I haven't declared integer to be NotAnIntegerso (in a closed
world)this would seem to exclude the contradictionbut...
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo
a
instance (NotAnInteger x) = Foo Bar x
I haven’t declared integer to be “NotAnInteger”….so (in a closed
world)….this would seem to exclude the contradiction….but…
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
”….so (in a closed
world)….this would seem to exclude the contradiction….but…
** **
** **
Functional dependencies conflict between instance declarations:
instance Foo Integer Integer -- Defined at liam1.lhs:7:12
instance NotAnInteger x = Foo Bar x -- Defined at liam1
Functional dependencies
conflict between instance declarations:.
You're running into the open worldassumption--anybody could come along and
make Integer part of your NotAnInteger class, and there's nothing you can do to
stop them. This is a design tradeoff for typeclasses: typeclass instances
= Range2
unpack = unRange2
but there is compilation error:
Duplicate instance declarations:
instance [incoherent] (Num a, Ord a, Rangable range a,
Packable range a) =
SubtypeOf range a
-- Defined at ...:22:10
instance
On Fri, Jan 25, 2013 at 3:18 PM, s9gf4...@gmail.com wrote:
Duplicate instance declarations:
instance [incoherent] (Num a, Ord a, Rangable range a,
Packable range a) =
SubtypeOf range a
-- Defined at ...:22:10
instance [incoherent] (Integral a, Packable range a,
MultipleTo range
This has the code smell of trying to use typeclasses for OOP. That won't
work. (Yes, really.)
I am not trying to use OOP, I am just writing some typecasting at all.
This would be correct. Constraints on an instance are applied *after* the
instance is selected, so when Haskell is looking
http://ideone.com/v2CrAm
I has posted to ideone to show what is wrong.___
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On Fri, Jan 25, 2013 at 12:39 PM, s9gf4...@gmail.com wrote:
**
This has the code smell of trying to use typeclasses for OOP. That
won't work. (Yes, really.)
I am not trying to use OOP, I am just writing some typecasting at all.
This would be correct. Constraints on an instance
be at the same level or lower than the context of any instance of any super-class of C. The purpose of this rule is to guarantee that the required super-class methods exist. [1] 4.3.2 Instance Declarations http://www.haskell.org/onlinereport/haskell2010/haskellch4.htmlClass hierarchy
Eq1 Show1
)
Yi/Prelude.hs:182:9:
Duplicate instance declarations:
instance Category Accessor.T -- Defined at Yi/Prelude.hs:182:9-38
instance Category Accessor.T
-- Defined in data-accessor-0.2.1:Data.Accessor.Private
cabal: Error: some packages failed to install:
yi-0.6.1 failed during
] ( Yi/Editor.hs-boot,
dist/build/Yi/Editor.o-boot )
[ 18 of 120] Compiling Yi.Debug ( Yi/Debug.hs, dist/build/Yi/Debug.o )
[ 19 of 120] Compiling Yi.Prelude ( Yi/Prelude.hs,
dist/build/Yi/Prelude.o )
Yi/Prelude.hs:182:9:
Duplicate instance declarations:
instance
Nicolas Pouillard wrote:
Excerpts from Marcelo Sousa's message of Sun Sep 27 14:13:43 +0200 2009:
Hey guys,
I'm trying to install yi using cabal but I got this error. Any ideas
how to solve it?!
I'm using ghc-6.10.1 and cabal-install version 0.6.2 using version
1.6.0.2 of the Cabal library.
with:
{-# OPTIONS -fglasgow-exts -fallow-undecidable-instances #-}
class A a
class R a
class S a
instance R a = A a
instance S a = A a
--
GHC gives
*Duplicate instance declarations*
* instance R a = A a *
* instance S a = A a *
**
*Why
Hello Alberto,
Friday, October 24, 2008, 12:20:39 PM, you wrote:
instance R a = A a
instance S a = A a
Duplicate instance declarations
Why?
because you may write in other module
instance R Int
instance S Int
if class A includes functions, it may be problematic to determine
which
Instance instantiation is *not* search, and *not* similar to
subclassing in OO languages.
Both your instance declarations simply add constraints to functions that use it.
Here's a more concrete example:
class A a where
doA :: a - String
class R a where
doR :: a - String
instance R Int
Never mind, that GHC compiler was again more clever than me, sigh.
That's really frustrating about Haskell: the compiler captures so many
errors at compile time, that newbies hardly get anything done, it's
a constant battle against the errors. But once it compiles, it usually
works at runtime
: Functional dependencies conflict between
instance declarations
Never mind, that GHC compiler was again more clever than me, sigh.
That's really frustrating about Haskell: the compiler captures so many
errors at compile time, that newbies hardly get anything done, it's
a constant battle against
[EMAIL PROTECTED] wrote:
Are you kidding, or has automatic proving of programs evolved that far?
Aaarrrggg, soon we're all out of job ;-)
Experts have been proclaiming this since high-level programming was
invented many decades ago. We're still waiting. ;-)
The Haskell mailing list seems to be filled with people requesting
information about this error, so I cannot resist to include myself in it ;-)
I've read the information about funcdeps in the GHC user guide, and I
think I understand how it works, but I get the error a lot, without
having a
Never mind, that GHC compiler was again more clever than me, sigh.
That's really frustrating about Haskell: the compiler captures so many
errors at compile time, that newbies hardly get anything done, it's a
constant battle against the errors. But once it compiles, it usually
works at runtime
Peter Verswyvelen wrote:
Never mind, that GHC compiler was again more clever than me, sigh.
That's really frustrating about Haskell: the compiler captures so many
errors at compile time, that newbies hardly get anything done, it's a
constant battle against the errors. But once it compiles, it
hello,
I have a question about context in type signature. I would like to
write a function, say (f :: T - T) which also relies on an instance of
class C being defined for T. The problem is, I don't want this instance
defined at the same time f is defined, instead I would like to defer
this
in anyway.
My question: Is there a technique to allow such innocuous ambiguity in
instance declarations?
I experimented with introducing something akin to a trace in a third
parameter to the class which would disambiguate the derivation (by
specifying the order of the transformations; reminded me
Nicolas Frisby posed a problem about controlling the order of
instance selection rules (or, the application of type improvement rules)
Given the following code
newtype IdL a = IdL a
newtype IdR a = IdR a
class C f g where nest :: f a - g a
instance C IdL IdR where nest (IdL x) = IdR x
Hi there,
but GHC complains:
Illegal instance declaration for `Foo (Either b b)'
(The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
In the instance declaration for `Foo (Either b b)'
unless I'm totally
but GHC complains:
Illegal instance declaration for `Foo (Either b b)'
(The instance type must be of form (T a b c)
where T is not a synonym, and a,b,c are distinct type variables)
In the instance declaration for `Foo (Either b b)'
unless I'm totally mistaken, your
Am Samstag, 25. Juni 2005 21:22 schrieb Josh Hoyt:
Hello,
I'm a new Haskeller, and I'm running into a problem attempting to
declare certain types as instances. I was attempting something that's
effectively equivalent to:
class Foo a
instance Foo (Either b b)
but GHC complains:
On Sun, 26 Jun 2005, Daniel Fischer wrote:
Am Samstag, 25. Juni 2005 21:22 schrieb Josh Hoyt:
Hello,
I'm a new Haskeller, and I'm running into a problem attempting to
declare certain types as instances. I was attempting something that's
effectively equivalent to:
class Foo a
Hello,
I'm a new Haskeller, and I'm running into a problem attempting to
declare certain types as instances. I was attempting something that's
effectively equivalent to:
class Foo a
instance Foo (Either b b)
but GHC complains:
Illegal instance declaration for `Foo (Either b b)'
Hi Marcin,
| There's no solid technical reason for this, but Haskell doesn't allow
| it at the moment because there isn't an easy way to name an instance
| declaration.
|
| There is another problem: even if we created a syntax to name them,
| if they would not be exported by default then
sense for
the class instance to be, too.
(This leads to the question of why we need to have instance
declarations at all :-) (My guesses would be: compiler
implementation issues, code clarity, error detection, partially
implemented classes))
Problems arise when a data type needs
Fri, 7 Dec 2001 11:38:14 -0800, Mark P Jones [EMAIL PROTECTED] pisze:
There's no solid technical reason for this, but Haskell doesn't allow
it at the moment because there isn't an easy way to name an instance
declaration.
There is another problem: even if we created a syntax to name them,
if
Marcin 'Qrczak' Kowalczyk wrote:
There is another problem: even if we created a syntax to name them,
if they would not be exported by default then current programs would
have to be changed.
Well, the default could be to export, unless explicitly hidden. If it
_is_ exported, you could have
(sorry to mess up mail threading, but I couldn't properly reply to the
message the way I'm using email right now--broken mail clients)
Recently, however, there has been some interest in using named instance
declarations in other ways, so perhaps we will see features like this
creeping
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