Re: [Ocaml-lib-devel] unknown length, user defined enumerations?

[Remi V. <van...@la...>]

Brian Hurt <bri...@ql...> writes:

> Something just occured to me, and I don't know if this is a good idea or 
> not.  But we were talking about reading a file and tail recursion over in 
> Ocaml beginners just now.  And I whipped up a quick example of reading a 
> file into a list of strings, using the standard List.rev trick.
>
> Now, that's "correct" for Ocaml beginners, but not what you really want to 
> do.  What you really want to do is use setcdr to construct the list 
> forwards, instead of backwards.  But setcdr is dangerous, and not 
> something I want to blithely recommend beginners play around with.
>
> What I thought would be neat would be to be able to allow the user to 
> define their own enumeration.  Define a function like:
>
>     let next chan () = (* partially apply to define chan *)
>         try
>             input_line chan
>         with
>             End_of_file -> raise No_more_elements
>     ;;
>
> You then pass this next function to list_of_enum and it uses setcdr in a 
> safe way to create your list of the lines of a file.  This allows us to 
> have safe setcdr-based list building in a generic way- *without* having to 
> discuss the dangers of setcdr with beginners.  Enumerations are a simple 
> enough idea- Java and C++ both have their iterators.
>
> Here's the problem: you don't know, ahead of time, how many lines are in 
> the file.  So there's no sane way to implement a count function.  The 
> count function, if meaningful, can be usefull- for example, Xarray could 
> make sure only one reallocation is necessary to add an entire enumeration.  
> But it's not *necessary*.

Well, is there I want to remind that those enumeration really look like
Stream. one could use Stream for this.

>
> So I propose that count be allowed to return -1 to indicate "length 
> unknown".  Throwing an exception might also be an idea. 

Throwing an exception is much better.

-- 
Rémi Vanicat
va...@la...
http://dept-info.labri.u-bordeaux.fr/~vanicat