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|
signature SEQUENCE = sig
type 'a t
val fold: ('a * 'b -> 'b) -> 'b -> 'a t -> 'b
val from: (unit -> 'a option) -> 'a t
val map: ('a -> 'b) -> 'a t -> 'b t
val app: ('a -> unit) -> 'a t -> unit
val fromArray: 'a array -> 'a t
val fromList: 'a list -> 'a t
val toList: 'a t -> 'a list
val toArray: 'a t -> 'a array
end
structure Sequence :> SEQUENCE = struct
type 'a t = unit -> 'a option
fun fold f seed t =
let
fun loop x =
case t () of
NONE => x
| SOME y => loop (f (y, x))
in
loop seed
end
fun from t = t
fun map f t =
fn () => (
case t () of
NONE => NONE
| SOME x => SOME (f x)
)
fun app f t =
let
fun loop () =
case t () of
NONE => ()
| SOME x => (
f x
; loop ()
)
in
loop ()
end
fun fromArray a =
let
val i = ref 0
fun f () =
if !i >= Array.length a then NONE
else
SOME (Array.sub (a, !i))
before i := !i + 1
in
from f
end
fun fromList l =
let
val c = ref l
fun f () =
case !c of
x :: y => (c := y; SOME x)
| nil => NONE
in
from f
end
fun toList t =
let
fun loop l =
case t () of
NONE => rev l
| SOME x => loop (x :: l)
in
loop nil
end
fun toArray t = Array.fromList (toList t)
end
|
