[Haskell-cafe] How to unify these three types with identical structures into one definition?

[Tobias Brandt]

Hello,

my adhoc solution would be to add a type parameter:

data ExprG a = Var String
   | Enclosed String a String
   | Prefix a a
   | Ternary a String a String a deriving Show


type Expr = ExprG (ExprG ())

type ExprL = ExprG [Expr]

Cheers,

Tobias


On 12/14/18 1:56 PM, ducis wrote:
> Hello,
>
> Say I have a certain type for parse trees:
> data Expr = Var String
>    | Enclosed String Expr String
>    | Prefix Expr Expr
>    | Ternary Expr String Expr String Expr
>    deriving (Show,Eq,Ord,Generic)--,Typeable)
>
> Then I want a new type where every occurence of "Expr" definition 
> changed to "[Expr]":
> data ExprL = VarL String
>    | EnclosedL String [Expr] String
>    | PrefixL [Expr] [Expr]
>    | TernaryL [Expr] String [Expr] String [Expr]
>    deriving (Show,Eq,Ord,Generic)
>
> Sometimes, I also want to use a DList instead of list.
> data ExprD = VarD String
>    | EnclosedD String (DList Expr) String
>    | PrefixD (DList Expr) (DList Expr)
>    | TernaryD (DList Expr) String (DList Expr) String (DList Expr)
>    deriving (Show,Eq,Ord,Generic)
>
> They have exactly the same structure, is there a way to unify the 
> three definitions into one?
> Furthurmore, is it possible to generalise the latter two for all 
> Functors ?
>
> Thanks,
> ducis
>
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