multi parameter classes and instance declarations
Good Afternoon, i'am trying to express something like this here:
-- CI.hs http://pastebin.com/f5b17cf57 {-# OPTIONS -XMultiParamTypeClasses #-}
module CI(D,f) where
class B b
class (B b) => A a b where f :: b
data D = D
instance B D
instance A D D where f = D -- CI.hs
-- MultiClassTest.hs http://pastebin.com/f1c481e8d import CI
a = f -- MultiClassTest.hs
ghc output: MultiClassTest.hs:4:4: No instance for (CI.A a D) arising from a use of `f' at MultiClassTest.hs:4:4 Possible fix: add an instance declaration for (CI.A a D) In the expression: f In the definition of `a': a = f Failed, modules loaded: CI. when opening MultiClassTest.hs it looks like the instance declaration gets not important, but I think it might have a reson, that the ghc rejects that code... Regards Markus -- Markus Barenhoff - MÃŒnster Germany,Europe,Earth sip:alios@alios.org - iax2:alios@alios.org - xmpp:alios@jabber.ccc.de gpg:0xAE7C7759 fp:79 64 AA D9 B7 16 F5 06 6A 88 5F A9 4D 49 45 BB
On Mon 16.02 15:28, Markus Barenhoff wrote:
Good Afternoon,
i'am trying to express something like this here:
-- CI.hs http://pastebin.com/f5b17cf57 {-# OPTIONS -XMultiParamTypeClasses #-}
module CI(D,f) where
class B b
class (B b) => A a b where f :: b
data D = D
instance B D
instance A D D where f = D -- CI.hs
-- MultiClassTest.hs http://pastebin.com/f1c481e8d import CI
a = f -- MultiClassTest.hs
I forgot to mention that constrain a :: D does leed to the same ghc messages.
ghc output: MultiClassTest.hs:4:4: No instance for (CI.A a D) arising from a use of `f' at MultiClassTest.hs:4:4 Possible fix: add an instance declaration for (CI.A a D) In the expression: f In the definition of `a': a = f Failed, modules loaded: CI.
Markus
Am Montag, 16. Februar 2009 15:40 schrieb Markus Barenhoff:
On Mon 16.02 15:28, Markus Barenhoff wrote:
Good Afternoon,
i'am trying to express something like this here:
-- CI.hs http://pastebin.com/f5b17cf57 {-# OPTIONS -XMultiParamTypeClasses #-}
module CI(D,f) where
class B b
class (B b) => A a b where f :: b
data D = D
instance B D
instance A D D where f = D -- CI.hs
-- MultiClassTest.hs http://pastebin.com/f1c481e8d import CI
a = f -- MultiClassTest.hs
I forgot to mention that constrain a :: D does leed to the same ghc messages.
ghc output: MultiClassTest.hs:4:4: No instance for (CI.A a D) arising from a use of `f' at MultiClassTest.hs:4:4 Possible fix: add an instance declaration for (CI.A a D) In the expression: f In the definition of `a': a = f Failed, modules loaded: CI.
GHC doesn't know that there's no other instance A a D besides the given instance A D D, so it doesn't know which instance to select (suppose there were an instance A Bool D where f = undefined, then it's clear that the instance to use can't be decided). Since class A is not exported, there can't be without changing CI, so one might think it would be reasonable to use the only existing instance of A and be done with it. However, GHC treats type classes with an open world assumption (i.e. doesn't select instances according to which instances it currently knows about, but what instances there potentially might be). To make f usable at all, you must make the type a somehow reachable from f (e.g. by adding a functional dependency: {-# LANGUAGE FunctionalDependencies #-} class (B b) => A a b | b -> a where f :: b -- the | b -> a part means that type b uniquely determines type a )
Markus
Cheers, Daniel
participants (2)
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Daniel Fischer -
Markus Barenhoff