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Merge two sorted lists by a key

Merges two sorted lists by a key.

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// Example: mergeBy (fun (key,_)->key) [("a", 1); ("c", 2)] [("c", 2); ("d", 3)];;
// val it : ((string * int) option * (string * int) option) list =
// [(Some ("a", 1), None); (Some ("c", 2), Some ("c", 2)); (None, Some ("d", 3))]

let mergeBy keyselector ls rs = 
    let rec aux ls rs acc =
        match ls, rs with
        | [], [] -> acc
        | l::ls', [] -> aux ls' [] ((Some l, None)::acc)
        | [], r::rs' -> aux [] rs' ((None, Some r)::acc)
        | l::ls', r::rs' -> 
            match compare (keyselector l) (keyselector r) with
            | n when n < 0 -> aux ls' rs ((Some l, None)::acc)
            | 0 -> aux ls' rs' ((Some l, Some r)::acc)
            | _ -> aux ls rs' ((None, Some r)::acc)
            
    aux ls rs [] |> List.rev
val mergeBy : keyselector:('a -> 'b) -> ls:'a list -> rs:'a list -> ('a option * 'a option) list (requires comparison)

Full name: Script.mergeBy
val keyselector : ('a -> 'b) (requires comparison)
val ls : 'a list
val rs : 'a list
val aux : ('a list -> 'a list -> ('a option * 'a option) list -> ('a option * 'a option) list)
val acc : ('a option * 'a option) list
val l : 'a
val ls' : 'a list
union case Option.Some: Value: 'T -> Option<'T>
union case Option.None: Option<'T>
val r : 'a
val rs' : 'a list
val compare : e1:'T -> e2:'T -> int (requires comparison)

Full name: Microsoft.FSharp.Core.Operators.compare
val n : int
Multiple items
module List

from Microsoft.FSharp.Collections

--------------------
type List<'T> =
  | ( [] )
  | ( :: ) of Head: 'T * Tail: 'T list
  interface IEnumerable
  interface IEnumerable<'T>
  member Head : 'T
  member IsEmpty : bool
  member Item : index:int -> 'T with get
  member Length : int
  member Tail : 'T list
  static member Cons : head:'T * tail:'T list -> 'T list
  static member Empty : 'T list

Full name: Microsoft.FSharp.Collections.List<_>
val rev : list:'T list -> 'T list

Full name: Microsoft.FSharp.Collections.List.rev
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More information

Link:http://fssnip.net/ck
Posted:5 years ago
Author:Simon Weijgers
Tags: lists