(4,a)(1,b)(2,c)(2,a)(1,d)(4,e) --> ((4 a) b (2 c) (2 a) d (4 e))

let rec transform l =
 match l with
 | (x,y)::t -> if x = 1 then y::transform(t) else [x; y]::transform(t)
 | [] -> []

Error: This expression has type int list
   but an expression was expected of type int

if x = 1
then y :: transform (t)
else [x; y] :: transform t

type 'a element =
  | Single of 'a
  | Count of int * 'a

let rec transform = function
  | [] -> []
  | (x,y)::t ->
      if x = 1 then Single y::transform t
      else Count (x, y)::transform t

let compact (x, y) =
  if x = 1 then Single y else Count (x, y)

let transform list = List.map compact list