Hi all, SML conveniently contains the type "exn" which is an instance of an "extensible data type". In other words, unlike normal data types that are "closed" (can't admit new constructors once defined), SML's exn type is "open," allowing programmers to keep adding new alternatives as often as they choose. Like normal datatypes, the elimination form for an extensible data type is a case statement (or match function). Friends have told me that Haskell doesn't have extensible data types. However, it seems fairly straightforward to code them up using type classes....though the solution I'm thinking of has a little bit of boilerplate I'd like to scrap (you have to define a new type declaration *and* an instance of a type class with a "match" method) and matching occurs through a string comparison (which can lead to silly programmer errors if there is accidentally a typo in the string). Anyway, it's possible with some thought I could come up with a better solution, but before worrying about it, I figured I'd ask if anybody else already has a package that does this. It seems like a pretty natural feature to want to have. Thanks in advance, Dave
princedpw:
Hi all,
SML conveniently contains the type "exn" which is an instance of an "extensible data type". In other words, unlike normal data types that are "closed" (can't admit new constructors once defined), SML's exn type is "open," allowing programmers to keep adding new alternatives as often as they choose. Like normal datatypes, the elimination form for an extensible data type is a case statement (or match function).
Friends have told me that Haskell doesn't have extensible data types. However, it seems fairly straightforward to code them up using type classes....though the solution I'm thinking of has a little bit of boilerplate I'd like to scrap (you have to define a new type declaration *and* an instance of a type class with a "match" method) and matching occurs through a string comparison (which can lead to silly programmer errors if there is accidentally a typo in the string).
You should probably use Typeable here, for the type matching, rather than a custom matcher. class Typeable a => Extensible a, this leads to a fairly straighforward extensible data type, where the open instance definition lets you add variants on the fly.
Anyway, it's possible with some thought I could come up with a better solution, but before worrying about it, I figured I'd ask if anybody else already has a package that does this. It seems like a pretty natural feature to want to have.
There's a number of ways to do this, including fully statically via type classes and existential types, or via the Dynamic type. Googling for "expression problem Haskell" will turn up some things. Some implementions of open data types in use can be found in xmonad, and the extensible exceptions proposal here, http://209.85.173.104/search?q=cache:xeXhle5KAqkJ:www.haskell.org/~simonmar/... -- Don
thanks for everyone's help -- it seems the simplest solution is to use
the Typeable class, existential types and type-safe cast.
Cheers,
Dave
On Sun, Jul 6, 2008 at 9:18 PM, Don Stewart
princedpw:
Hi all,
SML conveniently contains the type "exn" which is an instance of an "extensible data type". In other words, unlike normal data types that are "closed" (can't admit new constructors once defined), SML's exn type is "open," allowing programmers to keep adding new alternatives as often as they choose. Like normal datatypes, the elimination form for an extensible data type is a case statement (or match function).
Friends have told me that Haskell doesn't have extensible data types. However, it seems fairly straightforward to code them up using type classes....though the solution I'm thinking of has a little bit of boilerplate I'd like to scrap (you have to define a new type declaration *and* an instance of a type class with a "match" method) and matching occurs through a string comparison (which can lead to silly programmer errors if there is accidentally a typo in the string).
You should probably use Typeable here, for the type matching, rather than a custom matcher. class Typeable a => Extensible a, this leads to a fairly straighforward extensible data type, where the open instance definition lets you add variants on the fly.
Anyway, it's possible with some thought I could come up with a better solution, but before worrying about it, I figured I'd ask if anybody else already has a package that does this. It seems like a pretty natural feature to want to have.
There's a number of ways to do this, including fully statically via type classes and existential types, or via the Dynamic type.
Googling for "expression problem Haskell" will turn up some things. Some implementions of open data types in use can be found in xmonad, and the extensible exceptions proposal here,
http://209.85.173.104/search?q=cache:xeXhle5KAqkJ:www.haskell.org/~simonmar/...
-- Don
I like the approach the "Data Types a la Carte" paper takes to solve
this problem.
There's a small discussion here:
http://wadler.blogspot.com/2008/02/data-types-la-carte.html
Summary: if you model your data types as functors, typeclass machinery
lets you combine them into an extensible whole, while maintaining type
safety. You then create an "interpretation" class which allows data
to choose how it interacts with a particular computation.
The biggest weakness is that you need a type annotation at the point
of calling the "interpretation" function. An example (leaving out the
"library" bits):
class Functor a => EvalSimple a where
evalSimple :: a Int -> Int
instance (EvalSimple a, EvalSimple b) => EvalSimple (a :+: b) where
evalSimple (Inl a) = evalSimple a
evalSimple (Inr b) = evalSimple b
-- foldExpr :: Functor e => (e a -> a) -> Expr e -> a
-- eval :: EvalSimple e => Expr e -> Int
eval e = foldExpr evalSimple e
newtype Val a = Val Int -- "trivial" functor
instance Functor Val where fmap _ (Val x) = (Val x)
instance EvalSimple Val where evalSimple (Val x) = x
val x = inject (Val x)
-- inject :: a :<: e => a (Expr e) -> Expr e
-- val :: Val :<: e => Int -> Expr e
data Add a = Add a a -- "pair" functor
instance Functor Add where fmap f (Add a b) = Add (f a) (f b)
add a b = inject (Add a b)
instance EvalSimple Add where evalSimple (Add a b) = a + b
-- here is where we need the type annotation
sample :: Expr (Val :+: Add)
sample = add (add (val 1) (val 2)) (val 3)
sampleResult = eval sample -- is 6
On 7/6/08, David Walker
Hi all,
SML conveniently contains the type "exn" which is an instance of an "extensible data type". In other words, unlike normal data types that are "closed" (can't admit new constructors once defined), SML's exn type is "open," allowing programmers to keep adding new alternatives as often as they choose. Like normal datatypes, the elimination form for an extensible data type is a case statement (or match function).
Friends have told me that Haskell doesn't have extensible data types. However, it seems fairly straightforward to code them up using type classes....though the solution I'm thinking of has a little bit of boilerplate I'd like to scrap (you have to define a new type declaration *and* an instance of a type class with a "match" method) and matching occurs through a string comparison (which can lead to silly programmer errors if there is accidentally a typo in the string).
Anyway, it's possible with some thought I could come up with a better solution, but before worrying about it, I figured I'd ask if anybody else already has a package that does this. It seems like a pretty natural feature to want to have.
Thanks in advance, Dave _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
I prefer Bruno's approach, though. It allows meta-level type-checking of expressions and there's the possibility of closing the extension with a wrapper: (References: "Generics as a Library" and his PhD thesis) - GADT as a type class (or encode the type as it's fold): class Exp e where lit :: TyRange a => a -> e a plus :: e Int -> e Int -> e Int and :: e Bool -> e Bool -> e Bool - Notice we cannot construct an ill-typed expression, the Haskell type-checker complains. - |TyRange| is the class of indices: class TyRange a instance TyRange Int instance TyRange Bool - The behaviour is given by instances: newtype Eval a = Eval {eval :: a} instance Exp Eval where lit = Eval plus x y = Eval (eval x + eval y) and x y = Eval (eval x && eval y) Extension is easy: class Exp e => ExpIf e where ifOp :: TyRange a => e Bool -> e a -> e a -> e a instance ExpIf Eval where ifOp c t e = Eval (if (eval c) then (eval t) else (eval e)) class Exp e => ExpMult e where mult :: e Int -> e Int -> e Int instance ExpMult Eval where mult x y = Eval (eval x * eval y) - Adding new meta-level types is easy: instance TyRange a => TyRange [a] instance TyRange Char class Exp e => ExpConcat e where conc :: e [Char] -> e [Char] -> e [Char] instance ExpConcat Eval where conc x y = Eval (eval x ++ eval y) - Closing expressions is also easy: wrap around a type and provide new functions: data TyRange a => Wrap a = Wrap (forall e. (Exp e, ExpIf e, ExpMult e, ExpConcat e) => e a)
evalExp :: TyRange a => Wrap a -> a evalExp (Wrap x) = eval x
- Some expresions: Compare: exp1 :: Exp e => e Int exp1 = plus (lit 3) (lit 3) val1 = eval exp1 With: exp1' :: Eval Int exp1' = plus (lit 3) (lit 3) va1' = eval exp1
participants (4)
-
David Walker -
Don Stewart -
Pablo Nogueira -
Ryan Ingram