Redundant constraints

[Michael Sloan]

One option for avoiding the warning without runtime overhead would be
to do something like this:

     f :: Ord a => a -> a -> Bool
     f x y = True
       where
         _ = x < y


On Wed, Jan 7, 2015 at 7:27 AM, Johan Tibell <johan.tibell at gmail.com> wrote:
> I think this probably makes sense, especially since you can silence the
> warning when you intend to add an unnecessary constraint.
>
> I had one thought though: consider an abstract data type with functions that
> operates over it. I might want to require e.g Ord in the definition of a
> function so I have freedom to change my implementation later, even though
> the current implementation doesn't need Ord. Think of it as separating
> specification and implementation. An example is 'nub'. I initially might
> implement it as a O(n^2) algorithm using only Eq, but I might want to leave
> the door open to using Ord to create something better, without later having
> to break backwards compatibility.
>
> On Wed, Jan 7, 2015 at 4:19 PM, Simon Peyton Jones <simonpj at microsoft.com>
> wrote:
>>
>> Friends
>>
>> I’ve pushed a big patch that adds –fwarn-redundant-constraints (on by
>> default).  It tells you when a constraint in a signature is unnecessary,
>> e.g.
>>
>>      f :: Ord a => a -> a -> Bool
>>
>>      f x y = True
>>
>> I think I have done all the necessary library updates etc, so everything
>> should build fine.
>>
>> Four libraries which we don’t maintain have such warnings (MANY of them in
>> transformers) so I’m ccing the maintainers:
>>
>> o   containers
>>
>> o   haskeline
>>
>> o   transformers
>>
>> o   binary
>>
>>
>>
>> Enjoy!
>>
>>
>>
>> Simon
>>
>>
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>>
>
>
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