Re: variables in 'let rec'

From: John Max Skaller (skaller@maxtal.com.au)
Date: Thu Mar 30 2000 - 11:52:37 MET DST

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Sven LUTHER wrote:
>
> On Tue, Mar 28, 2000 at 11:03:36AM +1000, Max Skaller wrote:
> > Damien Doligez wrote:
> > >
> > > >From: Max Skaller <maxs@in.ot.com.au>
> > > >
> > > >Alternatively, you can just use
> > > >
> > > > let rec x' () = ...
> > > > and ...
> > > > let x = x()
> > > >
> > > >in lieu of what you really wanted: (that is,
> > > >there is a workaround).
> > >
> > > But that doesn't work for "let rec x = 1 :: x", which is the most
> > > interesting feature of O'Caml's recursive value definitions.
> >
> > It doesn't?
> >
> > let rec x'() = 1 :: x()'
> > let x = x'()
>
> FYI :
>
> bash-2.03$ ocaml
> Objective Caml version 2.99 (99/12/08)
>
> # let rec x = 1 :: x ;;
> val x : int list =
> [1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1; 1;
> ...]

Ah! I withdraw my last comment: the list is constructed as instructed,
resulting in a circular list. The printer routines recognizes this,
or at least is smart enough to apply a printing limit.

_Traversing_ the list may take infinite time, but examining
the first n elements will not. (And the construction is
constant time :-)

-- 
John (Max) Skaller, mailto:skaller@maxtal.com.au
10/1 Toxteth Rd Glebe NSW 2037 Australia voice: 61-2-9660-0850
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Next message: John Max Skaller: "Re: to have labels or not"
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