Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)

17 Sep 2009


      Hey Andy,

On Thu, Sep 17, 2009 at 15:40, Andy Gimblett  wrote:
...
Now, some of those algebraic data type types happen to be
enumerations; in this case, my idea is to list the constructors, with
the rule that each constructor's position in the list is the Int which
gets converted into that constructor.
...
class Enumerated a where
    constructors :: [a]
E.g. here's a type Bar with three constructors:
...
data Bar = X | Y | Z deriving (Show)
instance Enumerated Bar where
    constructors = [X, Y, Z]
(This is certainly ugly.  Any suggestions?)
|constructors| is expressible in SYB:

{-# LANGUAGE ScopedTypeVariables #-}
...
{-# LANGUAGE FlexibleContexts #-}
module Test where
import Data.Data
import Data.Generics.Aliases (extB)
-- | Construct the empty value for a datatype. For algebraic datatypes, the
-- leftmost constructor is chosen.
empty :: forall a. Data a => a
empty = general
      `extB` char
      `extB` int
      `extB` integer
      `extB` float
      `extB` double where
  -- Generic case
  general :: Data a => a
  general = fromConstrB empty (indexConstr (dataTypeOf general) 1)
-- Base cases
  char    = '\NUL'
  int     = 0      :: Int
  integer = 0      :: Integer
  float   = 0.0    :: Float
  double  = 0.0    :: Double
-- | Return a list of values of a datatype. Each value is one of the
possible
-- constructors of the datatype, populated with 'empty' values.
constrs :: forall a. Data a => [a]
constrs = general
      `extB` char
      `extB` int
      `extB` integer
      `extB` float
      `extB` double where
  -- Generic case
  general :: Data a => [a]
  general = map (fromConstrB empty)
              (dataTypeConstrs (dataTypeOf (unList general))) where
    unList :: Data a => [a] -> a
    unList = undefined
-- Base cases
  char    = "\NUL"
  int     = [0   :: Int]
  integer = [0   :: Integer]
  float   = [0.0 :: Float]
  double  = [0.0 :: Double]
|constrs| is similar to your |constructors|, but in this way you get it for
free for any datatype with a |Data| instance. Then I guess your |convert|
is:

convert :: forall a. Data a => Int -> Maybe a
...
convert n = let cs :: [a]
                cs = constrs
            in if (length cs > n) then (Just (cs !! n)) else Nothing
Note that ScopedTypeVariables are essential to typecheck this code.


Cheers,
Pedro

Re: [Haskell-cafe] Ambiguous type variable with subclass instance (also: is there a better way to do this?)

José Pedro Magalhães