I would not use -XMonoLocalBinds for all modules -- that will force you to do
more work.
Instead use it just for the offending Pol3_ module, via {-# LANGUAGE
MonoLocalBinds #-}
Or, probably better, give a type signature inside moduloBasisx, as suggested in
the http://hackage.haskell.org/trac/ghc/ticket/4361. The signature is better
because it also makes the program easier to understand!
Simon
| -----Original Message-----
| From: Serge D. Mechveliani [mailto:mech...@botik.ru]
| Sent: 21 December 2012 10:08
| To: Simon Peyton-Jones
| Cc: glasgow-haskell-bugs@haskell.org
| Subject: Re: DoCon and GHC
|
| On Wed, Jun 20, 2012 at 04:56:01PM +0000, Simon Peyton-Jones wrote:
| > Serge
| >
| > I hope you are well.
| >
| > I'm making a significant simplification to the type inference engine,
| > which will have a small knock-on effect in DoCon.
| >
| > I implemented a VERY DELICATE HACK to solve your problem before, but
| > it has become a significant problem to maintain the hack, so I'm
| taking it out.
| >
| > See http://hackage.haskell.org/trac/ghc/ticket/4361, and the comments
| > I have added there, which tell you what change to make. It's very
| minor!
| >
| > This will take effect from GHC 7.6 onwards. Thanks
| >
| > Simon
|
|
| This is on Ticket #4361 "(closed bug: fixed)"
| about compiling DoCon.
|
| Its story is as follows.
|
| * 7.4.0 failed to compile the module Pol3_ in DoCon due to
| 1) a certain GHC manner of constraint simplification (as Simon
| wrote)
| 2) a due to complex enough constraints used in DoCon, in particular,
| instance (LinSolvRing (Pol a), CommutativeRing a) =>
| LinSolvRing (UPol (Pol a))
| set in Pol3_.hs
| (I do not know how to simplify this constraint without loosing
| generality).
|
| * 7.4.1 does compile it, but, as Simon wrote, applies for this a very
| specific and unstable compilation method.
| * 7.6+ removes this latter method, and Simon P. Jones concludes
|
| > Happily, it's extremely easy to fix your source code in either of
| > these two ways:
| >
| > * If you use -XMonoLocalBinds (which is in any case implied by -XGADTs
| > and -XTypeFamilies), then GHC won't generalise the definition of x
| > in the example, and all will be well.
| >
| > * Alterantively, give a type signature for x, thus (in this case)
| >
| > moduloBasisx p = let x :: ()
| > x = upLinSolvRing p
| > in ()
|
|
|
| Now, I am trying ghc-7.6.1.20121207 built from source on Debian Linux.
|
|
| I. I try adding -XMonoLocalBinds for compiling DoCon:
|
|
| -- doco.cabal ------------------------------- ...
| ghc-options:
| -fno-warn-overlapping-patterns -fwarn-unused-binds
| -fwarn-unused-matches -fwarn-unused-imports -XMonoLocalBinds
| -O
| ----------------------------------------------
|
| `make build' fails at the first module:
|
| ------------------------------------------------------
| module Prelude_
| where
| ...
| instance (DShow a, DShow b) => DShow (a, b)
| ...
| where
| dShows opts (x, y) = showChar '(' . shows1 x . showString sep .
| shows1 y . showChar ')' --
| line 628
| where
| opts' = addToShowOptions (- 1) $ opts {parInShow = Parens}
| sep = fieldSeparator opts
| shows1 x = (case parInShow opts of Parens -> id
| _ -> unparensUpper "()")
| . dShows opts' x
|
| aShows (a, b) = showString "(pair " . aShows a . showChar ' ' .
| aShows b . showChar ')'
| ------------------------------------------------------
|
| The report is
|
| runghc Setup.hs build
| Building docon-2.12...
| Preprocessing library docon-2.12...
| [ 1 of 84] Compiling Prelude_ ( Prelude_.hs,
| dist/build/Prelude_.o )
|
| Prelude_.hs:628:32:
| Could not deduce (a ~ b)
| from the context (DShow a, DShow b)
| bound by the instance declaration at Prelude_.hs:622:10-43
| `a' is a rigid type variable bound by
| the instance declaration at Prelude_.hs:622:10
| `b' is a rigid type variable bound by
| the instance declaration at Prelude_.hs:622:10
| In the first argument of `shows1', namely `y'
| In the first argument of `(.)', namely `shows1 y'
| In the second argument of `(.)', namely shows1 y . showChar ')'
| ...
| -----------------------
|
| The line 628 is marked in the code as "-- line 628".
|
|
| I suspect that besides -XMonoLocalBinds, I need also to add some
| explicit type signatures, for example,
| dShows opts (x, y) = showChar '(' . shows1 (x :: DShow a => a) .
| ...
|
| Because GHC, probably, finds some contradiction in applying shows1 to
| x :: a and to y :: b.
|
|
| II. My another attempt will be removing -XMonoLocalBinds
| (with this, it compiles many modules and stops at Pol3.hs) and
| inserting a type signature in an appropriate place (this will also need
| to add ScopedTypeVariables + `forall'
| -- thanks to people for hints
| ).
|
| If (II) works, then it will be, probably less restrictive than (I).
|
| This is just the current report of my attempt to compile DoCon with GHC.
|
| ------
| Sergei
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