module Main where import Control.Monad import Control.Concurrent class Processor p where ready :: p b c → [b → c] instance Processor (→) where ready = repeat --instance Processor [b → c] where --ready = id newtype FunList b c = FunList [b → c] instance Processor FunList where ready (FunList fl) = fl broadcast :: Processor p ⇒ p b c → [b] → [c] broadcast p bs = bcast ps bs [] where ps = ready p bcast [] _ cs = cs bcast _ [] cs = cs bcast ps bs cs = let (cp,nps) = rotate [] ps (cb,nbs) = rotate [] bs in bcast nps nbs (cp cb:cs) rotate :: [a] → [a] → (a,[a]) rotate os (x:[]) = (x,os) rotate os (x:xs) = (x,xs) main :: IO () main = do let pid = id let ppm = FunList [ (x +) | x ← [1..10] ] print $ broadcast ppm [1..10] ------ Thank you for your answer! This comes close to solving the problem but in the last line of the above I want to be able to say: either
print $ broadcast id [1..10]
or
print $ broadcast [ (x +) | x ← [1..10] ] [1..10]
both need to be possible*.
So is there a way to make the FunList disappear completely?
Regards,
Cetin
P.S.: * broadcast is a dummy function, I need this for tidying up the
interface of a little experiment: http://corsis.blogspot.com/
2009/2/13 Benedikt Huber
Cetin Sert schrieb:
Hi,
class Processor a where ready :: (forall b c. a → [b → c])
instance Processor (b → c) where ready = repeat ... ------------------------------- Why can I not declare the above instances and always get: Hi Cetin, in your class declaration you state that a (Processor T) provides a function ready :: T -> [b -> c] so ready (t::T) has type (forall b c. [b -> c]), a list of functions from arbitrary types b to c.
The error messages tell you that e.g.
repeat (f :: t1 -> t2) has type (t1->t2) -> [t1->t2] and not the required type (t1->t2) -> [a -> b]
With your declarations,
head (ready negate) "hi" has to typecheck, that's probably not what you want.
Is there a way around this?
Maybe you meant
class Processor a where ready :: a b c -> [b -> c] instance Processor (->) where ready = repeat newtype FunList b c = FunList [b->c] instance Processor FunList where ready (FunList fl) = fl
I think the newtype FunList is neccessary here. benedikt
message.hs:229:10: Couldn't match expected type `b' against inferred type `b1' `b' is a rigid type variable bound by the instance declaration at message.hs:228:20 `b1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:19 Expected type: b -> c Inferred type: b1 -> c1 In the expression: repeat In the definition of `ready': ready = repeat
message.hs:229:10: Couldn't match expected type `c' against inferred type `c1' `c' is a rigid type variable bound by the instance declaration at message.hs:228:24 `c1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:21 Expected type: b -> c Inferred type: b1 -> c1 In the expression: repeat In the definition of `ready': ready = repeat
message.hs:232:10: Couldn't match expected type `b1' against inferred type `b' `b1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:19 `b' is a rigid type variable bound by the instance declaration at message.hs:231:20 Expected type: [b1 -> c] Inferred type: [b -> c1] In the expression: id In the definition of `ready': ready = id
message.hs:232:10: Couldn't match expected type `c1' against inferred type `c' `c1' is a rigid type variable bound by the type signature for `ready' at message.hs:226:21 `c' is a rigid type variable bound by the instance declaration at message.hs:231:24 Expected type: [b -> c1] Inferred type: [b1 -> c] In the expression: id In the definition of `ready': ready = id
Is there a way around this?
Regards, CS
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