build error (GHC 6.12.3)

[kowey]

Hi Thomas,

One of my users reports the following build error with GHC 6.12.3. Do you think it's an easy fix? Thanks! :-)

Data/AttoLisp.hs:1:0:
    Couldn't match expected type `c' against inferred type `b'
      `c' is a rigid type variable bound by
          the instance declaration at Data/AttoLisp.hs:308:34
      `b' is a rigid type variable bound by
          the instance declaration at Data/AttoLisp.hs:308:32
    When using functional dependencies to combine
      ParseList b b,
        arising from a use of `parseList' at Data/AttoLisp.hs:312:4-25
      ParseList b c,
        arising from the instance declaration at Data/AttoLisp.hs:308:9-59

Data/AttoLisp.hs:309:2:
    Couldn't match expected type `c' against inferred type `b'
      `c' is a rigid type variable bound by
          the instance declaration at Data/AttoLisp.hs:308:34
      `b' is a rigid type variable bound by
          the instance declaration at Data/AttoLisp.hs:308:32
    When using functional dependencies to combine
      ParseList a a,
        arising from the dependency `a -> b'
        in the instance declaration at Data/AttoLisp.hs:314:9
      ParseList b c,
        arising from a use of `parseList' at Data/AttoLisp.hs:312:4-25
    When generalising the type(s) for `parseList'
    In the instance declaration for `ParseList (a -> b) c'

Data/AttoLisp.hs:312:4:
    Overlapping instances for ParseList b b
      arising from a use of `parseList' at Data/AttoLisp.hs:312:4-25
    Matching instances:
      instance ParseList a a -- Defined at Data/AttoLisp.hs:314:9-21
      instance (FromLisp a, ParseList b c) => ParseList (a -> b) c
        -- Defined at Data/AttoLisp.hs:308:9-59
    (The choice depends on the instantiation of `b'
     To pick the first instance above, use -XIncoherentInstances
     when compiling the other instance declarations)
    In the expression: parseList msg (f y) xs
    In the expression:
        do { y <- parseLisp x;
             parseList msg (f y) xs }
    In the definition of `parseList':
        parseList msg f (x : xs)
                    = do { y <- parseLisp x;
                           parseList msg (f y) xs }
cabal: Error: some packages failed to install:
atto-lisp-0.2 failed during the building phase. The exception was:
ExitFailure 1

[kowey]

What's also odd about this is that I could have sworn I was using GHC 6.12.x on a Windows VM to build atto-lisp too. So I don't know what it should fail for my user...

[nominolo]

I don't think so.

The problem is with the 'struct' function and its use of the ParseList
type class.

The idea is to have the user provide a function (actually a
constructor) and from that we can automatically figure out the parser.
I'm not sure if this is even supposed to work, but it seems to work
fine with GHC 7+. I'll take a look.

On 9 September 2011 11:51, kowey
reply@reply.github.com
wrote:

Hi Thomas,

One of my users reports the following build error with GHC 6.12.3.  Do you think it's an easy fix?  Thanks! :-)

   Data/AttoLisp.hs:1:0:
       Couldn't match expected type c' against inferred typeb'
         c' is a rigid type variable bound by              the instance declaration at Data/AttoLisp.hs:308:34          b' is a rigid type variable bound by
             the instance declaration at Data/AttoLisp.hs:308:32
       When using functional dependencies to combine
         ParseList b b,
           arising from a use of `parseList' at Data/AttoLisp.hs:312:4-25
         ParseList b c,
           arising from the instance declaration at Data/AttoLisp.hs:308:9-59

   Data/AttoLisp.hs:309:2:
       Couldn't match expected type c' against inferred typeb'
         c' is a rigid type variable bound by              the instance declaration at Data/AttoLisp.hs:308:34          b' is a rigid type variable bound by
             the instance declaration at Data/AttoLisp.hs:308:32
       When using functional dependencies to combine
         ParseList a a,
           arising from the dependency a -> b'            in the instance declaration at Data/AttoLisp.hs:314:9          ParseList b c,            arising from a use ofparseList' at Data/AttoLisp.hs:312:4-25
       When generalising the type(s) for parseList'        In the instance declaration forParseList (a -> b) c'

   Data/AttoLisp.hs:312:4:
       Overlapping instances for ParseList b b
         arising from a use of parseList' at Data/AttoLisp.hs:312:4-25        Matching instances:          instance ParseList a a -- Defined at Data/AttoLisp.hs:314:9-21          instance (FromLisp a, ParseList b c) => ParseList (a -> b) c            -- Defined at Data/AttoLisp.hs:308:9-59        (The choice depends on the instantiation ofb'
        To pick the first instance above, use -XIncoherentInstances
        when compiling the other instance declarations)
       In the expression: parseList msg (f y) xs
       In the expression:
           do { y <- parseLisp x;
                parseList msg (f y) xs }
       In the definition of `parseList':
           parseList msg f (x : xs)
                       = do { y <- parseLisp x;
                              parseList msg (f y) xs }
   cabal: Error: some packages failed to install:
   atto-lisp-0.2 failed during the building phase. The exception was:
   ExitFailure 1

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#4

Push the envelope. Watch it bend.

[kowey]

OK, just to make sure there's no needless sense of urgency, my user just told me they upgraded to GHC 7, so problem solved for my purposes :-)

[nominolo]

OK. I still want to make sure that the current implementation doesn't just work by accident. I'll ask haskell-cafe and see if Oleg chimes in.

[nominolo]

Closing, because 6.12 is no longer supported.