Hi all,

{-# LANGUAGE TypeFamilies #-}

data CB = CB deriving (Show)

data CO = CO deriving (Show)

data CBParseResult = CBParseResult deriving (Show)

data COParseResult = COParseResult deriving (Show)

class ParseResult a where

type EntityType a :: *

toEntity :: a -> EntityType a

instance ParseResult CBParseResult where

type EntityType CBParseResult = CB

toEntity result = CB

instance ParseResult COParseResult where

type EntityType COParseResult = CO

toEntity result = CO

getParseResult :: (ParseResult r) => String -> IO r

getParseResult i = do

if someCond

then return CBParseResult

else return COParseResult

where

someCond = null i

Couldn't match expected type ‘r’ with actual type ‘COParseResult’

‘r’ is a rigid type variable bound by

the type signature for

getParseResult :: ParseResult r => String -> IO r

at Parse.hs:19:19

Relevant bindings include

getParseResult :: String -> IO r (bound at Parse.hs:20:1)

In the first argument of ‘return’, namely ‘COParseResult’

In the expression: return COParseResult

What I want to express is getParseResult to be able to return any instances of ParseResult type class. But what I understand from the error is that r has been specialized to CBParseResult, but in the next expression I'm returning COParseResult. Is my understanding correct? And, How would I express what I described?

Thanks,

Marco