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[Caml-list] Clarification of original post [still having problems]
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Date: -- (:)
From: Brian Hurt <brian.hurt@q...>
Subject: Re: [Caml-list] Clarification of original post [still having problems]
On Sun, 20 Apr 2003, Ryan Bastic wrote:

> 
> Can anyone take some time and help a struggling newbie out? :-(
> 
> This stuff is still failing to parse right for my brain.
> 
> ----- Original Message Follows -----
> > Hello all,
> >   A few weeks ago I posted a message asking how to create a program in
> > OCaml to shuffle  vector of strings into an array of arrays. Had some
> > problems understanding how to do it in Ocaml properly, and a kind soul
> > posted a very elegant solution to the problem :-) unfortunately, i
> > still can't understand some things from it.
> > 
> >   What follows is some code with comments on how it is expected to be
> > used, and also
> > where my confusion in the semantics of the code lay.
> > 
> >   (* Return first 'n' from input and the rest. *)
> > let firstN n input =
> >   let nInput = Array.length input in
> >     if n >= nInput
> >     then (input, [||])
> >     else (Array.sub input 0 n, Array.sub input n (nInput - n))
> > 
> > let group n input =
> >   let rec group' n input =
> >     if Array.length input = 0 then []
> >     else 
> >       let (front, rest) = firstN n input in
> >         (* the next line confuses me. i'm aware of :: being a list
> > concatenation operator, but in this case, shouldn't group 'n rest be
> > returning an array, because that's what firstN returns. I've
> experimented in
> > the REPL and had no luck in figuring it out. *)
> >         front :: group' n rest
> >   in
> >     Array.of_list (group' n input)
> > 
> 
> The specific problems that confuse me is that 
> 
> let rec group' ... in Array.of_list is all part of the same expression,
> so how does Ocaml
> let front :: group' n rest work?? firstN n input returns two arrays, and
> as far as my
> understanding goes, :: is a list concatenation operator. How then, does
> this work?

You can have lists of arrays just fine.  Which is what this function 
returns.  Basically, it (recursively) calls group', which returns a list 
of arrays.  When the recursive call returns, it prepends front to the list 
and returns the prepended-to list.

One trick I find myself doing a lot is to cut internal functions out into 
global functions to check their type signatures.  And given:
  let rec group' n input =
    if Array.length input = 0 then []
    else
      let (front, rest) = firstN n input in
        front :: group' n rest

Ocaml returns the type:
val group' : int -> 'a array -> 'a array list = <fun>

Which means a function that takes two arguments, an int and an 'a array, 
and returns a 'a array list.  group' returns a list- so prepending an 'a 
to the list makes perfect sense.

Does this help?

Brian


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