Can someone explain why the following doesn't work? Is there some other way to achieve the same effect (declaring a set of instances for pair-like types in one go)? Thanks. Dean swan(108)% cat Test1.hs {-# OPTIONS -fglasgow-exts #-} class R r where rId :: r -> String class (R r) => RT r t where rtId :: r -> t -> String data (R r1, R r2) => RPair r1 r2 = RPair r1 r2 instance (R r1, R r2) => R (RPair r1 r2) where rId (RPair r1 r2) = "RPair " ++ rId r1 ++ " " ++ rId r2 class TPair t t1 t2 where prj :: t -> (t1,t2) inj :: (t1,t2) -> t instance (RT r1 t1, RT r2 t2, TPair t t1 t2) => RT (RPair r1 r2) t where rtId (RPair r1 r2) t = "RT (RPair " ++ rtId r1 t1 ++ " " ++ rtId r2 t2 ++ ")" where (t1,t2) = prj t main = return () swan(109)% ghci Test1.hs ___ ___ _ / _ \ /\ /\/ __(_) / /_\// /_/ / / | | GHC Interactive, version 5.04.3, for Haskell 98. / /_\\/ __ / /___| | http://www.haskell.org/ghc/ \____/\/ /_/\____/|_| Type :? for help. Loading package base ... linking ... done. Loading package haskell98 ... linking ... done. Compiling Main ( Test1.hs, interpreted ) Test1.hs:19: Could not deduce (RT r2 t21, RT r1 t11) from the context (RT (RPair r1 r2) t, RT r1 t1, RT r2 t2, TPair t t1 t2, R (RPair r1 r2)) Probable fix: Add (RT r2 t21, RT r1 t11) to the class or instance method `rtId' arising from use of `rtId' at Test1.hs:19 In the first argument of `(++)', namely `rtId r2 t2' In the second argument of `(++)', namely `(rtId r2 t2) ++ ")"' Test1.hs:20: Could not deduce (TPair t t11 t21) from the context (RT (RPair r1 r2) t, RT r1 t1, RT r2 t2, TPair t t1 t2, R (RPair r1 r2)) Probable fix: Add (TPair t t11 t21) to the class or instance method `rtId' arising from use of `prj' at Test1.hs:20 In a pattern binding: prj t Failed, modules loaded: none. Prelude>
Dean Herington wrote:
Can someone explain why the following doesn't work?
{-# OPTIONS -fglasgow-exts #-}
class R r where rId :: r -> String
class (R r) => RT r t where rtId :: r -> t -> String
data RPair r1 r2 = RPair r1 r2
instance (R r1, R r2) => R (RPair r1 r2) where rId (RPair r1 r2) = "RPair " ++ rId r1 ++ " " ++ rId r2
class TPair t t1 t2 where prj :: t -> (t1,t2) inj :: (t1,t2) -> t
instance (RT r1 t1, RT r2 t2, TPair t t1 t2) => RT (RPair r1 r2) t where rtId (RPair r1 r2) t = "RT (RPair " ++ rtId r1 t1 ++ " " ++ rtId r2 t2 ++")" where (t1,t2) = prj t
You need a functional dependency. For example: class TPair t t1 t2 | t->t1 t2 where prj :: t -> (t1,t2) inj :: (t1,t2) -> t with this definition, the typechecker is satisfied. Without the dependency, the compiler assumes that there may be several instances: TPair t t1 t2 and TPair t t1' t2' You claimed that RT r1 t1 and RT r2 t2 holds. But you didn't promise that RT r1 t1' and RT r2 t2' will also hold. In other words, (RT r1 t1, RT r2 t2, TPair t t1 t2) reads as (exists t1 t2. RT r1 t1, RT r2 t2, TPair t t1 t2) rather than (forall t1 t2. RT r1 t1, RT r2 t2, TPair t t1 t2) (which you need to guarantee that the definition of (t1,t2) = prj t can be typechecked). Notice that forall is _inside_ of parentheses, on the assumption side (the negative side).
There's another possible fix which makes use of scoped variables. instance (RT r1 t1, RT r2 t2, TPair t t1 t2) => RT (RPair r1 r2) t where rtId (RPair r1 r2) t = "RT (RPair " ++ rtId r1 t1 ++ " " ++ rtId r2 t2 ++")" where (t1::t1,t2::t2) = prj t ^^^^^^^^^^^^^^ scoped variables Martin
Dean Herington wrote:
Can someone explain why the following doesn't work?
{-# OPTIONS -fglasgow-exts #-}
class R r where rId :: r -> String
class (R r) => RT r t where rtId :: r -> t -> String
data RPair r1 r2 = RPair r1 r2
instance (R r1, R r2) => R (RPair r1 r2) where rId (RPair r1 r2) = "RPair " ++ rId r1 ++ " " ++ rId r2
class TPair t t1 t2 where prj :: t -> (t1,t2) inj :: (t1,t2) -> t
instance (RT r1 t1, RT r2 t2, TPair t t1 t2) => RT (RPair r1 r2) t where rtId (RPair r1 r2) t = "RT (RPair " ++ rtId r1 t1 ++ " " ++ rtId r2 t2 ++")" where (t1,t2) = prj t
You need a functional dependency. For example:
class TPair t t1 t2 | t->t1 t2 where prj :: t -> (t1,t2) inj :: (t1,t2) -> t
with this definition, the typechecker is satisfied.
Without the dependency, the compiler assumes that there may be several instances: TPair t t1 t2 and TPair t t1' t2'
You claimed that RT r1 t1 and RT r2 t2 holds. But you didn't promise that RT r1 t1' and RT r2 t2' will also hold. In other words, (RT r1 t1, RT r2 t2, TPair t t1 t2) reads as (exists t1 t2. RT r1 t1, RT r2 t2, TPair t t1 t2) rather than (forall t1 t2. RT r1 t1, RT r2 t2, TPair t t1 t2) (which you need to guarantee that the definition of (t1,t2) = prj t can be typechecked). Notice that forall is _inside_ of parentheses, on the assumption side (the negative side).
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