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(*
* Constant time concatenable list.
*
* -- Allen
*)
signature CATNETABLE_LIST =
sig
type 'a catlist
val empty : 'a catlist
val null : 'a catlist -> bool
val length : 'a catlist -> int
val cons : 'a * 'a catlist -> 'a catlist
val unit : 'a -> 'a catlist
val append : 'a catlist * 'a catlist -> 'a catlist
val hd : 'a catlist -> 'a
val tl : 'a catlist -> 'a catlist
val fromList : 'a list -> 'a catlist
val toList : 'a catlist -> 'a list
val map : ('a -> 'b) -> 'a catlist -> 'b catlist
val app : ('a -> unit) -> 'a catlist -> unit
end
structure CatnetableList :> CATNETABLE_LIST =
struct
datatype 'a catlist = empty | unit of 'a | @ of 'a catlist * 'a catlist
fun null empty = true
| null _ = false
fun length empty = 0
| length (unit _) = 1
| length (a @ b) = length a + length b
fun hd empty = raise Empty
| hd (unit a) = a
| hd (a @ b) = hd a
fun tl empty = raise Empty
| tl (unit a) = empty
| tl ((unit _) @ a) = a
| tl ((a @ b) @ c) = tl(a @ (b @ c))
| tl (empty @ c) = tl c
fun cons(a,empty) = unit a
| cons(a,b) = unit a @ b
fun append(empty,a) = a
| append(a,empty) = a
| append(a,b) = a @ b
fun map f l =
let fun g empty = empty
| g (unit a) = unit(f a)
| g (a @ b) = (g a) @ (g b)
in g l end
fun app f l =
let fun g empty = ()
| g (unit a) = f a
| g (a @ b) = (g a; g b)
in g l end
fun fromList [] = empty
| fromList (a::b) = cons(a,fromList b)
fun toList l =
let fun g(empty,l) = l
| g(unit a,l) = a::l
| g(a @ b, l) = g(a,g(b,l))
in g(l,[]) end
end
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