Hi Joachim,

Here is a piece of code that produces the desired error:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
{-# LANGUAGE DataKinds, KindSignatures #-}

data P (x :: Bool) = P

class And a b c | a b -> c where
  op :: P a -> P b -> P c

instance And False False False where
  op _ _ = P

test = op (P :: P False) (P :: P False) :: P True

The `DataKinds` and `KindSigantures` are only used to match your example closely.
Here is a simpler version that causes essentially the same error:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}

class C a b | a -> b where
  op :: a -> b

instance C Bool Char where
  op _ = 'a'

test = op True :: Float



-Iavor



On Tue, Dec 3, 2013 at 4:09 AM, Joachim Breitner <mail@joachim-breitner.de> wrote:
Hi,

I have an error message, and I’m looking for code that produces it (how
is that for a change...)

While fixing https://ghc.haskell.org/trac/ghc/ticket/8576 I’d like to
clean up some error reporting in FunDeps.lhs, in particular code that is
involved in producing errors like

    Couldn't match type 'False with 'True
    When using functional dependencies to combine
      And 'False 'False 'False,
        arising from the dependency `a b -> c'
        in the instance declaration in `UnitTyped.Units'
      And 'False 'False 'True,
        arising from a use of `+' at <interactive>:14:7
    In the expression: meter + second
    In an equation for `it': it = meter + second

but unfortunately, the test suite does _not_ contain any code that
creates this error message. Also, the results obtained from googling for
that error message yield either no code, or only unhelpful code
fragments, or code that produces a different error message with current
HEAD.

Unfortunately, I cannot produce code that triggers it. Does anyone have
code lying around that triggers that error message?

Also: I found code that had this kind of error message in 7.6, e.g. the
attached code’s error changed from

        FunDepError.hs:86:27:
            Couldn't match type `F a1' with `U'
            When using functional dependencies to combine
              UpdateR (xs :> s) (S n) t (xs' :> s),
                arising from the dependency xs n t -> xs'
                in the instance declaration at FunDepError.hs:54:10
              UpdateR ((xs' :> F a0) :> F a1) (S O) U ((jj0 :> U) :> U),
                arising from a use of `var' at FunDepError.hs:86:27-29
            In the expression: var a
            In the first argument of `lam', namely `(\ b -> var a)'

(sorry for not finding something simpler) to

        FunDepError.hs:86:5:
            No instance for (Consume xs' jj) arising from a use of ‛lam’
            Possible fix:
              add (Consume xs' jj) to the context of
                the inferred type of x :: LLC t xs' jj (a :-> (a1 :-> a))
            In the expression: lam (\ a -> lam (\ b -> var a))
            In an equation for ‛x’: x = lam (\ a -> lam (\ b -> var a))

        FunDepError.hs:86:27:
            No instance for (UpdateR
                               ((xs' :> F a) :> F a1) (S O) U ((jj :> U) :> U))
              arising from a use of ‛var’
            In the expression: var a
            In the first argument of ‛lam’, namely ‛(\ b -> var a)’
            In the expression: lam (\ b -> var a)

Is that desired or a regression?

Greetings,
Joachim


--
Joachim “nomeata” Breitner
  mail@joachim-breitner.de • http://www.joachim-breitner.de/
  Jabber: nomeata@joachim-breitner.de  • GPG-Key: 0x4743206C
  Debian Developer: nomeata@debian.org

_______________________________________________
ghc-devs mailing list
ghc-devs@haskell.org
http://www.haskell.org/mailman/listinfo/ghc-devs