why is this legal

[Zhanyong Wan]

Martin Norbäck wrote:

| However, it's extremely common to write code like this:
|
| f expr = case expr of
|   EXPR_NOT expr -> something with expr
|   EXPR_UNARY_MINUS expr -> something with expr
|
| not allowing this would make the programmer have to invent  new names
| here. I had to turn off this warning for the project we do at work,
| because the compiler would emit hundreds of warnings for cases like
| this. Changing all bound names was not a good option.

I agree with Antony on that compilers should emit warnings on
potentially-dangerous usages.  Anyone working on a lint for Haskell?  :-)
It would be a valuable contribution to our community.

On the other hand, I think Martin has a reasonable concern.  However, note
that in Hal's original example:

| f x = f' 0 x
|     where f' acc [] = acc
|           f acc (x:xs) = f' (x+acc) xs

the inner f is never used.  Therefore I believe -fwarn-unused-local-binding
(if there is such an option) will detect the bug in Hal's code without
flooding stderr in Martin's case.

- Zhanyong