Here is a curiosity of the Haskell update mechanism;
you can map one type to another using it.
Consider this datatype and function...
data Foo a = Foo { foo :: a
, bar :: Int
} deriving Show
up s t = s { foo = t }
Now, what should the type of the function 'up' be?
up :: Foo a -> a -> Foo a
** OR **
up :: Foo a -> b -> Foo b
Both Hugs and GHC give the later type,
ie. the updated object can have
a different type than the parent object.
Pages 26,27 of the Haskell98 report does not
say anything about polymorphic fields in records.
I was somewhat surprised by the type of up, but it
did come in useful in a piece of code I was writing..
What do people think about this? Does it surprise anyone
else, or is this a known design decision?
Andy
-------------------------------------------------------------
-- [To complete the example]
obj = Foo { foo = 1 :: Int
, bar = 99
}
main = print (obj -- plain object :: Foo Int
,up obj () -- updated object :: Foo ()
,obj { foo = True } -- updated object :: Foo Bool
)
