known, I know: class contexts and mutual recursion
Hi folks I just tripped over the "Contexts differ in length" error message. I know it's not a new problem, but I thought I'd enquire as to its status. For those of you who haven't seen it, here's an example, contrived but compact.
data Thing = Val Int | Grok Thing (Maybe Int -> Int)
eval :: Monad m => Thing -> m Int eval (Val i) = return i eval (Grok t f) = return (f (eval t))
My eval function compiles ok. See? The recursive call to eval targets the Maybe monad, so I get *Mmm> eval (Grok (Val 5) (maybe 0 (1 +))) :: Maybe Int Just 6 However, when I try to decompose eval as a pair of mutually recursive functions, namely
foo :: Monad m => Thing -> m Int foo (Val i) = return i foo (Grok t f) = return (goo t f)
goo :: Thing -> (Maybe Int -> Int) -> Int goo t f = f (foo t)
I get Mmm.lhs:15:1: Contexts differ in length When matching the contexts of the signatures for foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int goo :: Thing -> (Maybe Int -> Int) -> Int The signature contexts in a mutually recursive group should all be identical Poking about on the web, I got the impression that this was a known infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, an early-adopting colleague with 6.6 alleges that foo-goo is still poisonous. I'm wondering what the story is. I mean, is there some nasty problem lurking here which prevents the lifting of this peculiar restriction? I'm not in a panic about this. I have a workaround for the problem as I encountered it in practice. Mind you, it's the sort of thing that's likely to happen more often, the more you localise the effects you tend to use. In the above, goo doesn't throw exceptions; rather, because goo has a handler, it can offer a /local/ exception-throwing capability to foo. Curious Conor
On Wed, Nov 29, 2006 at 06:14:56PM +0000, Conor McBride wrote:
Mmm.lhs:15:1: Contexts differ in length When matching the contexts of the signatures for foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int goo :: Thing -> (Maybe Int -> Int) -> Int The signature contexts in a mutually recursive group should all be identical
Poking about on the web, I got the impression that this was a known infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, an early-adopting colleague with 6.6 alleges that foo-goo is still poisonous.
You can compile it with 6.6 if you use -fglasgow-exts. It's not clear to me whether this will always work, e.g. if you have higher rank types floating around, but if it does then we should add a hint to the error; Simon?
I'm wondering what the story is. I mean, is there some nasty problem lurking here which prevents the lifting of this peculiar restriction?
I don't know either, but this sounds like a good thing to bring up for Haskell' if no-one has already. Thanks Ian
On Mon, Dec 04, 2006 at 05:21:15PM +0000, Ian Lynagh wrote:
On Wed, Nov 29, 2006 at 06:14:56PM +0000, Conor McBride wrote:
Mmm.lhs:15:1: Contexts differ in length When matching the contexts of the signatures for foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int goo :: Thing -> (Maybe Int -> Int) -> Int The signature contexts in a mutually recursive group should all be identical
Poking about on the web, I got the impression that this was a known infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, an early-adopting colleague with 6.6 alleges that foo-goo is still poisonous.
You can compile it with 6.6 if you use -fglasgow-exts. It's not clear to me whether this will always work, e.g. if you have higher rank types floating around, but if it does then we should add a hint to the error; Simon?
I'm wondering what the story is. I mean, is there some nasty problem lurking here which prevents the lifting of this peculiar restriction?
I don't know either, but this sounds like a good thing to bring up for Haskell' if no-one has already.
Haskell 98 mandates this behaviour: s4.5.2 says the types of all functions in a dependency group must have the same context, even if they have explicit signatures. Haskell' ticket #65 (RelaxedDependencyAnalysis) proposes that dependencies on variables with explicit signatures should be ignored by the dependency analysis. With that change, if you provided signatures for foo or goo, or both, they'd be in different dependency groups and not required to have the same context. One could argue that this change should have been made when polymorphic recursion was permitted.
| > Mmm.lhs:15:1: | > Contexts differ in length | > When matching the contexts of the signatures for | > foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int | > goo :: Thing -> (Maybe Int -> Int) -> Int | > The signature contexts in a mutually recursive group should all be | > identical | > | > Poking about on the web, I got the impression that this was a known | > infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, | > an early-adopting colleague with 6.6 alleges that foo-goo is still | > poisonous. | | You can compile it with 6.6 if you use -fglasgow-exts. It's not clear to | me whether this will always work, e.g. if you have higher rank types | floating around, but if it does then we should add a hint to the error; | Simon? Yes, it'll work in 6.6, and it's even documented (at the end of the section on type-system extensions). I'll add a suggestion to the error message to use -fglasgow-exts Simon
Hello,
I just tripped over the "Contexts differ in length" error message. I know it's not a new problem, but I thought I'd enquire as to its status.
I have run into that a number of times. There aren't any technical
issues with solving it, in fact, depending on how one implements the
type checker, it is more work to implement this extra restriction
(Hugs and THIH don't impose this restriction). The basic idea is that
we check values with explicit signatures after we have finished type
inference. While we do type inference we may simply assume that the
values have their specified types.
It is proposed for fixing in Haskell' but I was surprised that it did
not seem to appear on the "definitely in" list (as far as I can
recall---I might be wrong). Hopefully the report will be fixed
though. Otherwise, as Ian mentioned, it works in GHC 6.6 with
-fglasgow-exts. Also, there should be no problems with higher-ranked
types etc.
-Iavor
On 11/29/06, Conor McBride
For those of you who haven't seen it, here's an example, contrived but compact.
data Thing = Val Int | Grok Thing (Maybe Int -> Int)
eval :: Monad m => Thing -> m Int eval (Val i) = return i eval (Grok t f) = return (f (eval t))
My eval function compiles ok. See? The recursive call to eval targets the Maybe monad, so I get
*Mmm> eval (Grok (Val 5) (maybe 0 (1 +))) :: Maybe Int Just 6
However, when I try to decompose eval as a pair of mutually recursive functions, namely
foo :: Monad m => Thing -> m Int foo (Val i) = return i foo (Grok t f) = return (goo t f)
goo :: Thing -> (Maybe Int -> Int) -> Int goo t f = f (foo t)
I get
Mmm.lhs:15:1: Contexts differ in length When matching the contexts of the signatures for foo :: forall (m :: * -> *). (Monad m) => Thing -> m Int goo :: Thing -> (Maybe Int -> Int) -> Int The signature contexts in a mutually recursive group should all be identical
Poking about on the web, I got the impression that this was a known infelicity in ghc 6.4 (which I'm using), due to be ironed out. However, an early-adopting colleague with 6.6 alleges that foo-goo is still poisonous. I'm wondering what the story is. I mean, is there some nasty problem lurking here which prevents the lifting of this peculiar restriction?
I'm not in a panic about this. I have a workaround for the problem as I encountered it in practice. Mind you, it's the sort of thing that's likely to happen more often, the more you localise the effects you tend to use. In the above, goo doesn't throw exceptions; rather, because goo has a handler, it can offer a /local/ exception-throwing capability to foo.
Curious
Conor
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Hi Iavor Diatchki wrote:
Hello,
I just tripped over the "Contexts differ in length" error message. I know it's not a new problem, but I thought I'd enquire as to its status.
I have run into that a number of times. There aren't any technical issues with solving it, in fact, depending on how one implements the type checker, it is more work to implement this extra restriction
Yes, that's what I would expect. It's an inevitable but not threatening consequence of polymorphic recursion. It rather surprised me that just telling the machine what my plan was (something I always do anyway) produced such an objection.
It is proposed for fixing in Haskell' but I was surprised that it did not seem to appear on the "definitely in" list (as far as I can recall---I might be wrong).
From what I can tell, it's "probably in". Strikes me as fairly essential. I don't expect artificial restrictions to be placed on the types of my programs when I am willing to make those types explicit.
Hopefully the report will be fixed though. Otherwise, as Ian mentioned, it works in GHC 6.6 with -fglasgow-exts. Also, there should be no problems with higher-ranked types etc.
Thanks for the tip, Ian. I guess I shouldn't put off installing 6.6 any longer. All the best Conor
participants (5)
-
Conor McBride -
Ian Lynagh -
Iavor Diatchki -
Ross Paterson -
Simon Peyton-Jones