[GHC] #12885: "too many iterations" causes constraint solving issue.

28 Nov 2016

      #12885: "too many iterations" causes constraint solving issue.
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           Reporter:  judahj         |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.2-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 The following file compiled fine with ghc-7.10, but fails in ghc-8.0.2-rc1
 (as well as ghc-8.0.1).

 This is a simplified version of a compilation issue with ghc-8 and
 https://github.com/tensorflow/haskell.  It seems similar to #12175, but
 even though that was fixed in ghc-8.0.2-rc1, the below code still doesn't
 compile.

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE TypeOperators #-}
 module ConstraintTest where

 import Data.Int
 import Data.Word
 import GHC.Exts (Constraint)

 import Lens.Family2 ((.~), (&), Lens')
 import Lens.Family2.Unchecked (lens)

 class MyType a where

 instance MyType Int8
 instance MyType Int16
 instance MyType Int32
 instance MyType Int64
 instance MyType Word8
 instance MyType Word16
 instance MyType Word32

 -- Require every element in the list to be an instance of 'MyType'.
 type family MyTypes (as :: [*]) :: Constraint where
     MyTypes '[] = ()
     MyTypes (a ': as) = (MyType a, MyTypes as)

 data Foo = Foo { fooInt :: Int }

 class Attr a where
     attr :: Lens' Foo a

 instance Attr Int where
     attr = lens fooInt (\f n -> f { fooInt = n })

 test :: MyTypes '[Int8,Int16,Int32,Int64,Word8,Word16,Word32]
         => Foo
 test = attr .~ (3 :: Int) $ Foo 0
 }}}

 Compilation error:
 {{{
 tensorflow/tests/ConstraintTest.hs:1:1: error:
     solveWanteds: too many iterations (limit = 4)
       Unsolved: WC {wc_simple =
                       [W] hole{a282} :: b_a25A ~ Int (CNonCanonical)}
       New superclasses found
       Set limit with -fconstraint-solver-iterations=n; n=0 for no limit
 }}}

 Some ways to change the test to make the compilation succeed:
 - Pass "-fconstraint-solver-iterations=0" to ghc.
 - Shorten the type-level list in the constraint of `test`.
 - Replace `Lens.Family2` with `Lens.Micro` from the `microlens` package.
 I think this is because `Lens.Family2`'s version of (.~) is higher-order
 than `Lens.Micro`:
 https://hackage.haskell.org/package/lens-family-1.2.1/docs/Lens-
 Family2.html#v:.-126-
 https://hackage.haskell.org/package/microlens-0.4.7.0/docs/Lens-
 Micro.html#v:.-126-

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12885>
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