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