[Haskell-cafe] Reverse Show instance
artyom.kazak at gmail.com
Thu May 19 23:11:51 CEST 2011
And I can declare an instance for (x, y) which does NOT implies (Show x):
instance Show (x, y) where
show _ = "I'm tuple! Hooray!"
Andrew Coppin <andrewcoppin at btinternet.com> писал(а) в своём письме Fri,
20 May 2011 00:08:27 +0300:
> Cannot deduce (Show x) from context (Show (x, y)).
> Cannot deduce (Show y) from context (Show (x, y)).
> Um... seriously?
> From Prelude, we have
> Show x, Show y => Show (x, y)
> So clearly it works in the forward direction. But apparently not in the
> reverse direction.
> Is this a bug or a feature? (I.e., is there some obscure possibility I
> haven't thought of which means that doing the reverse inference would be
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
More information about the Haskell-Cafe