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src/Pure/General/seq.ML

author | wenzelm |

Wed, 10 Jun 1998 11:52:59 +0200 | |

changeset 5014 | 32e6cab5e7d4 |

child 5558 | 64a8495201d1 |

permissions | -rw-r--r-- |

moved seq.ML to General/seq.ML;

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