[Haskell-cafe] Polymorphic algebraic type constructors

Ralf Laemmel Ralf.Laemmel at cwi.nl
Wed Jun 23 12:19:23 EDT 2004

Graham Klyne wrote:

> If I have a polymorphic algebraic type (T a) with several type 
> constructors, only one of which actually references the type 
> parameter, is there any way to express type conversion for the 
> type-parameter-independent constructors without actually mentioning 
> all the constructors?

Just for the record, using gunfold (from boilerplate paper II)
and cast (from boilerplate paper I), one can do this in a weird
way. The default equation becomes:

f g s = just (shallow_rebuild s)
-- instead of f g s = s

The shallow_rebuild function rebuilds the top-layer of a term.
Polymorphism is no problem here because the constructor is built from 
The dirty bit is "just" which goes from Maybe to Certainly.
Code attached for fun. This particular solution is perhaps too untyped,
but some bits of this solution were surprising for me.


-------------- next part --------------
{-# OPTIONS -fglasgow-exts #-}

import Data.Typeable
import Data.Generics

-- Representation of kids
kids x = gmapQ Kid x -- get all kids
type Kids = [Kid]
data Kid  = forall k. Typeable k => Kid k

-- Build term from a list of kids and the constructor 
fromConstrL :: Data a => Kids -> Constr -> Maybe a
fromConstrL l = unIDL . gunfold k z
  z c = IDL (Just c) l
  k (IDL Nothing _) = IDL Nothing undefined
  k (IDL (Just f) (Kid x:l)) = IDL f' l
    f' = case cast x of
          (Just x') -> Just (f x')
          _         -> Nothing

-- Helper datatype
data IDL x = IDL (Maybe x) Kids
unIDL (IDL mx _) = mx

-- Two sample datatypes
data A = A String deriving (Read, Show, Eq, Data, Typeable)
data B = B String deriving (Read, Show, Eq, Data, Typeable)

-- Mediate between two "left-equal" Either types
f :: (Data a, Data b, Show a, Read b)
  => (a->b) -> Either String a -> Either String b

f g (Right a)    = Right $ g a       -- conversion really needed
-- f g (Left  s) = Left s            -- unappreciated conversion
-- f g s         = s                 -- doesn't typecheck 
-- f g s         = deep_rebuild s    -- too expensive
f g s            = just (shallow_rebuild s) -- perhaps this is Ok?

-- Get rid of maybies
just = maybe (error "tried, but failed.") id

-- Just mentioned for completeness' sake
deep_rebuild :: (Show a, Read b) => a -> b
deep_rebuild = read . show

-- For the record: it's possible.
shallow_rebuild :: (Data a, Data b) => a -> Maybe b
shallow_rebuild a = b 
  b      = fromConstrL (kids a) constr
  constr = indexConstr (dataTypeOf b) (constrIndex (toConstr a))

-- Test cases
a2b (A s) = B s            -- silly conversion
t1 = f a2b (Left "x")      -- prints Left "x"
t2 = f a2b (Right (A "y")) -- prints Right (B "y")

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