On Fri, May 24, 2013 at 10:41 AM, Niklas Hambüchen <m...@nh2.me> wrote:
> On Sat 25 May 2013 00:37:59 SGT, TP wrote: > > Is this the right way to go? Is there any other solution? > > I believe whether it's right or just depends on what you want to express. > > > Do you confirm that tilde in s~s1 means "s has the same type as s1"? > > It means: Both your s and s1 are "Eq"s but not necessarily the same one. > > No, it doesn't. s1 ~ s2 means the types are the same. ~ is the "equality constraint". http://www.haskell.org/ghc/docs/7.4.1/html/users_guide/equality-constraints.html To say that s1 and s2 are Eq's, but not necessarily the same one, we would write a constraint of the form: (Eq s1, Eq s2) => That is a completely different notion of equality than ~. Your first example allows that, so you could have one with an Int and > one with a String inside (both are Eqs). > > a = Box 1 > b = Box "hello" > > Now if that first code compiled, your code > > (Box s1) == (Box s2) = s1 == s2 > > would effectively perform > > ... = 1 == "hello" Nope. It would perform (Just 1) == (cast "hello"), which is completely possible, since (cast "hello") has the same type as (Just 1).
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