[nhc-bugs] nhc98 rejects legal rebinding

[Malcolm Wallace]

Thomas Hallgren <hallgren@cse.ogi.edu> writes:

>     module Rebind where
>     f x = x
>       where
>         x = True
> 
> nhc98-1.16 [1] produces the error message
> 
>     ======  Errors when renaming:
>     Redefinition of Identifier x at 5:5
> 
> While this is not completely unreasonable, it seems to be a deviation 
> from the Haskell 98 report and the behaviour of other Haskell 
> implementations.

It is quite difficult to be sure exactly what the Haskell Report
says on this matter, but I think you are probably right.  There is
no explicit section giving the semantics of `where' clauses at all!
As you say though, there is one rule in the formal semantics for
pattern matching that translates a `where' clause to a `let' binding.

However, I do not propose to fix this bug in nhc98.  The example
you give is extremely likely to be a programmer's mistake, so in
my opinion the error message from the compiler is more useful than
the strange runtime behaviour that might otherwise occur.  When the
rebinding is deliberate, it is easy for the author to find a different
way of coding the intended semantics.

Regards,
    Malcolm