I am trying to iterate through an IDictionary (reasons explained later...) in F#, and round each value to a specified precision. Essentially this is what I'm trying to do:
List.iter (fun(x) -> a.Item(x) <- Math.Round(a.Item(x), input.precision)) (ICollectionToDoubleList a.Keys)
(where ICollectionToDoubleList takes the ICollection a.Keys and casts it to a double list).
However since you can't alter mutable variables inside closures, this doesn't compile.
My first attempt at a solution was this:
List.iter (fun(x) -> let p = Math.Round(a.Item(x), input.precision)
a.Item(x) := p
) (ICollectionToDoubleList a.Keys)
However I'm getting the error:
This expression was expected to have type
'a ref
but here has type
double
on a.Item(x)
I could convert the IDictionary into two lists (or a list of tuples), perform the rounding, and re-cast into an IDictionary, but this seems a bit messy and convoluted.
Any guidance greatly appreciated.
EDIT:
I forgot to mention a was defined as:
let mutable (a : IDictionary<double,double>) = ...
I think you want
a.Item(x) <- p
In F# you use <- to assign to mutable values, whilst := assign to ref values.
You could even use
a.[x] <- p
for a slightly simpler version.
Explaination of what mutable means (it behaves like the opposite of const in C)
let mutable m = [|1|]
let t = [|1|]
m.[0] <- 0
t.[0] <- 0 //neither of these change m or t - only elements so they are fine
m <- [|1;2;3;|] //fine as m is mutable
t <- [|1;2;3;|] //not allowed as t is not mutable
If you are used to const in C, the above are roughly equivalent to
int* m = {1};
const int* t = {1}
note, neither is equivalent to
const int* q const = {1}
which is I think what you thought not mutable meant.
Ok so I've discovered the answer...
I have defined a as:
let mutable (a : IDictionary<double,double>) = ...
If I change this to
let (a : IDictionary<double,double>) = ...
then this compiles. It seems a little counter-intuative to me that a non-mutable value can be mutated, but a mutatable variable cannot!!
Related
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
Learning F# as part of my course, and can do some cool things, but something has been bugging me, whenever I use the val keyword, I get an error. I think it could be due to not declaring something in script, but I don't really know.
module Prime
#light
let nums = [1; 2; 3; 4; 5];;
val nums : list<int>
let rec sum list =
match list with
| h::tail -> (sum tail) + h
| [] -> 0
val sum : list<int> -> int
I get (line 5):
Error 1 Unexpected keyword 'val' in definition . Expected incomplete structured construct at or before this point or other token
Any ideas?
The val keyword in F# (unlike 'val' in ML) is used to declare a field in a class or structure type without initializing it.
http://msdn.microsoft.com/en-us/library/dd469494.aspx
if you want to define mutable value in the Module you can use
let mutable...
By the way, if you define the value with the same name (like 'nums') twice or more times then the effective value for the compiler will be latest defined in the scope.
So actually, I had misread the coursework set out, annoyingly the papers use val to define what the expected output of the function is, as opposed to using it as the keyword it is meant to be. Hence my confusion and lots of head scratching.
This looks like F# interactive output mixed in with code.
If I type this into FSI:
let nums = [1; 2; 3; 4; 5];;
The output is
val nums : int list = [1; 2; 3; 4; 5]
Note that ;; is where FSI parses and runs input. You wouldn't have this in non-interactive code. The output might differ because of an older version or editing, but nontheless, it doesn't belong in code.
Coincidentally, val is also a rarely used F# keyword for explicit fields. Hence the strange error message.
The val keyword is used to declare a field ; it must be used inside a type definition (class or structure). Since in your code the variable nums is already defined and as the list type inferred by F# type inference engine, there is no need for your val line.
An example of val keyword usage is (from msdn) :
type MyType() =
let mutable myInt1 = 10
[<DefaultValue>] val mutable myInt2 : int
[<DefaultValue>] val mutable myString : string
member this.SetValsAndPrint( i: int, str: string) =
myInt1 <- i
this.myInt2 <- i + 1
this.myString <- str
printfn "%d %d %s" myInt1 (this.myInt2) (this.myString)
say I've got a tuple:
let t = (1,'a')
and I want to create a triple/3-tuple:
let createTriple (a,b) c =
(a, b, c)
is there a syntax in F# where I can create an n-tuple from an n-1-tuple and a singular value?
let createNTuple (a..z) q =
(a..z, q)
You're asking about stepping outside the type system, so sure you can do it through reflection, but not in a type-safe way.
let createNTuple tuple x =
let fields = [|yield! FSharpValue.GetTupleFields(tuple); yield box x|]
let fieldsTy = [|yield! FSharpType.GetTupleElements(tuple.GetType()); yield x.GetType()|]
let ty = FSharpType.MakeTupleType(fieldsTy)
FSharpValue.MakeTuple(fields, ty) |> unbox
let (a: int, b: char, c: string) = createNTuple (1, 'a') "foo"
What I would suggest is defining a set of functions (perhaps as static members on a class, so that you can make use of overloading), that would handle appending to tuples in a type-safe way. Realistically, you should only need a 5- or 7-tuple at most, anything more feels like pushing it too far (though you can still try to handle those cases through reflection if you care).
Something like this:
type TupleAppender =
static member Append((a, b), x) = (a, b, x)
static member Append((a, b, c), x) = (a, b, c, x)
(* and so on *)
This is boilerplate, but at least you're within the type system.
No, what you're asking is impossible. What type signature would such a function have? As opposed to lists, tuples can have differently typed elements, but they are of fixed length - precisely because you need to know the type for each element. You can't specify a type for any n-element tuple. If you need variable-length structures, use a list.
I'm reading Expert F# book and I found this code
open System.Collections.Generic
let divideIntoEquivalenceClasses keyf seq =
// The dictionary to hold the equivalence classes
let dict = new Dictionary<'key,ResizeArray<'T>>()
// Build the groupings
seq |> Seq.iter (fun v ->
let key = keyf v
let ok,prev = dict.TryGetValue(key)
if ok then prev.Add(v)
else let prev = new ResizeArray<'T>()
dict.[key] <- prev
prev.Add(v))
dict |> Seq.map (fun group -> group.Key, Seq.readonly group.Value)
and the example use:
> divideIntoEquivalenceClasses (fun n -> n % 3) [ 0 .. 10 ];;
val it : seq<int * seq<int>>
= seq [(0, seq [0; 3; 6; 9]); (1, seq [1; 4; 7; 10]); (2, seq [2; 5; 8])]
first for me this code is really ugly, even if this is safe, It looks more similar to imperative languages than to functional lang..specially compared to clojure. But the problem is not this...I'm having problems with the Dictionary definition
when I type this:
let dict = new Dictionary<'key,ResizeArray<'T>>();;
I get this:
pruebafs2a.fs(32,5): error FS0030: Value restriction. The value 'dict' has been inferred to have generic type
val dict : Dictionary<'_key,ResizeArray<'_T>> when '_key : equality
Either define 'dict' as a simple data term, make it a function with explicit arguments or, if you do not intend for it to be generic, add a type annotation.
is It ok?...
thanks so much
improve question:
Ok I've been reading about value restriction and I found this helpfull information
In particular, only function definitions and simple immutable data
expressions are automatically generalized
...ok..this explains why
let dict = new Dictionary<'key,ResizeArray<'T>>();;
doesn't work...and show 4 different techniques, although in my opinion they only resolve the error but aren't solutions for use generic code:
Technique 1: Constrain Values to Be Nongeneric
let empties : int list [] = Array.create 100 []
Technique 3: Add Dummy Arguments to Generic Functions When Necessary
let empties () = Array.create 100 []
let intEmpties : int list [] = empties()
Technique 4: Add Explicit Type Arguments When Necessary (similar to tec 3)
let emptyLists = Seq.init 100 (fun _ -> [])
> emptyLists<int>;;
val it : seq<int list> = seq [[]; []; []; []; ...]
----- and the only one than let me use real generic code ------
Technique 2: Ensure Generic Functions Have Explicit Arguments
let mapFirst = List.map fst //doesn't work
let mapFirst inp = List.map fst inp
Ok, in 3 of 4 techniques I need resolve the generic code before can work with this...now...returning to book example...when the compile knows the value for 'key and 'T
let dict = new Dictionary<'key,ResizeArray<'T>>()
in the scope the code is very generic for let key be any type, the same happen with 'T
and the biggest dummy question is :
when I enclose the code in a function (technique 3):
let empties = Array.create 100 [] //doesn't work
let empties () = Array.create 100 []
val empties : unit -> 'a list []
I need define the type before begin use it
let intEmpties : int list [] = empties()
for me (admittedly I'm a little dummy with static type languages) this is not real generic because it can't infer the type when I use it, I need define the type and then pass values (not define its type based in the passed values) exist other way define type without be so explicit..
thanks so much..really appreciate any help
This line
let dict = new Dictionary<'key,ResizeArray<'T>>();;
fails because when you type the ;; the compiler doesn't know what 'key and 'T are. As the error message states you need to add a type annotation, or allow the compiler to infer the type by using it later or make it a function
Examples
Type annotation change
let dict = new Dictionary<int,ResizeArray<int>>();;
Using types later
let dict = new Dictionary<'key,ResizeArray<'T>>()
dict.[1] <- 2
using a function
let dict() = new Dictionary<'key,ResizeArray<'T>>();;
This actually doesn't cause an issue when it's defined all together. That is, select the entire block that you posted and send it to FSI in one go. I get this:
val divideIntoEquivalenceClasses :
('T -> 'key) -> seq<'T> -> seq<'key * seq<'T>> when 'key : equality
However, if you type these individually into FSI then as John Palmer says there is not enough information in that isolated line for the interpreter to determine the type constraints. John's suggestions will work, but the original code is doing it correctly - defining the variable and using it in the same scope so that the types can be inferred.
for me this code is really ugly, even if this is safe, It looks more similar to imperative languages than to functional lang.
I agree completely – it's slightly tangential to your direct question, but I think a more idiomatic (functional) approach would be:
let divideIntoEquivalenceClasses keyf seq =
(System.Collections.Generic.Dictionary(), seq)
||> Seq.fold (fun dict v ->
let key = keyf v
match dict.TryGetValue key with
| false, _ -> dict.Add (key, ResizeArray(Seq.singleton v))
| _, prev -> prev.Add v
dict)
|> Seq.map (function KeyValue (k, v) -> k, Seq.readonly v)
This allows sufficient type inference to obviate the need for your question in the first place.
The workarounds proposed by the other answers are all good. Just to clarify based on your latest updates, let's consider two blocks of code:
let empties = Array.create 100 []
as opposed to:
let empties = Array.create 100 []
empties.[0] <- [1]
In the second case, the compiler can infer that empties : int list [], because we are inserting an int list into the array in the second line, which constrains the element type.
It sounds like you'd like the compiler to infer a generic value empties : 'a list [] in the first case, but this would be unsound. Consider what would happen if the compiler did that and we then entered the following two lines in another batch:
empties.[0] <- [1] // treat 'a list [] as int list []
List.iter (printfn "%s") empties.[0] // treat 'a list [] as string list []
Each of these lines unifies the generic type parameter 'a with a different concrete type (int and string). Either of these unifications is fine in isolation, but they are incompatible with each other and would result in treating the int value 1 inserted by the first line as a string when the second line is executed, which is clearly a violation of type safety.
Contrast this with an empty list, which really is generic:
let empty = []
Then in this case, the compiler does infer empty : 'a list, because it's safe to treat empty as a list of different types in different locations in your code without ever impacting type safety:
let l1 : int list = empty
let l2 : string list = empty
let l3 = 'a' :: empty
In the case where you make empties the return value of a generic function:
let empties() = Array.create 100 []
it is again safe to infer a generic type, since if we try our problematic scenario from before:
empties().[0] <- [1]
List.iter (printfn "%s") (empties().[0])
we are creating a new array on each line, so the types can be different without breaking the type system.
Hopefully this helps explain the reasons behind the limitation a bit more.
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.