(* 目的：受け取った ekimei0 までの距離を lst から探して返す *) 
(* assoc : string -> (string * float) list -> float *) 
let rec assoc ekimei0 lst = match lst with 
    [] -> infinity 
  | (ekimei, kyori) :: rest -> 
      if ekimei = ekimei0 then kyori else assoc ekimei0 rest 
 
(* テスト *) 
let test1 = assoc "後楽園" [] = infinity 
let test2 = assoc "後楽園" [("新大塚", 1.2); ("後楽園", 1.8)] = 1.8 
let test3 = assoc "池袋" [("新大塚", 1.2); ("後楽園", 1.8)] = infinity