arising from the dependency ... in the instance declaration
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
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
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
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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@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org
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
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@joachim-breitner.de • http://www.joachim-breitner.de/ Jabber: nomeata@joachim-breitner.de • GPG-Key: 0x4743206C Debian Developer: nomeata@debian.org
participants (2)
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Iavor Diatchki -
Joachim Breitner