why in the world does the constructor for a dictionary in F# allow duplicate keys and just overwrites silently?
let ``you just got dict`` = dict [ "hello","goodbye";"hello","world"]
This is very counter-intuitive behavior.
You could shadow the built-in dict function with a version that behaves as you want. You could return the more fitting IReadOnlyDictionary interface while you're at it.
let dict source =
let d = Dictionary<'K, 'V>(HashIdentity.Structural)
source |> Seq.iter d.Add
{
new IReadOnlyDictionary<'K, 'V> with
member x.ContainsKey(key) = d.ContainsKey(key)
member x.TryGetValue(key, value) = d.TryGetValue(key, &value)
member x.Item with get key = d.[key]
member x.Keys = d.Keys :> _
member x.Values = d.Values :> _
interface IReadOnlyCollection<KeyValuePair<'K, 'V>> with
member x.Count = d.Count
interface IEnumerable<KeyValuePair<'K, 'V>> with
member x.GetEnumerator() = d.GetEnumerator() :> _
interface System.Collections.IEnumerable with
member x.GetEnumerator() = d.GetEnumerator() :> _
}
I can't explain the reason for this design - just as I can't explain why Dictionary<TKey, TValue> doesn't take a sequence of KeyValuePairs as input.
However, if you look at the implementation of dict, you'll see that it internally adds each element using the indexer, like this:
foreach (Tuple<TKey, TValue> tuple in keyValuePairs)
{
TValue local = tuple.Item2;
TKey local2 = tuple.Item1;
d[new RuntimeHelpers.StructBox<TKey>(local2)] = local;
}
where d is the Dictionary being created. The indexer silently updates the dictionary entry, so this explains the mechanics of it.
Not an entire answer, I admit, but perhaps a piece of the puzzle.
You aren't going to get a why unless some Microsoft engineer explains it to you why they chose to do it that way. Regardless, it is what it is and works just as the documentation says it should:
https://msdn.microsoft.com/en-us/library/k7z0zy8k(v=vs.110).aspx
Remarks
You can also use the Item property to add new elements by setting the value
of a key that does not exist in the Dictionary<TKey, TValue>; for example,
myCollection[myKey] = myValue (in Visual Basic, myCollection(myKey) =
myValue). However, if the specified key already exists in the
Dictionary<TKey, TValue>, setting the Item property overwrites the old
value. In contrast, the Add method throws an exception if a value with the
specified key already exists.
Related
Beginner in F# here
I want to create a type, which is a sequence of another concrete type (Event) with at least one element. Any other elements can be added anytime later. Normally in C# I would create a class with a private List<Event> and public methods.
But I want to do it with a functional approach and not imitate the C# approach. Or at least try.
My train of thought:
Let's create a type "of seq" and give it a constructor requiring instance of the Event type
type Event = Event of string
type PublishedEvents = EventList of seq<Event> with
static member create (event:Event) = EventList(Seq.singleton event)
Now let's add an "add" method for adding another optional Event instances
type PublishedEvents with
member this.add(event:Event) = Seq.append this [event]
But that doesn't work, F# complains that "this" is not compatible with seq<'a>.
So I tried this:
type PublishedEvents with
member this.add (event:Event) : PublishedEvents = EventList(Seq.append this [event])
Now it complains that "this" is not compatible with seq<Event>...which is confusing me now since few lines above it says EventList of seq<Event> ... so I guess I need to somehow convert EventList back to seq<Event> so I can then use Seq.append ?
let convertFunction (eventList:PublishedEvents) : seq<Event> = ???
But I have no idea how to do this.
Am I even going the right direction? Is it better for this to mimic a C# class with a backing field? Or am I missing something?
The actual sequence of events is wrapped inside an EventList discriminated union case.
You can unwrap it and re-wrap it like this:
type PublishedEvents with
member this.add(event:Event) =
match this with
| EventList events -> Seq.append events [event] |> EventList
However, I have to question the value of creating this PublishedEvents type in the first place, if it's just a single EventList case containing a sequence that requires you to wrap and unwrap values repeatedly.
Also, please be aware that this add method doesn't change the existing PublishedEvents. It creates a new one with a new sequence of events, because of the way that Seq.append works, because seq<'a> is actually just F#'s name for System.Collections.Generic.IEnumerable<'a>).
Furthermore, your approach does not prevent creation of a non-empty event sequence. EventList is a public constructor for PublishedEvents so you can just write:
EventList []
A simple way to make the type system enforce a non-empty sequence is this:
type NonEmptySeq<'a> = { Head : 'a; Tail : seq<'a> } with
static member Create (x:'a) = { Head = x; Tail = [] }
member this.Add x = { this with Tail = Seq.append this.Tail [x] }
let a = NonEmptySeq.Create (Event "A")
let b = a.Add (Event "B")
But again, these sequences are immutable. You could do something similar with a C# List<'a> if you need mutation. In F# it's called a ResizeArray<'a>:
type NonEmptyResizeArray<'a> = { Head : 'a; Tail : ResizeArray<'a> } with
static member Create (x:'a) = { Head = x; Tail = ResizeArray [] }
member this.Add x = this.Tail.Add x
let a = NonEmptyResizeArray.Create (Event "A")
a.Add (Event "B")
I propose that you go even more functional and not create members for your types - have it done in your functions. For example this would achieve the same and I would argue it's more idiomatic F#:
type Event = Event of string
type PublishedEvents = EventList of Event * Event list
let create e = EventList (e,[])
let add (EventList(head,tail)) e = EventList(e,head::tail)
let convert (EventList(head,tail)) = head::tail |> Seq.ofList
let myNewList = create (Event "e1")
let myUpdatedList = add myNewList (Event "e2")
let sequence = convert myUpdatedList
val sequence : seq = [Event "e2"; Event "e1"]
On the other hand if your aim is to interop with C# your approach would be easier to consume on C# side.
Can someone clarify when to use typedefof<'T> vs. typeof<'T>?
Both typedefof<System.String> and typeof<System.String> return the same Type instance.
However, they return different instances and different information for System.Collections.Generic.List<_>.
Can I think of typedefof as a new and improved typeof? Should I just switch to always using typedefof? Or is it more subtle than that?
This ought to illustrate the difference. When you use typeof, the compiler infers type arguments and constructs a concrete type. In this case, the inferred type argument is System.Object:
let t1 = typeof<System.Collections.Generic.List<_>>
let t2 = typedefof<System.Collections.Generic.List<_>>
printfn "t1 is %s" t1.FullName
printfn "t2 is %s" t2.FullName
Output:
t1 is System.Collections.Generic.List`1[[System.Object, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]
t2 is System.Collections.Generic.List`1
Because typeof can only return a constructed type, typedefof is necessary if you need a type object representing a generic type definition.
typeof is used when you want to get the System.Type object for a given type. typedefof is used when you want to get the System.Type that represents the type definition for a generic type. As an example that uses both, suppose you had a type called Generic<'a>, and you wanted to create a function that returned the System.Type object for the Generic of any given type.
type Generic<'a> = Value of 'a
let makeGenericOf<'a> () =
typedefof<Generic<_>>.MakeGenericType(typeof<'a>)
Here, you would use the typedefof function to get the type defintion, and typeof to get the type of 'a for constructing the generic Generic<'a> Type.
I really appreciate the answers from phoog, Aaron, and JLRishe. Here is what I have learned, based on their answers and my own experimentation.
There are two Type instances associated with generics.
There is a Type associated with a generic that has specific type parameters. For example, there is a Type associated with List<int> and a different Type associated with List<string>. This is what you get when you use typeof<>.
> typeof<List<string>>.ToString();;
val it : string = "Microsoft.FSharp.Collections.FSharpList`1[System.String]"
> typeof<List<int>>.ToString();;
val it : string = "Microsoft.FSharp.Collections.FSharpList`1[System.Int32]"
There is a Type associated with the generic type definition itself. For example, there is a single Type associated with List<'T>, which is the same for List<int>, List<string>, and List<_>. This is what you get when you use typedefof<>.
> typedefof<List<string>>.ToString();;
val it : string = "Microsoft.FSharp.Collections.FSharpList`1[T]"
> typedefof<List<int>>.ToString();;
val it : string = "Microsoft.FSharp.Collections.FSharpList`1[T]"
> typedefof<List<_>>.ToString();;
val it : string = "Microsoft.FSharp.Collections.FSharpList`1[T]"
By the way, the Type class has an instance method to GetGenericTypeDefinition(). That means, the following two return the same instance:
> Object.ReferenceEquals(typeof<List<int>>.GetGenericTypeDefinition(), typedefof<List<int>>);;
val it : bool = true
What happens if you call typeof<List<_>>? You get back the Type definition for List<Object>, as phoog mentioned.
> typeof<List<_>>.ToString();;
val it : string = "Microsoft.FSharp.Collections.FSharpList`1[System.Object]"
This is all helpful to understand. For example, suppose I need to know if an object is a generic list (of any type).
// does not give me the answer I naively expected
> o.GetType() = typeof<List<_>>;;
val it : bool = false
// does this reference point to a List<'T>?
> o.GetType().IsGenericType && o.GetType().GetGenericTypeDefinition() = typedefof<List<_>>;;
val it : bool = true
Additionally, if you want to late-bound instantiate a generic type, you can use the MakeGenericType(...) method which Aaron mentioned.
> let myList = typedefof<List<_>>.MakeGenericType(typeof<int>);;
val myList : Type = Microsoft.FSharp.Collections.FSharpList`1[System.Int32]
This question is in follow up to an earlier question, Preserving Field names across the F#/C# boundary
Because of the current limitation encountered with F# type providers (see the earlier question), I want to map the type-provider-generated list to my own list of records, in which the record is, in part,
type inspection = {
inspectionID : string;
inspectorID : int;
EstablishmentID : string;
EstablishmentName : string; // other members elided
}
I think the way to do this will use Seq.map, but I am not certain. (Recall I am doing a learning exercise.) So here is what I tried:
type restaurantCsv = CsvProvider<"C:\somepath\RestaurantRatings2013.csv",HasHeaders=true>
// which generates a type, but it is an "erased" type, so member names do not propogate
// over to C#.
type RawInspectionData(filename : string) =
member this.allData = restaurantCsv.Load(filename) // works fine
member this.allInspections =
this.allData.Data
|> Seq.map(fun rcrd -> new inspection[{inspectionID = rcrd.InspectionID;}])
and, of course, the complete statement would have the other member names as part of the inspection, here elided for brevity. Someone pointed me to p 43 of F# For Scientists, which is why I thought to use this format with the curly braces. But this yields a syntax error, "Unexpected symbol '{' in expression. Expected ',', ']' or other token."
Hopefully, though, this snippet is adequate to show what I would like to do, create a Generated Type from the Erased Type. How can I accomplish this?
Your code is going in the right direction. When using Seq.map (which is like Select in LINQ), you need to turn a single element of the original sequence into a single element of the new sequence. So the lambda function just needs to create a single instance of the record.
A record is constructed using { Field1 = value1; Field2 = value2; ... } so you need:
type RawInspectionData(filename : string) =
let allData = restaurantCsv.Load(filename) // works fine
member this.allInspections =
allData.Data
|> Seq.map(fun rcrd -> {inspectionID = rcrd.InspectionID})
I also changed allData from a member to a local let definition (which makes it private field of the class). I suppose that your original code new inspection[{...}] tried to create a singleton array with the element - to create an array you'd write [| { Field = value; ... } |] (and the compiler would infer the type of the array for you). But in this case, no arrays are needed.
For a project I am working on I need a global variable(technically I don't, I could build it and then pass it to every single function call, and let every single function call know about it, but that seems just as hacky, less readable and more work.)
The global variables are look up tables(endgame, opening book and transpositions/cache) for a game.
The fact that some of the code may lose some of it's indempotent behavior is actually the point(speedups) in short, yes I know global mutable state is bad, it's really worth it in this case(10x+ performance improvement)
So here's the question, "build a singleton or use a static value in a static class with combinators"
They are effectively identical but I am curious what people have done before on this sort of problem
Or alternatively, should I be passing the thing around to everyone(or at least a reference to it anyways),is that really the best answer?
Here is a solution similar to the one posted by #Yin Zhu's, but using abstract types to specify a usage interface for the mutable value, a local definition to encapsulate it and object literals to provide an implementation (this is taken from Expert F#--which is co-authored by Don Syme):
type IPeekPoke =
abstract member Peek: unit -> int
abstract member Poke: int -> unit
let makeCounter initialState =
let state = ref initialState
{ new IPeekPoke with
member x.Poke(n) = state := !state + n
member x.Peek() = !state }
You can also do it with static fields, like this:
type Common() =
static let mutable queue : CloudQueue = null
static let mutable storageAccount : CloudStorageAccount = null
static member Queue
with get() = queue
and set v = queue <- v
static member StorageAccount
with get() = storageAccount
and set v = storageAccount <- v
In another module, just:
open Common
Common.Queue <- xxxx
here is the convention used in F# PowerPack Matrix library (\src\FSharp.PowerPackmath\associations.fs):
// put global variable in a special module
module GlobalAssociations =
// global variable ht
let ht =
let ht = new System.Collections.Generic.Dictionary<Type,obj>()
let optab =
[ typeof<float>, (Some(FloatNumerics :> INumeric<float>) :> obj);
typeof<int32>, (Some(Int32Numerics :> INumeric<int32>) :> obj);
...
typeof<bignum>, (Some(BigRationalNumerics :> INumeric<bignum>) :> obj); ]
List.iter (fun (ty,ops) -> ht.Add(ty,ops)) optab;
ht
// method to update ht
let Put (ty: System.Type, d : obj) =
// lock it before changing
lock ht (fun () ->
if ht.ContainsKey(ty) then invalidArg "ty" ("the type "+ty.Name+" already has a registered numeric association");
ht.Add(ty, d))
Consider my first attempt, a simple type in F# like the following:
type Test() =
inherit BaseImplementingNotifyPropertyChangedViaOnPropertyChanged()
let mutable prop: string = null
member this.Prop
with public get() = prop
and public set value =
match value with
| _ when value = prop -> ()
| _ ->
let prop = value
this.OnPropertyChanged("Prop")
Now I test this via C# (this object is being exposed to a C# project, so apparent C# semantics are desirable):
[TestMethod]
public void TaskMaster_Test()
{
var target = new FTest();
string propName = null;
target.PropertyChanged += (s, a) => propName = a.PropertyName;
target.Prop = "newString";
Assert.AreEqual("Prop", propName);
Assert.AreEqual("newString", target.Prop);
return;
}
propName is properly assigned, my F# Setter is running, but the second assert is failing because the underlying value of prop isn't changed. This sort of makes sense to me, because if I remove mutable from the prop field, no error is generated (and one should be because I'm trying to mutate the value). I think I must be missing a fundamental concept.
What's the correct way to rebind/mutate prop in the Test class so that I can pass my unit test?
As a side-note, I would probably use if .. then instead of the match construct as it makes the code more succinct (patterh matching is especially valuable when you need to test the value agains multiple complex patterns). Also, public is the default access for member, so you can make the code a bit more succinct:
type Test() =
inherit BaseImplementingNotifyPropertyChangedViaOnPropertyChanged()
let mutable prop : string = null
member this.Prop
with get() = prop
and set(value) =
if value <> prop then
prop <- value
this.OnPropertyChanged("Prop")
Try this:
type Test() =
inherit BaseImplementingNotifyPropertyChangedViaOnPropertyChanged()
let mutable prop: string = null
member this.Prop
with public get() = prop
and public set value =
match value with
| _ when value = prop -> ()
| _ ->
prop <- value
this.OnPropertyChanged("Prop")
You need to make the binding mutable and then alter its value in your setter. In your initial code, you were just creating a new binding (also called prop) within your setter, so no change was visible.
In your pattern match you are actually binding a new value with
let prop = value
When you bind a value like this with the same name, it will shadow the other value for the scope of the newly declared one. I believe what you actually want to do is this:
prop <- value