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Recursion on List of Custom Type
f#

I have a list of items, which are of a type CustomType, a type that is defined externally. I write a recursive function:

let rec function arg (custom_type_list : List <CustomType>) =
match eq_div_proj_list with
| [] | [_] -> []
| h::t -> let remainder = function arg List.tl custom_type_list
match h.attribute1 with
| arg -> (h.attribute2)::remainder
| _ -> remainder

So this should pull out all the instances where custom_type_list.attribute1 is equal to arg. The output is the list of the corresponding attribute2's , in reverse order of course.

The error I get is on the third line

| [] | [_] -> []

"This expression has type 'a list but here is used with type List<CustomType>" .

It seems I don't know how to express an empty list of a custom type. Or is there some other trick to doing recursion on lists of custom types?

.

My guess is that you're using the System.Collections.Generic.List instead of F#'s list. Perhaps you opened System.Collections.Generic?

The BCL's List is an auto-resizing array, and F# aliases "ResizeArray" to it. If you want F# lists, then just use "type list" to specify the type.

let rec function arg (custom_type_list : CustomType list) =

Should work fine.

By on 3/16/2009 2:47 PM ()

Yes I do have System.Collections.Generic.List open, and your solution fixed it. Thanks!

By on 3/16/2009 2:55 PM ()

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