Different ways to implement a filter then map

map-filter.fsx

let isEven num = num % 2 = 0
let square num = num * num
let printResults = List.iter <| printfn "%d"

// Filter then map
[1..5]
  |> List.filter isEven
  |> List.map square
  |> printResults

// Filter then map using `fold`
let foldFn predicate mapFn state = function
  | i when predicate i -> state @ [mapFn i]
  | _ -> state

[1..5]
  |> List.fold (foldFn isEven square) []
  |> printResults

// Filter then map using function composition
let filterMap predicate mapFn = 
  List.filter predicate >> List.map mapFn

[1..5]
  |> filterMap isEven square
  |> printResults

// Implement map then filter using fold, just for fun :)
let mapFilter mappingFn predicate state current = 
  let mappedResult = mappingFn current
  
  match predicate mappedResult with
    | true -> state @ [mappedResult]
    | false -> state

[1..5]
  |> List.fold (mapFilter square isEven) []
  |> printResults

// These outputs should match those above
[1..5]
  |> List.map square
  |> List.filter isEven
  |> printResults