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best and fastest way to read lines from a file?
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Date: -- (:)
From: kirillkh <kirillkh@g...>
Subject: Re: [Caml-list] best and fastest way to read lines from a file?
2007/10/2, Olivier Andrieu <oandrieu@gmail.com>:
>
> On 10/2/07, kirillkh <kirillkh@gmail.com> wrote:
> > OK, so I'll give up the parsing/buffering part and only leave efficient
> > exception handling. This should leave the user free to do anything with
> it,
> > but prevent performance pitfalls. The following is based on Mattias
> > Engdegard's code:
> >
> > (* I couldn't figure out, how to declare a polymorphic exception
> properly *)
> > exception Done of 'a
> >
> >  let fold_file (file: in_channel)
> >               (read_func: in_channel->'a)
> >               (elem_func: 'a->'b->'b)
> >               (seed: 'b) =
> >   let rec loop prev_val =
> >     let input =
> >       try read_func file
> >       with End_of_file -> raise (Done prev_val)
> >     in
> >       let combined_val = elem_func input prev_val in
> >       loop combined_val
> >   in
> >     try loop seed with Done x -> x
> >
> > And the usage for line counting:
> >
> > let line_count filename =
> >    let f = open_in filename in
> >    let counter _ count = count + 1 in
> >    fold_file f readline counter 0
> >
> > Since it's library code, we can tolerate the little annoyance of the
> second
> > try-catch. As far as I can tell, this code has the same performance
> > characteristics as yours: no consing + tail recursion. Any other
> problems
> > with it?
>
> well apart from the fact that you cannot have "polymorphic exceptions"
> in OCaml, this kind of code is IMHO much more natural with an
> imperative loop instead of a functional one:
>
>
> let fold_file read chan f init =
>   let acc = ref init in
>   begin
>     try while true do
>       let d = read chan in
>       acc := f d !acc
>     done
>     with End_of_file -> ()
>   end ;
>   !acc
>
> --
>   Olivier
>

A little weird to see such inherently functional construct as fold
implemented imperatively. But it's fine with me, as long as it does the job.
I wonder, though, how would the performance of a line counter based on your
code compare with the one suggested by Brian.