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 incorrect?)
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@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 incorrect?)
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On 19/05/2011 10:11 PM, Artyom Kazak wrote:
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!"
Ah. So it's a feature. Fortunately I refactored the program where this came up, so it's no longer an issue. I just wanted to see whether or not it was a bug. PS. Wouldn't such an instance require FlexibleContexts or something?
If you have nested type, then it usually makes sense to have Show defined for the inside types, too, but it's not a requirement. Technically, only when you call 'show' for something in the data type you are defining Show for, *then* you need a Show instance defined for that inside-type. On Fri, May 20, 2011 at 12:15 AM, Andrew Coppin <andrewcoppin@btinternet.com
wrote:
On 19/05/2011 10:11 PM, Artyom Kazak wrote:
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!"
Ah. So it's a feature.
Fortunately I refactored the program where this came up, so it's no longer an issue. I just wanted to see whether or not it was a bug.
PS. Wouldn't such an instance require FlexibleContexts or something?
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-- Markus Läll
On Thursday 19 May 2011 23:15:06, Andrew Coppin wrote:
On 19/05/2011 10:11 PM, Artyom Kazak wrote:
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!"
Ah. So it's a feature.
Fortunately I refactored the program where this came up, so it's no longer an issue. I just wanted to see whether or not it was a bug.
PS. Wouldn't such an instance require FlexibleContexts or something?
No, the instance head is a type constructor applied to two distinct type variables, it's a perfectly valid H98 instance declaration. It won't be accepted though, because there is already instance (Show x, Show y) => Show (x,y) where ... I think basically the only way to make GHC accept the above instance involves NoImplicitPrelude (but I may be wrong).
Think of it this way: -- Here is some data representing the typeclass 'Show' data ShowDict a = ShowD (a -> String) show :: ShowDict a -> a -> String show (ShowD f) a = f a -- Here's a sample implementation for Strings showString :: ShowDict String showString = ShowD (\s -> "\"" ++ escape s ++ "\"") where escape = concatMap escapeChar escapeChar '\\' = "\\\\" escapeChar '"' = "\\\"" escapeChar c = [c] -- Here's an implementation for pairs that uses the implementation for each piece of the pair showPair :: ShowDict a -> ShowDict b -> ShowDict (a,b) showPair (ShowD sa) (ShowD sb) = ShowD (\(a,b) -> "(" ++ sa a ++ ", " ++ sb b ++ ")") -- Here is what you are asking for implementMe :: ShowDict (a,b) -> ShowDict a implementMe = ???? On Thu, May 19, 2011 at 2:08 PM, Andrew Coppin <andrewcoppin@btinternet.com>wrote:
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 incorrect?)
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participants (5)
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Andrew Coppin -
Artyom Kazak -
Daniel Fischer -
Markus Läll -
Ryan Ingram