fsharp-learning/exercism/fsharp/beer-song/BeerSong.fs

module BeerSong

let stateLine n =
    if n > 1
    then sprintf "%d bottles of beer on the wall, %d bottles of beer." n n
    else if n = 1
    then "1 bottle of beer on the wall, 1 bottle of beer."
    else "No more bottles of beer on the wall, no more bottles of beer."

let takeDownLine n =
    if n > 2
    then sprintf "Take one down and pass it around, %d bottles of beer on the wall." (n - 1)
    else if n = 2
    then "Take one down and pass it around, 1 bottle of beer on the wall."
    else if n = 1
    then "Take it down and pass it around, no more bottles of beer on the wall."
    else "Go to the store and buy some more, 99 bottles of beer on the wall."

let verse (n: int) (left: int) =
    if left > 1 then [ stateLine n; takeDownLine n; "" ] else [ stateLine n; takeDownLine n ]

let rec beerLines (n: int) (left: int): list<string> =
    let newBottles = if n = 0 then 99 else n - 1 // wrap around when get to 0
    if left > 0 && n >= 0 then
        List.concat
            [ (verse n left)
              (beerLines newBottles (left - 1)) ]
    else
        []

let recite (startBottles: int) (takeDown: int) = beerLines startBottles takeDown