On Fri, May 24, 2013 at 9:37 AM, TP
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).
Yes. http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/equality-constraints.... Is this (Typeable) the right way to go? Is there any other solution?
Using typeable is a perfectly reasonable way to go.