Back to Questions
781
views1
comment
Newbie error: Expecting a Stream<'a> but given a Stream<'a2>
f#compiler

Hi,

I'm completely new to F# and I've been struggling a bit with type constraints. I'm defining a Stream type that contains tuples like

1
2

(Company: "IBM", Price: 200€)
(Company: "AAPL", Price: 100€)

The Stream class contains a groupby method that should partition all its tuples according to some criteria. For instance, I can partition by Company and obtain something like:

1
2
3
4

{
    "IBM" => [(Company: "IBM", Price: 200€), ... other IBM tuples],
     "APPL" => [(Company: "AAPL", Price: 100€), ... other AAPL tuples]
}

where { key => value } is my notation for a Map.

Anyway, here is the full code:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38

#light

open System

type ITuple =
    abstract Timestamp : DateTime

type Stocks(timestamp, company : string, price : int) =
    member e.Company = company
    member e.Price = price
    override e.ToString() = sprintf "%A" (company, price)
    interface ITuple with
        member e.Timestamp = timestamp
 
type Stream<'a>(parent : Stream<'a> option) =
    let triggerUpdate, update = Event.create()
    let mutable tuples = []
    member self.Update : IEvent<'a> = update
    member self.add(item : 'a) : unit = tuples <- item :: tuples
                                        triggerUpdate item
    member self.groupby(part : ('a -> 'g)) =                     =====> COMPILER FAILS HERE
        let result = GroupMap<'group, 'a>(part)
        self.Update.Add(fun t -> result.add(t))
        result
    override self.ToString() = sprintf "%A" tuples
   
and GroupMap<'group, 'a>(partitioner : ('a -> 'group)) =
    let mutable map : Map<'group, 'a Stream> = Map.empty
    member self.add(tuple) =
        let g = partitioner(tuple)
        map.[g].add(tuple)

let x = Stream<Stocks>(parent = None)
let groupped : GroupMap<string, Stocks> = x.groupby((fun t -> t.Company))
for i in seq { 1 .. 5 } do
    x.add(Stocks(DateTime.Now, "AAPL", i))
    x.add(Stocks(DateTime.Now, "IBM", i))

The compiler fails with "Expecting a Stream<'a> but given a Stream<'a2>. The type 'a does not match the type 'a2". Is there any way to relate the type 'a in Stream with the type 'a in GroupMap?

Thanks!

.

Hi,

Make the types more specific and after all compiles start removing the types constraints and let type inference kick in.

Is that helpful ? :)

The code below builds and runs:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48

#light

open System

type ITuple =
    abstract Timestamp : DateTime

type Stocks(timestamp, company, price) =
    member e.Company = company
    member e.Price = price

    override e.ToString() = sprintf "%A" (company, price)

    interface ITuple with
        member e.Timestamp = timestamp

type Stream(parent : Stream option) =
    let triggerUpdate, update = Event.create()
    let mutable tuples = []

    member self.Update : IEvent<Stocks> = update
    member self.add(item : Stocks) : unit = tuples <- item :: tuples

                                            triggerUpdate item
    member self.groupby(part) =                               
        let result = GroupMap<'group>(part)
        self.Update.Add(fun t -> result.add(t))
        result

    override self.ToString() = sprintf "%A" tuples

and GroupMap<'group>(partitioner) =
    let mutable map : Map<'group, Stream> = Map.empty

    member self.add(tuple) =
        let g = partitioner(tuple)
        if (not( map.ContainsKey(g) )) then 
            map <- map.Add (g, Stream(parent = None))
            map.[g].add(tuple)
        else
            map.[g].add(tuple)

let x = Stream(parent = None)
let groupped : GroupMap<string> = x.groupby((fun t -> t.Company))

for i in seq { 1 .. 5 } do
    x.add(Stocks(DateTime.Now, "AAPL", i))
    x.add(Stocks(DateTime.Now, "IBM", i))
By on 10/27/2008 3:21 PM ()

    Topic tags

    IntelliFactory Offices Copyright (c) 2011-2012 IntelliFactory. All rights reserved.
    Home | Products | Consulting | Trainings | Blogs | Jobs | Contact Us | Terms of Use | Privacy Policy | Cookie Policy     		Built with WebSharper