arising from the dependency ... in the instance declaration

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arising from the dependency ... in the instance declaration

Joachim Breitner-2
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 at joachim-breitner.de ? http://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  ? GPG-Key: 0x4743206C
  Debian Developer: nomeata at debian.org
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arising from the dependency ... in the instance declaration

Iavor Diatchki
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 at 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 at joachim-breitner.de ? http://www.joachim-breitner.de/
>   Jabber: nomeata at joachim-breitner.de  ? GPG-Key: 0x4743206C
>   Debian Developer: nomeata at debian.org
>
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://www.haskell.org/mailman/listinfo/ghc-devs
>
>
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arising from the dependency ... in the instance declaration

Joachim Breitner-2
Hi,

Am Dienstag, den 03.12.2013, 11:08 -0800 schrieb Iavor Diatchki:

> Here is a piece of code that produces the desired error:
>
> [..]
>
> The `DataKinds` and `KindSigantures` are only used to match your
> example closely.
> Here is a simpler version that causes essentially the same error:
>
> [..]
>

thanks for these examples. Both, though, give different error messages
in GHC HEAD. I?m more and more confident that this is an inadvertent
error message, see
https://ghc.haskell.org/trac/ghc/ticket/8592

Once that is sorted out, I?ll make sure these examples (or at least a
representative one) makes it into the testsuite.

Greetings,
Joachim


--
Joachim ?nomeata? Breitner
  mail at joachim-breitner.de ? http://www.joachim-breitner.de/
  Jabber: nomeata at joachim-breitner.de  ? GPG-Key: 0x4743206C
  Debian Developer: nomeata at debian.org
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arising from the dependency ... in the instance declaration

Iavor Diatchki
Hi,

oh, I didn't realize that you meant using HEAD.

I'd guess that the new behavior is a result of the changes Simon made to
the constraint solver a little while ago (maybe about a month ago?).
I haven't had a chance to look at the code yet, but I believe he eliminated
some constraint interactions, which did not contribute towards
type-checking more programs, but lead to more confusing error messages.

By the way, I am not sure that this is a regression---I kind of like new
the new behavior.  For my second example we get:

test.hs:9:8:
    No instance for (C Bool Float) arising from a use of ?op?
    In the expression: op True :: Float
    In an equation for ?test?: test = op True :: Float

This tells us exactly what the problem is, namely the missing instance,
without going into stuff about functional dependencies.  I guess we _could_
also say that such an instance can't exist because of the other instance
and the functional dependency, but I am not sure how relevant that is...

-Iavor







On Tue, Dec 3, 2013 at 1:40 PM, Joachim Breitner
<mail at joachim-breitner.de>wrote:

> Hi,
>
> Am Dienstag, den 03.12.2013, 11:08 -0800 schrieb Iavor Diatchki:
>
> > Here is a piece of code that produces the desired error:
> >
> > [..]
> >
> > The `DataKinds` and `KindSigantures` are only used to match your
> > example closely.
> > Here is a simpler version that causes essentially the same error:
> >
> > [..]
> >
>
> thanks for these examples. Both, though, give different error messages
> in GHC HEAD. I?m more and more confident that this is an inadvertent
> error message, see
> https://ghc.haskell.org/trac/ghc/ticket/8592
>
> Once that is sorted out, I?ll make sure these examples (or at least a
> representative one) makes it into the testsuite.
>
> Greetings,
> Joachim
>
>
> --
> Joachim ?nomeata? Breitner
>   mail at joachim-breitner.de ? http://www.joachim-breitner.de/
>   Jabber: nomeata at joachim-breitner.de  ? GPG-Key: 0x4743206C
>   Debian Developer: nomeata at debian.org
>
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