For John

gistfile1.txt

type Building = 
  { left : int
    right : int
    height : int }

let buildings = []
let getHeight building = building.height
let getWidth building = building.right - building.left
let getArea building = getHeight building * getWidth building

type Op = 
  { map : Building -> int
    reduce : int -> int -> int
    zero : int }

let maxHeight = 
  { map = getHeight
    reduce = 
      (fun l r -> 
      if l < r then l
      else r)
    zero = System.Int32.MinValue }

let minWidth = 
  { map = getWidth
    reduce = 
      (fun l r -> 
      if l > r then l
      else r)
    zero = System.Int32.MaxValue }

let totalArea = 
  { map = getArea
    reduce = (+)
    zero = 0 }

let fs = [ maxHeight; minWidth; totalArea ]

// Use the foldBack pattern:
//     foldBack (extractor >> combiner) elements state

let limits = 
  [ for f in fs -> List.foldBack (f.map >> f.reduce) buildings f.zero ]