Re: [Caml-list] queasiness about Exit

["Marcin 'Qrczak' Kowalczyk" <qrczak@knm.org.pl>]

Thu, 15 Nov 2001 17:58:32 +0100, Andreas Rossberg <rossberg@ps.uni-sb.de> pisze:

> It is not too difficult to provide variations of iter and folds for
> your data structures that allow early breaks. For example, consider:
> 
> 	iter' : ('a -> bool) -> 'a array -> unit
> 
> Unlike the standard version of iter, the argument function returns a
> boolean saying whether iteration should continue. Similarly for folds:
> 
> 	fold_left' : ('a -> 'b -> 'a * bool) -> 'a -> 'b array -> 'a

An alternative is

fold_right' : ('a -> (unit -> 'b) -> 'b) -> 'a array -> (unit -> 'b) -> 'b

which can also replace iter (with 'b = unit).

When fold in either direction could be used, fold_left is usually
beter if the folding function is strict, and fold_right if it's lazy.

> let exists p = fold_left' (fun b x -> let b' = b || p x in (b', not b')) true

let exists p xs = fold_right' (fun x bf -> p x || bf ()) xs (fun () -> false)

Wrapping "init" (shouldn't it be called "last" here?) in a closure is
unnecessary for this case but it's important e.g. for concatenation
of lazy lists.

-- 
 __("<  Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/
 \__/
  ^^
QRCZAK

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