RE: [Haskell-cafe] Hiding functions
It's an explicit Haskell 98 design choice http://haskell.org/onlinereport/modules.html "5.6.2 Shadowing Prelude Names The rules about the Prelude have been cast so that it is possible to use Prelude names for nonstandard purposes; however, every module that does so must have an import declaration that makes this nonstandard usage explicit." It's a decision one could debate, but it was an explicit choice. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Lyle | Kopnicky | Sent: 12 August 2004 20:23 | To: Haskell Cafe | Subject: [Haskell-cafe] Hiding functions | | Hi all, | | I'm working on a program that uses my own brand of exceptions, and I | created two functions called 'throw' and 'catch'. In order for this to | work, I hide the Prelude 'catch' in my module, called 'Cont.hs'. Thus: | | module Cont where | import Prelude hiding (catch) | ... | throw = ... | catch = ... | | This works hunky-dory until I create another file that imports Cont. I | get a conflict when I use 'catch', so I have to hide the Prelude one again: | | import Prelude hiding (catch) | import Cont | ... | ... throw ... | ... catch ... | | So I'm a bit annoyed by this 'propagation' of hiding clauses. Then I | created a new file, that redefined throw: | | module ResumableExceptions where | import Cont hiding (throw) | import qualified Cont (throw) | ... | throw = ... Cont.throw ... | | Finally, I created a file using ResumableExceptions: | | import Cont hiding (throw) | import ResumableExceptions | ... | ... throw ... | | If I wanted to also use 'catch' I'd have to hide that from the Prelude | as well. | | I can't use type classes to solve this problem, because the types of the | two 'throw' functions are different. | | Perhaps I should just make up new names for these things, eh? But there | might be code that uses 'throw' and doesn't really care which one is | used, and it would be nice to just modify the import line and be done | with it. | | Any opinions? | | Thanks, | Lyle | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe
Simon, That makes good sense, as it's hard to read code that contains standard terms used in a nonstandard way. I was just concerned that the function name I wanted to use was already in the Prelude! Perhaps the Prelude 'catch', I reasoned, could be called 'catchIO', since it is specific to the IO monad, allowing people to write their own 'catch'. Or 'catch' could be a member of a type class, which could be overloaded for any new monad. But then I realized this argument could be made for practically every function in the Prelude. To dilute it with such abstraction would be a waste of resources. And there are loads of synonyms... I could use 'fling' and 'nab', 'punt' and 'snare', 'toss' and 'capture'.... Thanks for the point. - Lyle Simon Peyton-Jones wrote:
It's an explicit Haskell 98 design choice http://haskell.org/onlinereport/modules.html
"5.6.2 Shadowing Prelude Names
The rules about the Prelude have been cast so that it is possible to use Prelude names for nonstandard purposes; however, every module that does so must have an import declaration that makes this nonstandard usage explicit."
It's a decision one could debate, but it was an explicit choice.
Simon
| -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On Behalf Of Lyle | Kopnicky | Sent: 12 August 2004 20:23 | To: Haskell Cafe | Subject: [Haskell-cafe] Hiding functions | | Hi all, | | I'm working on a program that uses my own brand of exceptions, and I | created two functions called 'throw' and 'catch'. In order for this to | work, I hide the Prelude 'catch' in my module, called 'Cont.hs'. Thus: | | module Cont where | import Prelude hiding (catch) | ... | throw = ... | catch = ... | | This works hunky-dory until I create another file that imports Cont. I | get a conflict when I use 'catch', so I have to hide the Prelude one again: | | import Prelude hiding (catch) | import Cont | ... | ... throw ... | ... catch ... | | So I'm a bit annoyed by this 'propagation' of hiding clauses. Then I | created a new file, that redefined throw: | | module ResumableExceptions where | import Cont hiding (throw) | import qualified Cont (throw) | ... | throw = ... Cont.throw ... | | Finally, I created a file using ResumableExceptions: | | import Cont hiding (throw) | import ResumableExceptions | ... | ... throw ... | | If I wanted to also use 'catch' I'd have to hide that from the Prelude | as well. | | I can't use type classes to solve this problem, because the types of the | two 'throw' functions are different. | | Perhaps I should just make up new names for these things, eh? But there | might be code that uses 'throw' and doesn't really care which one is | used, and it would be nice to just modify the import line and be done | with it. | | Any opinions? | | Thanks, | Lyle | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe
On Fri, 13 Aug 2004 15:32:51 -0700, Lyle Kopnicky
Simon,
That makes good sense, as it's hard to read code that contains standard terms used in a nonstandard way. I was just concerned that the function name I wanted to use was already in the Prelude! Perhaps the Prelude 'catch', I reasoned, could be called 'catchIO', since it is specific to the IO monad, allowing people to write their own 'catch'. Or 'catch' could be a member of a type class, which could be overloaded for any new monad.
If you don't mind non-haskell98 code, you could always use the MonadError class in Control.Monad.Error: class Monad m => MonadError m e | m -> e where throwError :: e -> m a catchError :: m a -> (e -> m a) -> m a /Martin
Simon, It also appears that if I don't call 'catch' from my module, I can import both and not have a conflict. - Lyle Simon Peyton-Jones wrote:
It's an explicit Haskell 98 design choice http://haskell.org/onlinereport/modules.html
"5.6.2 Shadowing Prelude Names
The rules about the Prelude have been cast so that it is possible to use Prelude names for nonstandard purposes; however, every module that does so must have an import declaration that makes this nonstandard usage explicit."
It's a decision one could debate, but it was an explicit choice.
Simon
participants (3)
-
Lyle Kopnicky -
Martin Sjögren -
Simon Peyton-Jones