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Date: -- (:)
From: David Allsopp <dra-news@m...>
Subject: Value shadowing
[ 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've been stung a couple of times recently by non-contrived versions of the
function [foo] (through careless coding, of course - but that's what
warnings and type-checkers are about otherwise we'd all be using BCPL!)

Thoughts?


David


[1]
http://caml.inria.fr/pub/ml-archives/caml-list/2007/02/f60c7ea8cc0ebdbf9d1d5
6f0623b45a2.en.html