(* 目的：ふたつの文字列をつなげる *) 
(* append_string : string -> string -> string *) 
let append_string first rest_result = first ^ rest_result 
 
(* 目的：リスト中の文字列をつなげた文字列を返す *) 
(* concat : string list -> string *) 
let concat lst = List.fold_right append_string lst "" 
 
(* テスト *) 
let test1 = concat [] = "" 
let test2 = concat ["春"] = "春" 
let test3 = concat ["春"; "夏"; "秋"; "冬"] = "春夏秋冬"