[Haskell-cafe] transform function for a GADT

[Ryan Ingram]

I'd add another parameter to the ExprType class and give an explicit
representation to your types.

data EType a where
    EInt :: EType Int
    EBool :: EType Bool

data TypeEq a b where Refl :: TypeEq x x

eqEType :: ExprType a -> ExprType b -> Maybe (TypeEq a b)
eqEType EInt EInt = Just Refl
eqEType EBool EBool = Just Refl
eqEType _ _ = Nothing

class ExprType a where
    toExpr :: a -> Expr a
    eType :: EType a

eTypeOf :: Expr a -> EType a
eTypeOf (Bin _) = EBool
eTypeOf (Num _) = EInt
eTypeOf (_ :+: _) = EInt
-- etc.

transform :: (ExprType a, ExprType b) => (Expr b -> Expr b) -> Expr a -> Expr a
transform = transform' eType

-- uses LANGUAGE PatternGuards
transform' :: EType b -> (Expr b -> Expr b) -> Expr a -> Expr a
transform' t f e | Just Refl <- eqEType t (eTypeOf e) = f e   -- this
is the magic line
transform' t f (Bin b) = Bin b
tranfsorm' t f (Num i) = Num i
transform' t f (e1 :+: e2) = transform' t f e1 :+: transform' t f e2
-- etc.

The magic line checks if the type of the expression matches the type
of the function, and if so, applies it.

  -- ryan

On Tue, Jul 27, 2010 at 8:28 AM, Ozgur Akgun <ozgurakgun at gmail.com> wrote:
> Café,
>
> I've tried several things already, but I am not including any of them for
> now.
> My question is, how would you define the 'transform' function for a GADT,
> say the one in the linked gist: http://gist.github.com/492364 (also attached
> to this e-mail)
>
> To be concise, I want 'transform' to apply the transformation function (its
> first parameter) to every immidiate child of its second parameter as long as
> the types match. Similar to what the 'tranform' function of Uniplate does
> for normal ADTs. (But just one level, so I guess it is more similar to the
> 'descend' of Uniplate. See
> http://hackage.haskell.org/packages/archive/uniplate/1.2.0.1/doc/html/Data-Generics-UniplateStr.html)
>
> I think I got closest to a sensible solution using multi-param type classes,
> and defining many instances for different combinations of ExprType's but
> still there were problems.
>
> Waiting for suggestions and/or insights.
>
> Best,
> Ozgur Akgun
>
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