[GHC] #12183: Do not display global bindings with -fno-max-relevant-binds (was: Display all relevant local bindings)

[GHC]

#12183: Do not display global bindings with -fno-max-relevant-binds
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        Reporter:  ezyang            |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      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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Description changed by ezyang:

@@ -54,1 +54,2 @@
- these anyway.)
+ these anyway.) So really, to see all local bindings I just want to set
+ `-fmax-relevant-binds=100000` (i.e., to a large number.)
@@ -56,3 +57,6 @@
- Thus, I propose that `-fmax-relevant-binds=N` be changed to apply to
- GLOBAL bindings only, and that GHC should unconditionally print all local
- bindings. Furthermore, I suggest that by default `-fmax-relevant-binds=0`.
+ So, I think there should be another flag `-frelevant-global-binds` which
+ triggers display of global relevant bindings, making `-fno-max-relevant-
+ binds` equivalent to `-fmax-relevant-binds=BIGNUM`; i.e., it does NOT
+ report global bindings. I don't know if the error message should mention
+ `-frelevant-global-binds` by default, but for now I would say it
+ shouldn't.

New description:

 Presently, "Relevant bindings" messages (e.g., from holes) will attempt to
 report all local bindings, and relevant global bindings (by some
 heuristic), up to the number of bindings specified by `-fmax-relevant-
 binds=N`. For example, here is an error I recently had with a small number
 of relevant bindings:

 {{{
 Productive.hs:162:16: error:
     • Found hole: _ :: Par (D k0 (Stream k0 b))
       Where: ‘k0’ is an ambiguous type variable
              ‘b’ is a rigid type variable bound by
                the type signature for:
                  maapM :: forall a b.
                           (a -> Par b)
                           -> (forall (k :: Clock). Stream k a) -> Par
 (Forall1 Stream b)
                at Productive.hs:155:10
     • In the second argument of ‘(<$>)’, namely ‘_’
       In a stmt of a 'do' block: Cons b <$> _
       In the expression:
         do { (a, s') <- deStreamCons s;
              b <- f a;
              b' <- f (scar (unForall1 s'));
              Cons b <$> _ }
     • Relevant bindings include
         b' :: b (bound at Productive.hs:159:5)
         b :: b (bound at Productive.hs:158:5)
         s' :: Forall1 Stream a (bound at Productive.hs:157:9)
         (Some bindings suppressed; use -fmax-relevant-binds=N or -fno-max-
 relevant-binds)
 }}}

 When I set `-fno-max-relevant-binds)`, this extends with:

 {{{
         a :: a (bound at Productive.hs:157:6)
         s :: forall (k :: Clock). Stream k a (bound at
 Productive.hs:156:9)
         f :: a -> Par b (bound at Productive.hs:156:7)
         maapM :: (a -> Par b)
                  -> (forall (k :: Clock). Stream k a) -> Par (Forall1
 Stream b)
           (bound at Productive.hs:156:1)
         take :: forall a.
                 Int -> (forall (k :: Clock). Stream k a) -> Par [a]
           (bound at Productive.hs:143:1)
 ...and a pile more global binds...
 }}}

 I think this is not really the behavior we want. If I'm doing an Agda/Coq
 development, what I want is for the compiler to tell me all LOCAL
 bindings. Heuristically picked global bindings might be nice, but it's not
 a priority (after all, I probably have explicit type signatures for all of
 these anyway.) So really, to see all local bindings I just want to set
 `-fmax-relevant-binds=100000` (i.e., to a large number.)

 So, I think there should be another flag `-frelevant-global-binds` which
 triggers display of global relevant bindings, making `-fno-max-relevant-
 binds` equivalent to `-fmax-relevant-binds=BIGNUM`; i.e., it does NOT
 report global bindings. I don't know if the error message should mention
 `-frelevant-global-binds` by default, but for now I would say it
 shouldn't.

 As a minor addendum, I don't find the context information `In the second
 argument of ‘(<$>)’, namely ‘_’`, etc. very useful. Just makes the message
 longer and harder to view.

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12183#comment:2>
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