Re: [Haskell-cafe] A Foldable binary search tree
Simon Peyton-Jones wrote: As others have pointed out, GHC allows you to put a context on any data constructor. I prefer this where syntax: But, in 6.8.x, you will need -XGADTS to permit this. 6.6.x permitted it anyway, arguably in error. http://hackage.haskell.org/trac/ghc/ticket/1901 Jules ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: [Haskell-cafe] A Foldable binary search tree
| data (Ord a) = BST a = Empty | BST (BST a) a (BST a) | | Experience has taught me to _never_ put class contexts on data | definitions. Correct. Haskell-98 contexts on data-type declarations are a mis-feature of Haskell, which I resisted at the time but failed to eliminate. As others have pointed out, GHC allows you to put a context on any data constructor. I prefer this where syntax: data BST a where Empty :: BST a BST :: Ord a = BST a - a - BST a - BST a but the other (existential-like) syntax also works: data BST a = Empty | Ord a = BST (BST a) a (BST a) The constructors have the signature they advertise. When you use BST as a constructor, you must supply an Ord context. But when you pattern-match on it, you *get* an Ord context. For example f :: a - BST a - Bool f x Empty = False f x (BST t1 y t2) = xy Note the absence of an Ord context on f. This works properly when combined with GADTs. The uniform rule is: you must supply the context when you use the constructor as a function, and you bring the context into scope when you pattern match on the constructor. Having this context on data constructors work right all the time is relatively new. Previously it really only worked for existentials; but now there are no restrictions on the contexts you write on data constructors. The Haskell 98 behaviour remains when you use the Haskell-98 syntax. | I would accept this pain if it meant I could write: | | insert :: a - BST a - BST a And so you can! But to get this, you'll have to write an Ord context on the Empty constructor too. Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A Foldable binary search tree
Hi Brad, Experience has taught me to _never_ put class contexts on data definitions. Now you can't write something as simple as Empty - you have to give it a class context. This is just plain annoying. With the class context in the BST definition, ghc gives no complaints when I evaluate Empty: In some circumstances, you need to give a type sig. For example using Hugs: Main Empty ERROR - Cannot find show function for: *** Expression : Empty *** Of type: BST a I guess GHC has enough defaulting to display this anyway. You'll also have the dreaded-evil-horrid monomorhpism restriction if you type. empty = Empty Thanks Neil ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A Foldable binary search tree
On Sun, 23 Dec 2007 06:42:52 -0500, Neil Mitchell [EMAIL PROTECTED] wrote: Hi Brad, Experience has taught me to _never_ put class contexts on data definitions. Now you can't write something as simple as Empty - you have to give it a class context. This is just plain annoying. With the class context in the BST definition, ghc gives no complaints when I evaluate Empty: In some circumstances, you need to give a type sig. For example using Hugs: Main Empty ERROR - Cannot find show function for: *** Expression : Empty *** Of type: BST a I guess GHC has enough defaulting to display this anyway. Sorry, forgot to mention that my BST derives Show. You'll also have the dreaded-evil-horrid monomorhpism restriction if you type. empty = Empty Thanks Neil More stuff for me to read about :-) Brad ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
