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From: Brighten Godfrey <pbg@cs.berkeley.edu>
Subject: Re: [Caml-list] Value shadowing
Date: Wed, 13 Aug 2008 02:15:53 -0700
To: David Allsopp <dra-news@metastack.com>
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X-Spam: no; 0.00; foo:01 checker:01 bool:01 ocaml:01 shadows:98 wrote:01 caml-list:01 tail:01 tail:01 match:02 let:03 let:03 raise:03 berkeley:04 exit:04 

On Aug 13, 2008, at 1:54 AM, David Allsopp wrote:

> [ There's been at least one recent(ish) discussion on this - [1] ]
>
> Suppose I have this piece of code:
>
> let foo xs =
>   match xs with
>     x::xs -> if x
>              then xs (* Return the tail of the list *)
>              else xs (* Return the entire list *)
>   | []    -> raise Exit
>
> Now, the comments show what's supposed to happen in this function  
> but that's
> obviously not how it will behave because the entire list [xs] on  
> line 1 is
> shadowed by the tail of the list [xs] on line 3 so in both cases  
> the tail of
> the list will be returned. The type checker can do nothing as both  
> have type
> [bool list].
>
> What would general opinion be if OCaml were to have a warning in this
> instance - "[xs] on line 3 shadows [xs] on line 1 with the same type"?
>
> As noted in the thread below, I too find
>
> let x = _
> in
>   let x = _
>   in
>     ...
>
> a useful style and would be greatly irritated by a warning on all  
> shadowing
> - but I think that in most cases for me the type of each [x] is  
> different.

I also find that style useful.  Sometimes the type changes, but I can  
recall useful cases where the type doesn't change, e.g., a sequence  
of various operations on a list.  Mock-up:

     let lst = generate_list_somehow () in
     let lst = List.filter (fun x -> x > 0) in
     let lst = List.sort compare lst in
     ...

~Brighten