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List<string> and string list (and adding new list item)
f#

Hi,
I'm having an issue figuring out if these two are compatible (List<string> and string list) and how and which one to use

the following code fails
let l1 = List.empty<string>
let mutable l2 = new List<string>()
l2 <- l1
[Error: This expression has type List<string> but is here used with type string list.]

The reason for this is that when I use List<string>(), I can write things like
let l = new List<string>()
l.Add("something")
l.[0] <- "something else"

but then pattern matching below fails
match l with
| [] -> "list is empty"
| h::t -> "list has something"

If I use the List.empty<string>, I can add using the :: operator but it will put it in front though the pattern matching works

Questions:
1. Are the two types compatible and how do I convert to one another if necessary
2. How do I add to the end of "string list"? I wanted to use the @ operator by putting the new element into a list and concatenating the two list but I read from a post here that the @ operator had performance issues
3. what is wrong with my pattern matching for List<string>()? Or I should have used "match l.Count with ..."?

thank you.

.

The problem is that your getting mixed up between System.Generics.List<A> (the list in BCL) and Microsoft.FSharp.Collections.List<A>, F#'s native list. In F# its common to refer to System.Generics.List<A> as ResizeArray to avoid this sort of confusion. You can only pattern match over the F# native list. These two types are not comptable, both implement the IEnumerable interface so its easy to transform between the two types. Use "List.of_seq" to create a F# native list from a ResizeArray and use "new ResizeArray(list)" to create a ResizeArray from a list. Cheers, Robert

By on 11/28/2008 2:56 AM ()

Hi.
Thanks for the reply. I've changed my code to use ResizeArray() instead. When using a list, you can say

let l = List.empty<string>
let h,t = (List.hd l), (List.tl l)
t |> List.fold_left (fun ...) h

The ResizeArray also has the fold_left function.
1. Is there a way to get the head and tail?
2. Currently, I'm using
let r = new string ResizeArray()
... add items ...
let l = List.of_seq r
... same as above ..
Is this the way to go?

Thank you.

By on 11/28/2008 6:11 AM ()

A ResizeArray is an array like structure, so while you can get it's head using Seq.hd, it doesn't really make sense to get it tail.

What your doing makes some sense, but it maybe simpler just stick with native F# list and define the contents as a literal:

let l = [ "toto"; "titi"; "tata" ]

Or use a Seq.map or a list comprehension if your reading the items from a file/database.

Cheers,

Rob

By on 11/28/2008 7:50 AM ()

Hi,
I really want to stick to the f# list as I'm new to f# and I want to use its types as often as possible. The issue is I have a class [Table] that contains columns [Column]. I used the column list at first but then in order to add new columns, I added the method addColumn. The definition is like

type Column(name) =
let mutable name = name
...
member x.Name
with get() = name
and set y = name <- y
....

type Table(name) =
let name = name
let mutable columns = List.empty<Column>
...
member x.Name ... same as above...
member x.addColumn col =
//this is where I have the issue

//option 1: This appends the new item to the front. Not what is required
columns <- col::columns

//option 2: This works fine except I read here that the @ operator is pretty expensive
columns <- columns@[col]

//option 3: change to a more familiar environment - .net list
[if .net list] columns.Add(col)

... later on ...
pattern matching on the list.

I'm currently learning f# so I really do not know most of the methods available for use. I'm currently familiar with map, fold_left, generate_using, iter and choose

Thank you.

By on 11/29/2008 4:09 PM ()

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