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(* Title: Pure/seq.ML
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ID: $Id$
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Author: Lawrence C Paulson, Cambridge University Computer Laboratory
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Unbounded sequences implemented by closures. RECOMPUTES if sequence
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is re-inspected. Memoing, using polymorphic refs, was found to be
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slower! (More GCs)
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*)
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signature SEQ =
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sig
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type 'a seq
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val make: (unit -> ('a * 'a seq) option) -> 'a seq
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val pull: 'a seq -> ('a * 'a seq) option
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val empty: 'a seq
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val cons: 'a * 'a seq -> 'a seq
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val single: 'a -> 'a seq
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val hd: 'a seq -> 'a
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val tl: 'a seq -> 'a seq
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val chop: int * 'a seq -> 'a list * 'a seq
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val list_of: 'a seq -> 'a list
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val of_list: 'a list -> 'a seq
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val map: ('a -> 'b) -> 'a seq -> 'b seq
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val mapp: ('a -> 'b) -> 'a seq -> 'b seq -> 'b seq
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val append: 'a seq * 'a seq -> 'a seq
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val filter: ('a -> bool) -> 'a seq -> 'a seq
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val flat: 'a seq seq -> 'a seq
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val interleave: 'a seq * 'a seq -> 'a seq
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val print: (int -> 'a -> unit) -> int -> 'a seq -> unit
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val it_right : ('a * 'b seq -> 'b seq) -> 'a seq * 'b seq -> 'b seq
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end;
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structure Seq: SEQ =
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struct
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datatype 'a seq = Seq of unit -> ('a * 'a seq) option;
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(*the abstraction for making a sequence*)
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val make = Seq;
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(*return next sequence element as None or Some (x, xq)*)
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fun pull (Seq f) = f ();
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(*the empty sequence*)
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val empty = Seq (fn () => None);
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(*prefix an element to the sequence -- use cons (x, xq) only if
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evaluation of xq need not be delayed, otherwise use
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make (fn () => Some (x, xq))*)
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fun cons x_xq = make (fn () => Some x_xq);
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fun single x = cons (x, empty);
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(*head and tail -- beware of calling the sequence function twice!!*)
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fun hd xq = #1 (the (pull xq))
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and tl xq = #2 (the (pull xq));
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(*the list of the first n elements, paired with rest of sequence;
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if length of list is less than n, then sequence had less than n elements*)
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fun chop (n, xq) =
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if n <= 0 then ([], xq)
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else
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(case pull xq of
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None => ([], xq)
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| Some (x, xq') => apfst (Library.cons x) (chop (n - 1, xq')));
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(*conversion from sequence to list*)
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fun list_of xq =
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(case pull xq of
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None => []
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| Some (x, xq') => x :: list_of xq');
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(*conversion from list to sequence*)
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fun of_list xs = foldr cons (xs, empty);
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(*map the function f over the sequence, making a new sequence*)
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fun map f xq =
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make (fn () =>
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(case pull xq of
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None => None
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| Some (x, xq') => Some (f x, map f xq')));
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(*map over a sequence xq, append the sequence yq*)
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fun mapp f xq yq =
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let
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fun copy s =
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make (fn () =>
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(case pull s of
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None => pull yq
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| Some (x, s') => Some (f x, copy s')))
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in copy xq end;
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(*sequence append: put the elements of xq in front of those of yq*)
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fun append (xq, yq) =
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let
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fun copy s =
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make (fn () =>
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(case pull s of
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None => pull yq
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| Some (x, s') => Some (x, copy s')))
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in copy xq end;
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(*filter sequence by predicate*)
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fun filter pred xq =
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let
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fun copy s =
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make (fn () =>
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(case pull s of
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None => None
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| Some (x, s') => if pred x then Some (x, copy s') else pull (copy s')));
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in copy xq end;
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(*flatten a sequence of sequences to a single sequence*)
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fun flat xqq =
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make (fn () =>
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(case pull xqq of
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None => None
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| Some (xq, xqq') => pull (append (xq, flat xqq'))));
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(*interleave elements of xq with those of yq -- fairer than append*)
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fun interleave (xq, yq) =
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make (fn () =>
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(case pull xq of
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None => pull yq
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| Some (x, xq') => Some (x, interleave (yq, xq'))));
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(*functional to print a sequence, up to "count" elements;
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the function prelem should print the element number and also the element*)
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fun print prelem count seq =
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let
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fun pr (k, xq) =
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if k > count then ()
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else
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(case pull xq of
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None => ()
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| Some (x, xq') => (prelem k x; writeln ""; pr (k + 1, xq')))
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in pr (1, seq) end;
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(*accumulating a function over a sequence; this is lazy*)
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fun it_right f (xq, yq) =
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let
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fun its s =
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make (fn () =>
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(case pull s of
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None => pull yq
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| Some (a, s') => pull (f (a, its s'))))
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in its xq end;
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end;
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