1 : int ref
let cell2 = cell1;;
val cell2 : int ref
let cell3 = cell2;;
val cell3 : int ref
!cell1;;
val it : int = 7
!cell2;;
val it : int = 7
!cell3;;
val it : int = 7
cell1 := 10;;
val it : unit = ()
!cell1;;
val it : int = 10
!cell2;;
val it : int = 10
!cell3;;
val it : int = 10
I was taught, that data, by default, is immutable in F#.
When we reassign value to some variable, what really happens is that it rebinds the value of variable, but setting a new value is different thing.
Rebinding is called Shadowing whilst setting new value is impossible if we don't say explicitly, that value of the variable is mutable.
Can anyone explain to me this concept in a bit more details?
What is the difference between shadowing (rebinding):
let var = "new_value"
and setting a new value, as:
var <- "new_value"
Is this a moment, that during rebinding we create another object and we assign that object's address to the variable, whereas in the second example we change the value itself? I brought that from heap/stack concept.. but I may be wrong.
Thanks.
Shadowing is when you create a new binding that uses the same name as a previous binding. This "shadows" the original name, which hides it but doesn't change or replace it. Try this in FSI to see:
let foo = 42
let printFoo () =
printfn "%i" foo
printFoo() ;;
This will print:
42
val foo : int = 42
val printFoo : unit -> unit
val it : unit = ()
Then add:
// ... more code
let foo = 24
printfn "%i" foo // prints 24
printFoo ();;
This will print:
24
42
val foo : int = 24
val it : unit = ()
Note that it still prints 42 when you call printFoo() - the function sees the original (unshadowed) binding, but the new print shows the new value.
Using <- to mutate a value requires a mutable binding:
let mutable bar = 42
let printBar () =
printfn "%i" bar
printBar ();;
This, like above, prints 42. Note that you override the default immutable behavior here with the mutable keyword.
You then change the value within the mutable binding:
bar <- 24
printfn "%i" bar
printBar ();;
This will print 24 twice, since, unlike the shadowed version, the mutation changes the original binding. If you leave mutable off in the original binding, you'll get an error when using <-.
To add on Reed Copsey's excellent answer, if you're writing a loop where you change the value of an accumulator of some sort, you'd set the original value as mutable. For example, you can do this.
let mutable acc = 0 // declaration
for i in 1..100 do
acc <- acc + i // assignment
This is more or less equivalent to the C# code :
var acc = 0;
for (int i = 1; i <= 100; i++)
{
acc = acc + i; // assignment
// Another way to write this:
// acc += i;
}
However, in Shadowing, as in this F# snippet:
let acc = 0 // declaration
for i in 1..100 do
let acc = acc + i // another declaration only within the loop
You're not actually doing anything useful!! The second declaration has scope only within the for loop, and it doesn't change the value of the original acc.
A rough C# equivalent would be this:
var acc = 0; // declaration
for (int i = 1; i <= 100; i++)
{
var acc2 = acc + i; // another declaration only within the loop
}
Note that using acc (instead of acc2) for the inside variable wouldn't compile in C#, as it doesn't have Shadowing in this context.
The use of shadowing is that, it prevents from using the original variant in a block of code where you don't want it. So there is one less variable to worry about.
Whenever I wonder what actually is happening I use tools like ILSpy
For example:
let f () =
let x = Dictionary<int, string> ()
let mutable x = ResizeArray<int> 16
x <- ResizeArray<int> 16
Using ILSpy to decompile it in C# code it becomes:
public static void f()
{
Dictionary<int, string> x = new Dictionary<int, string>();
List<int> x2 = new List<int>(16);
x2 = new List<int>(16);
}
Here it's more obvious what the difference between shadowing and setting.
Shadowing x creates a new variable with name x2.
Setting is normal assignment.
I have written two versions of code. The first one works as expected and print "Hi". the second one gives me error that "block following this let is unfinished"
1st version
#light
let samplefn() =
let z = 2
let z = z * 2
printfn "hi"
samplefn()
2nd version
#light
let samplefn() =
let z = 2
let z = z * 2
samplefn()
Only difference is the printfn is absent in the second version. I am using Visual Studio 2010 as my IDE. I am very new to F# but this error seems very strange to me. I guess I am missing some very important concept. Please explain.
Edit: Also if I do it outside the function I get error even with the first version of code.
#light
let z = 2
let z = z * 2
printfn "Error: Duplicate definition of value z"
let binds a value to a label but otherwise doesn't do much else. Your function contains two bindings but doesn't use them, and so you get an error.
To think of it another way, all functions in F# need a return value, which is the value of the last executed expression in your function. let doesn't have a return value so your function is invalid. To fix this you can add a return value, for example:
let samplefn() =
let z = 2
let z = z * 2
()
which defines a function that does absolutely nothing (returns unit). Perhaps a better example is this:
let samplefn() =
let z = 2
let z = z * 2
z
which will return 4 (the value of the binding for label z).
I think it is helpful to understand the non-light syntax here. Let's translate:
1st Version (let binding expressions)
let samplefn() =
let z = 2 in
let z = z * 2 in
printfn "hi";;
samplefn();;
The important thing to understand here is that all non-top-level let bindings are actually expressions of the form let <variable> = <expression1> in <expression2> where <variable> is bound to the result of <expression1> in a new scope <expression2>, and <expression2> is the return value of the entire expression. The light syntax makes you believe such let bindings are variable assignments / statements, when in fact it really is true that almost everything in F# is an expression.
Perhaps the following illustrates this more clearly:
let a = (let b = 3 in b + 2) //a is 5
2nd Version (top-level let bindings)
let z = 2;;
let z = z * 2;;
printfn "Error: Duplicate definition of value z";;
Top level let-bindings are terminated with ;;, indicating the completion of what may be thought of as a statement. The top-level is a single scope, and here we get an error for trying to bind z twice within the same scope. Whereas using the expression form of let bindings in Example 1, we bind z anew for each sub-scope in the expression chain. Note that we could do something like this at the top-level:
let z = (let z = 2 in z * 2);;
A let that is not at the top level (e.g. your indented ones) has to have a statement (actually an expression, as pst notes) called a "body" following the assignment. In the first example the body is printfn "hi", while the second example has no body. That's what the compiler is complaining about.
Note that in your function definitions the inner let expressions actually create nested scopes. That is, the let z = z * 2 actually creates a new value called z and binds to it the value of the outer z times 2, then uses it in the body of the let (which is the printfn in this case). A nested let will always have a body. It is the nesting which allows the seemingly duplicate definition.
Since an outermost let does not need a body, the compiler thinks you're trying to redefine z in the same scope, which is an error. You can use parentheses to tell the compiler how to properly interpret the last example:
let z = 2
(let z = z * 2
printfn "z = %d" z)
printfn "z = %d" z
The above will print z = 4
z = 2
Is there a way to have mutable function arguments in F#, that would allow something like
let mutable i = 9
let somefun n = n <- 12; ()
somefun i
(* *not* a real-world example *)
I do understand that this can be made to work by wrapping it into a record type
type SomeRec = { mutable i: int }
let ri = { i = 9 }
let someotherfun r = r.i <- 12; ()
and that this can be done in a similar fashion for class members. However, even after browsing through the whole F# Language Specification (yes, I did!), there seems to be no syntax to allow the first case, and the compiler appears to be quite unhappy about my trying this. I was hoping there would be some sort of type annotation, but mutable cannot be used in such.
I also know that I should not be doing this sort of thing in the first place, but the first case (int binding) and the second (record type) are semantically identical, and any such objection would hold for both cases equally.
So I think that I am missing something here.
You can use ref as arguments
let v = ref 0
let mutate r =
r := 100
mutate v
printfn "%d" !v
Or byref keyword
let mutable v = 0
let mutate (r : byref<_>) =
r <- 100
mutate &v
printfn "%d" v
Use byref keyword which is equivalent to C# ref.
See Passing by reference.