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how to filter and apply some operation from list of union
f#

I have a union and list of that union like below.

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type ABC=|A|B|C

let abcList=[A;A;B;C;...]

First question is ,

I wanna filter just item of A and apply some map operation to filtered items.

I don't know another way except applying pattern matching twice for filter and map like below,but it's not succinct and elegant.

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abc list
|>List.filter (fun item->match item with |A->true|_->false)
|>List.map (fun item->match item with |A->someFunc item|_->failwith "invalid")

Is there another good way to do so?

Second qustion is ,

I wanna extract fist item of A and apply some operation from that list too.

I face again same problem to use pattern matching twice.

I suppose same solution will be applied for both questions.

Regards,

.

You can use List.choose instead of the List.filter + List.map combination:

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abcList

|> List.choose (function | A -> Some(someFunc A) | _ -> None)
By on 4/4/2011 7:01 AM ()

bravo!!

By on 4/4/2011 4:59 PM ()

hai hapeens,
The filter adn apply some operation from list of union.

val length : 

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'a list -> int

Return the length (number of elements) of the given list.

val hd : 

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'a list -> 'a

Return the first element of the given list. Raise

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Failure "hd"

if the list is empty.

val tl : 

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'a list -> 'a list

Return the given list without its first element. Raise

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Failure "tl"

if the list is empty.

val nth : 

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'a list -> int -> 'a

Return the n-th element of the given list.
The first element (head of the list) is at position 0.
Raise 

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Failure "nth"

if the list is too short.
Raise 

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Invalid_argument "List.nth"

if n is negative.

val rev : 

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'a list -> 'a list

List reversal.

<i> <i> I have indicated <i>some<i> incompatibilities in the following text.

By on 9/29/2011 5:35 AM ()

Hi,

we can remove pattern matching code from List.map in this case.

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abc list
|>List.filter (fun item->match item with |A->true|_->false)
|>List.map someFunc
List.filter (fun item->match item with |A->true|_->false)

will return only the items of A. So we don't need to do it again.

Regards,
Nyi Nyi

By on 4/2/2011 9:29 PM ()

Hi,

Thxs for your reply.

Compiler issues warning for the map function of the source like below

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type ABC= |A of int|B|C

let abcList=[A(1);B;A(2);C]

let filtered=abcList|>List.filter (fun items->match items with |A(x)->true|_->false)

filtered|>List.map(fun (A(x))->Console.WriteLine("A:{0}",x))

but it's still valid.right?

By on 4/3/2011 12:08 AM ()

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