[Haskell-cafe] coding standard question

Malcolm Wallace Malcolm.Wallace at cs.york.ac.uk
Mon Jun 22 08:30:25 EDT 2009

Johan Tibell <johan.tibell at gmail.com> wrote:

> > Example:
> >  f a b = g (a+b) (b-a)
> >        where g a c = a*c
> > ---->
> >  f a b = g (a+b) (b-a)
> >        where g a' c = a*c
> Actually there's a warning:
> <interactive>:1:34: Warning: Defined but not used: `a''

Clearly I simplified the example too far.  Try this, only slightly more
complex, example instead.  Remember, the larger the example, the more
likely you are to miss an occurrence.

     f a b = g (a+b) (b-a)
           where g a c = a*(c-a)
     f a b = g (a+b) (b-a)
           where g a' c = a'*(c-a)

Perhaps I should advocate for a new warning in GHC to cover this case:
-fwarn-mixed-scopes, which could flag the use of the unprimed a, due to
being bound at an outer scope.

Because warn-mixed-scopes is almost the opposite of warn-name-shadowing,
then the only way to avoid warnings would be to manually lambda-lift all
functions.  :-)


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