CODEWITHSUNDEEP
listcastingf#
citykid
citykid

Reputation: 11060

Is it possible to implement a List.cast function in F# similar to System.Linq.Enumerable.Cast?

To convert a list of ints into a list of IComparables I write

[ 1; 2; 3 ] |> List.map (fun x -> x :> IComparable)

System.Linq offers the Cast method to cast a collection. Trying to implement a similar List.cast<'newtype> function in F# appears more tricky than I thought.

let listcast<'i> ls = ls |> List.map (fun x -> x :> 'i) // error indeterminate type

let listcast<'t, 'i when 't :> 'i> (ls: 't list) = // error: could not be generalized
    ls
    |> List.map (fun x -> x :> 'i)

let listcast<'i when 'i :> Object> (ls: Object list) = // error indeterminate type
    ls
    |> List.map (fun x -> x :> 'i)

Solution thx to the hint of pblasucci:

module List =
    let cast<'t> = Seq.cast<'t> >> Seq.toList

[1..4] |> List.cast<IComparable>

Upvotes: 2

Views: 78

Answers (1)

pblasucci
pblasucci

Reputation: 1778

Why not just use Seq.cast<_> (either directly or as the basis for a List.cast<_> function)?

Upvotes: 3

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