On 10/19/2012 06:14 AM, AntC wrote:
Roman Cheplyaka <roma <at> ro-che.info> writes:
* Dmitry Vyal <akamaus <at> gmail.com> [2012-10-18 17:31:13+0400]
On 10/18/2012 03:20 PM, MigMit wrote:
Why do you need "ALike x", "BLike x" etc.? Why not just "Like u x"?
Hmm, looks like a nice idea. I tried it, unfortunately I can't cope
with compiler error messages:
tst.hs:32:15:
Context reduction stack overflow; size = 201
Use -fcontext-stack=N to increase stack size to N
Upcast a b
In the first argument of `(.)', namely `(upcast :: b -> a)'
In the expression: (upcast :: b -> a) . (upcast :: c -> b)
In the expression: (upcast :: b -> a) . (upcast :: c -> b) $ x
instance (Upcast a b, Upcast b c) => Upcast a c where
upcast = (upcast :: b -> a) . (upcast :: c -> b)
This is the offending instance. Remember, GHC only looks at the instance
head ("Upcast a c" here) when it decides which instance to use.
Roman
Hi Dmitry, looks like you've got the classic (show . read) difficulty. In
your "Upcast a c" instance, the compiler is trying to figure out the type of b.
You might think there's only one 'chain' to get from (say) type A to type D --
that is via Upcast A B to Upcast B C to Upcast C D; but there's also an
instance Upcast x x -- which means there could be any number of Upcast A A,
Upcast B B, etc links in the chain.
(And this doesn't count all the other possible instances that might be defined
in other modules -- for all the compiler knows at that point.)
The modern way to handle this is using type functions (aka type families aka
associated types), but I'm not sure how that would apply here. (And, for the
record, the old-fashioned way would use functional dependencies, as per the
Heterogenous Collections paper aka 'HList's).
AntC
Hello Antony,
do I understand you correctly, that the error message is the result of
compiler using depth first search of some kind when calculating
instances? Also can you please elaborate a bit more on using functional
dependencies for this problem? Upcast x y is not a function, it's a
relation, y can be upcasted to different x'es and different y's can be
upcasted to single x.
Dmitry
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