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Herbert Valerio Riedel | 19 Jul 2012 12:50

Why GHC doesn't warn about LHS nullary-constructor pattern bindings?

Hello,

Recently, I was a bit suprised that GHC didn't warn about useless
`where` definitions such as the following when using `-Wall` (and I
couldn't find a respective warning GHC CLI flag which would have enabled
reporting a warning in this case -- unless I missed it)

  module Foo where

  foo :: Int -> Int
  foo n = n + 1
    where
      Nothing = Just n

...wouldn't it be a sensible thing for GHC to warn in such cases
(i.e. when the LHS of a pattern binding is a nullary constructor), or is
there a useful application for this construct?

(In my original case, I ended up with such a "dead" construct after some
"unsound" code refactoring, and it would have helped me catch my error
earlier, if GHC would have pointed out that somethings fishy with my
code)

headers

Henning Thielemann | 19 Jul 2012 12:56

Re: Why GHC doesn't warn about LHS nullary-constructor pattern bindings?


On Thu, 19 Jul 2012, Herbert Valerio Riedel wrote:

> Recently, I was a bit suprised that GHC didn't warn about useless
> `where` definitions such as the following when using `-Wall` (and I
> couldn't find a respective warning GHC CLI flag which would have enabled
> reporting a warning in this case -- unless I missed it)
>
>  module Foo where
>
>  foo :: Int -> Int
>  foo n = n + 1
>    where
>      Nothing = Just n

I think that

   where
     x <at> Nothing = Just n

could be useful, if 'x' is evaluated somewhere.

headers

Christopher Done | 19 Jul 2012 13:07

Re: Why GHC doesn't warn about LHS nullary-constructor pattern bindings?

In your case the Nothing is unused so will never be a problem.

Perhaps more worrying:

foo :: Int -> Int
foo n = x + 1
    where
      Just x = Nothing

This gives no warnings.

headers

Christian Maeder | 19 Jul 2012 14:20

Re: Why GHC doesn't warn about LHS nullary-constructor pattern bindings?

You're right. That's a good case for a feature request.

In fact any binding that binds no variables should be warned about:

   let Just _ = ...

Cheers Christian

Am 19.07.2012 12:50, schrieb Herbert Valerio Riedel:
> Hello,
>
> Recently, I was a bit suprised that GHC didn't warn about useless
> `where` definitions such as the following when using `-Wall` (and I
> couldn't find a respective warning GHC CLI flag which would have enabled
> reporting a warning in this case -- unless I missed it)
>
>    module Foo where
>
>    foo :: Int -> Int
>    foo n = n + 1
>      where
>        Nothing = Just n
>
> ...wouldn't it be a sensible thing for GHC to warn in such cases
> (i.e. when the LHS of a pattern binding is a nullary constructor), or is
> there a useful application for this construct?
>
> (In my original case, I ended up with such a "dead" construct after some
> "unsound" code refactoring, and it would have helped me catch my error
> earlier, if GHC would have pointed out that somethings fishy with my

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