Hello,
I continue my learning of "not so obvious" Haskell/GHC topics when
encountering problems in the code I write.
Below is a small example of an heterogeneous list, using GADT, inspired
from:
http://en.wikibooks.org/wiki/Haskell/Existentially_quantified_types#Example:_heterogeneous_lists
----------
{-# LANGUAGE GADTs #-}
data Box where
Box :: Eq s => s -> Box
instance Eq Box where
(Box s1) == (Box s2) = s1 == s2
----------
This code does not compile, because GHC is not sure that s1 and s2 have the
same type:
----------
Could not deduce (s ~ s1)
from the context (Eq s)
bound by a pattern with constructor
Box :: forall s. Eq s => s -> Box,
in an equation for `=='
at test_eq_GADT_before.hs:8:6-11
[and more lines...]
----------
(Do you confirm that tilde in s~s1 means "s has the same type as s1"? I have
not found this information explicitly in the Haskell stuff I have read).
I have googled on the topic, and found:
http://stackoverflow.com/questions/6028424/defining-eq-instance-for-haskell-gadts
>From the proposed solution, I wrote:
----------
{-# LANGUAGE GADTs #-}
{-# LANGUAGE DeriveDataTypeable #-}
import Data.Typeable
data Box where
Box :: (Typeable s, Eq s) => s -> Box
deriving Typeable
instance Eq Box where
(Box s1) == (Box s2) = Just s1 == cast s2
----------
that seems to work correctly:
----------
let a = Box (2::Int)
let b = Box (2::Float)
print $ a == b
print $ a == a
----------
Is this the right way to go? Is there any other solution?
Thanks,
TP
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