[GHC] #11451: Inconsistent warnings for unused binders

[GHC]

#11451: Inconsistent warnings for unused binders
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        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by ekmett):

 > What I'm missing is this: why would it not be more perspicuous to say

 {{{#!hs
 Category * (Indexed _)
 }}}

 So that in 3-4 years when I can stop supporting older compilers, I can
 finally lose the signaling of intent that comes from the conventions that
 we follow elsewhere of `f` and `m` and `a`? There is a lot of information
 packed in those conventions.

 Nobody reading the documentation cares whether the instance uses the
 argument or not, but this warning would force everyone to put that front
 and center in the documentation users see and seems to clutter about 70%
 of the variables in instances out there with a perl-like `_` sigil. I like
 having names for things rather than Miranda's `*`, `**`, `***`.  That
 said, one of the biggest sources of teasing from the outside world about
 haskell is how short our type variable names tend to be, getting rid of
 them entirely like this ''is'' one way to avoid the argument. ;)

 > If it is, why do you not object to the warning you get (for unused y)
 when you write this?

 {{{#!hs
 f x y = x
 }}}

 Nothing we do at the value level is reflected in the documentation;
 everything we do at the type level is reflected in the documentation.

 The fact that the value-level unused binding warning is on or not is
 invisible to the user of the library. Whether I turn on the warning about
 unused bindings at the value level or not the user never sees this fact.
 It remains entirely internal to the source file as a local concern.

 > That is, do you see class instances and type-function instances the
 same?

 Going from the current state where ''can't'' use `_` in that position at
 all, to a new state where you ''must'' use `_` or `_a` in a place that
 affects generated documentation and affects the vast majority of instances
 that have ever been written seems like a big jump, when the existing style
 has been in use for 20+ years. This seems to indicate to me that doing
 this to instances is quite a big deal and the amount of work for the user
 community boggles my mind.

 As for type instances, this then hoists you on the dilemma of copying the
 value level warning style or matching the behavior of instances. The
 resolution you seem to be reaching for is to change the behavior of
 instances to match, and it would cleanly resolve the dilemma, but at a
 rather great cost.

 In many ways the same "it shows up in the documentation" argument applies
 to the `type instance` case as well, now that haddock actually shows type
 families, but there is a lot less code affected.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11451#comment:15>
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