Dear,
Recently I was wrapping something in a typeclass (you may argue the
reason for the typeclass, I will admit that's no longer needed, yet the
compile error still begs questioning). Basically if you have only one
global instance of a typeclass (so there is no overlapping), GHC will
still complain about overlapping. Is there any reason this is so or is
this a remnant of more complicated overlapping situations?
The sample code is:
{-# OPTIONS_GHC -fglasgow-exts #-}
module Main where
data Located a = L a
class Locatable a where
value :: Located a -> a
wrap :: a -> Located a
instance Locatable a where
value (L a) = a
wrap a = L a
main :: IO ()
main = do
print . value . wrap $ 1
-- Illegal instance declaration for `Locatable a'
-- (There must be at least one non-type-variable in the instance head
-- Use -fallow-undecidable-instances to permit this)
-- In the instance declaration for `Locatable a'
Christophe(vincenz)
--
Christophe Poucet
Ph.D. Student
Phone:+32 16 28 87 20
E-mail: Christophe (dot) Poucet (at) imec (dot) be
Website: http://notvincenz.com/
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