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:... ---------- {-# 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-...
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