On 6/6/11 1:52 AM, Yitzchak Gale wrote:
You almost never want to use UndecidableInstances
when writing practical programs in Haskell.
When GHC tells you that you need them, it almost
always means that your types are poorly designed,
usually due to influence from previous experience
with OOP.
That's a bit unfair. There are many kinds of type-level hackery which
require UndecidableInstances but are (a) perfectly safe for practical
use, and (b) have nothing to do with OOP.
One particularly trivial example that comes to mind is:
newtype Mu f = Mu (f (Mu f))
instance Show (f (Mu f)) => Show (Mu f) where
show (Mu x) = "Mu (" ++ show x ++ ")"
-- Or however you'd like to show it
This can be solved for any f=F by,
instance Show a => Show (F a) where...
--
Live well,
~wren
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