On Apr 12, 2006, at 4:09 PM, Scott Weeks wrote:
Or carry an instance in along with a type parameter, using
existentials or GADT.
Brandon Moore
Do you know of an example that would apply to my situation?
I think I neglected to state part of my problem. I am storing the
root nodes of btree indexes in a heterogeneous list using Typeable.
When they come out of the list they need to be unwrapped
I think this will clarify my situation because I've been doing a
poor job of explaining:
import Data.Dynamic
data Foo a = FVal a
deriving (Show, Typeable)
type FooStr = Foo String
type FooInt = Foo Integer
main = do
fooType <- getLine
fooVal <- getLine
let foo = toDyn (FVal fooVal)
fs = [foo]
(f:_) = fs
Just dynFoo = fromDynamic f
dostuff dynFoo
dostuff :: (Show a) => Foo a -> IO ()
dostuff (FVal x) = print x
This fails with:
Ambiguous type variable `a' in the constraints:
`Typeable a'
arising from use of `fromDynamic' at /Users/weeksie/workspace/
haskell/Main.hs:243:20-30
`Show a'
arising from use of `dostuff' at /Users/weeksie/workspace/haskell/
Main.hs:247:2-8
Probable fix: add a type signature that fixes these type
variable(s)
However, changing main:
main = do
fooType <- getLine
fooVal <- getLine
let foo = toDyn (FVal fooVal)
fs = [foo]
(f:_) = fs
Just dynFoo = fromDynamic f
if fooType == "str"
then dostuff (dynFoo::FooStr)
else dostuff (dynFoo::FooInt)
You are trying to assign two distinct types to dynFoo; that's a no-
no. You need to move the usage of the polymorphic function out of
the let so that the use at each distinct type doesn't get unified.
{-# OPTIONS -fglasgow-exts #-}
import Data.Dynamic
data Foo a = FVal a
deriving (Show, Typeable)
type FooStr = Foo String
type FooInt = Foo Integer
main = do
fooType <- getLine
fooVal <- getLine
let foo = toDyn (FVal fooVal)
fs = [foo]
(f:_) = fs
if fooType == "str"
then dostuff (fromDyn f undefined :: Foo String)
else dostuff (fromDyn f undefined :: Foo Int)
dostuff :: (Show a) => Foo a -> IO ()
dostuff (FVal x) = print x
BTW, this little example always stuffs a string into the FVal, so
trying to get anything else out will fail.
Rob Dockins
Speak softly and drive a Sherman tank.
Laugh hard; it's a long way to the bank.
-- TMBG
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