Hi! I would just skip the `Value` type and go with this type Object = Map String data Expr t where ExprBool :: Bool -> Expr Bool ExprBoolOr :: Expr Bool -> Expr Bool -> Expr Bool ExprText :: String -> Expr String ExprTextAppend :: Expr String -> Expr String -> Expr String ExprObject :: Object (Expr t) -> Expr (Object t) ExprLookup :: Expr (Object t) -> String -> Expr t eval :: Expr t -> t eval (ExprBool value) = value eval (ExprBoolOr lft rgt) = eval lft || eval rgt eval (ExprText value) = value eval (ExprTextAppend lft rgt) = eval lft <> eval rgt eval (ExprObject map) = eval <$> map eval (ExprLookup map fieldName) = eval map ! fieldName `ExprObject` constructs an object expression from a mapping that has expressions in the range. `ExprLookup` looks up a field name from an object expression. This is more general, because it lets you have expressions of arbitrary type in objects. But it gets harder if you need to restrict the types that can appear in objects. / Emil Den 2019-08-07 kl. 05:28, skrev Hilco Wijbenga:
Hi all,
I'm trying to implement an evaluator with GADTs. This is about as far as I've gotten: https://pastebin.com/XjWBzgw7 .
data Value = ValueBool Bool | ValueText String | ValueObject (Map String Value)
data Expr t where ExprBool :: Bool -> Expr Bool ExprBoolOr :: Expr Bool -> Expr Bool -> Expr Bool ExprText :: String -> Expr String ExprTextAppend :: Expr String -> Expr String -> Expr String ExprObject :: Map String Value -> String -> Expr Value
eval :: Expr t -> t eval (ExprBool value) = value eval (ExprBoolOr lft rgt) = eval lft || eval rgt eval (ExprText value) = value eval (ExprTextAppend lft rgt) = eval lft <> eval rgt eval (ExprObject map fieldName) = map ! fieldName
Note that the Value data type was just an attempt and not (necessarily) what I'm looking for. And I'm ignoring all error handling for the moment to keep the example small.
This compiles but obviously the object type is completely separate from the Expr Bool and Expr String types. Apparently, Type Familiies might help here? I could not find anything relevant that really explained it.
I've been thinking about changing ExprObject to something like
ExprObjectBool :: Map String Value -> String -> Expr Bool ExprObjectString :: Map String Value -> String -> Expr String ExprObjectObject :: Map String Value -> String -> Expr ???
but I can't figure out what ??? would be. And this would seem to explode if I add more "primitive" types, especially if I want to support lists and maps as well. (Maps and objects are very similar but not the same.)
How would I go about making the object type useful here? Or should I go back to plain "Expr" and just error out when trying to, e.g., "or" 2 Strings?
Cheers, Hilco _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.