On Thu, Sep 17, 2009 at 15:40, Andy Gimblett <haskell@gimbo.org.uk> 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]

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