Hi Cafe. I was wondering if it's possible to have automatic deriving for some classes for the Void type. This would be classes that require a parameter of the type in its functions. This should obviously be trivial as the definition can be undefined since there's no way to supply a value of the type. I guess the obvious one would be Show. data Nada deriving Show {* instance Show Nada where show _ = undefined *} Thoughts?
Lyndon Maydwell
writes: I was wondering if it's possible to have automatic deriving for some classes for the Void type. ...
Hi Lyndon, I'm struggling to see any use case for that. You'd 'achieve' `show (undefined :: Nada)` crashing your program at run- time; rather than failing to compile (if you didn't have the instance). Just bite the bullet and give your Void type a constructor. AntC
I guess the obvious one would be Show.
data Nada deriving Show
{* instance Show Nada where show _ = undefined *}
The use case I'm interested in is for exploratory programming, I define a
type-driven compositional data-flow, and use void types until I'm ready to
start filling in the gaps. Some data-dependencies may require class
instances, and for this I currently use a stand-alone instance with
undefined definitions.
Maybe there's a better way to go about this though?
On Tue, Oct 29, 2013 at 3:31 PM, AntC
Lyndon Maydwell
writes: I was wondering if it's possible to have automatic deriving for some classes for the Void type. ...
Hi Lyndon, I'm struggling to see any use case for that.
You'd 'achieve' `show (undefined :: Nada)` crashing your program at run- time; rather than failing to compile (if you didn't have the instance).
Just bite the bullet and give your Void type a constructor.
AntC
I guess the obvious one would be Show.
data Nada deriving Show
{* instance Show Nada where show _ = undefined *}
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On Tue, Oct 29, 2013 at 04:31:49AM +0000, AntC wrote:
Lyndon Maydwell
writes: I was wondering if it's possible to have automatic deriving for some classes for the Void type. ...
Hi Lyndon, I'm struggling to see any use case for that.
You'd 'achieve' `show (undefined :: Nada)` crashing your program at run- time; rather than failing to compile (if you didn't have the instance).
Just bite the bullet and give your Void type a constructor.
It was considered a worthwhile enough idea to add a Show instance to the "void" package http://hackage.haskell.org/package/void-0.6.1/docs/Data-Void.html (where the instance was written by hand, of course, not derived). Lyndon: perhaps import Data.Void newtype Nada = Nada Void deriving (Show) will suit you use case. Tom
Hey that's a great idea! On Tue, Oct 29, 2013 at 7:18 PM, Tom Ellis < tom-lists-haskell-cafe-2013@jaguarpaw.co.uk> wrote:
On Tue, Oct 29, 2013 at 04:31:49AM +0000, AntC wrote:
Lyndon Maydwell
writes: I was wondering if it's possible to have automatic deriving for some classes for the Void type. ...
Hi Lyndon, I'm struggling to see any use case for that.
You'd 'achieve' `show (undefined :: Nada)` crashing your program at run- time; rather than failing to compile (if you didn't have the instance).
Just bite the bullet and give your Void type a constructor.
It was considered a worthwhile enough idea to add a Show instance to the "void" package
http://hackage.haskell.org/package/void-0.6.1/docs/Data-Void.html
(where the instance was written by hand, of course, not derived).
Lyndon: perhaps
import Data.Void
newtype Nada = Nada Void deriving (Show)
will suit you use case.
Tom _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Hi.
On Tue, Oct 29, 2013 at 5:15 AM, Lyndon Maydwell
I was wondering if it's possible to have automatic deriving for some classes for the Void type. This would be classes that require a parameter of the type in its functions. This should obviously be trivial as the definition can be undefined since there's no way to supply a value of the type.
I guess the obvious one would be Show.
data Nada deriving Show
{* instance Show Nada where show _ = undefined *}
This actually works with StandaloneDeriving: {-# LANGUAGE StandaloneDeriving #-} data Nada deriving instance Show Nada deriving instance Eq Nada deriving instance Ord Nada And I agree, all of these are useful, in particular if Nada is used as an argument to a parameterized datatype. Trying to derive Read triggers a compiler bug with 7.6.3, but this seems to be fixed in HEAD. Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com
participants (4)
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Andres Löh -
AntC -
Lyndon Maydwell -
Tom Ellis