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Today's Topics:
1. Re: Cabal: Upgrading to the latest version of library
(Yucheng Zhang)
2. Re: Haskell wants the type, but I only know the class.
(Antoine Latter)
3. Re: Haskell wants the type, but I only know the class.
(Amy de Buitl?ir)
4. Re: Haskell wants the type, but I only know the class.
(Antoine Latter)
5. Re: Haskell wants the type, but I only know the class.
(aditya siram)
6. Re: Haskell wants the type, but I only know the class.
(Amy de Buitl?ir)
7. Re: Haskell wants the type, but I only know the class.
(Amy de Buitl?ir)
8. Re: Haskell wants the type, but I only know the class.
(Brent Yorgey)
----------------------------------------------------------------------
Message: 1
Date: Fri, 4 Nov 2011 19:13:24 +0800
From: Yucheng Zhang <[email protected]>
Subject: Re: [Haskell-beginners] Cabal: Upgrading to the latest
version of library
To: [email protected]
Message-ID:
<CANgDTog2=Hk3M1bkE4nPbCN-rGKVsV5GGGb-=fyjrntgdws...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Fri, Nov 4, 2011 at 6:30 PM, Giovanni Tirloni <[email protected]> wrote:
> On Wed, Nov 2, 2011 at 1:05 AM, Rustom Mody <[email protected]> wrote:
>>
>> I wonder if you could throw some light on this?
>> Say you just have to use the 'sub-optimal' solution and cabal install some
>> package foo.
>> Later it appears in the rpm list and you can yum install it.
>> How do you now cabal uninstall foo?
>
> I don't know if this is the "right way" but it seems to work:
> ghc-pkg unregister <package>
> rm ?the contents from $HOME/.cabal/package/hackage.haskell.org/<package>
Maybe 'cab' on Hackage is the right thing to try.
--
Yucheng Zhang
------------------------------
Message: 2
Date: Fri, 4 Nov 2011 07:06:43 -0500
From: Antoine Latter <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: Amy de Buitl?ir <[email protected]>
Cc: [email protected]
Message-ID:
<CAKjSnQG4Hf1EMdCF=x7hsvkcn-dnwxyia5uutnt3ndzm94e...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Fri, Nov 4, 2011 at 4:40 AM, Amy de Buitl?ir <[email protected]> wrote:
> I could put a header in the file that tells me what the type of the object is.
> Then I would know at run-time (but not compile-time). Would that help?
>
A really simple and common way to do this would be using a sum-type:
data TypeOne = ...
data TypeTwo = ...
data TypeThree = ...
data AllOfTheTypes = T1 TypeOne
| T2 TypeTwo
| T3 TypeThree
instance Binary AllOfTheTypes where
put (T1 x) = putWord8 0; put x
put (T2 x) = putWord8 1; put x
put (T3 x) = putWord8 2; put x
get = do
tag <- getWord8
case tag of
0 -> T1 <$> get
1 -> T2 <$> get
2 -> T3 <$> get
Antoine
------------------------------
Message: 3
Date: Fri, 4 Nov 2011 12:51:23 +0000 (UTC)
From: Amy de Buitl?ir <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Antoine Latter <aslatter <at> gmail.com> writes:
> A really simple and common way to do this would be using a sum-type:
Here's what I'm trying to accomplish. I want to write a daemon that will cycle
through the files, load each one in turn, and invoke the doSomething method. I
would like to be able to allow people to use my daemon with any assortment of
new
types that they create, as long as that type implements the doSomething,
readThing, and writeThing methods.
I could make the sum-type approach work, but then users would have to modify the
type. And there might be dozens of different types in use, some provided by me,
and some provided by the user, so it would get messy. But I might have to go
with that approach.
------------------------------
Message: 4
Date: Fri, 4 Nov 2011 08:26:08 -0500
From: Antoine Latter <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: Amy de Buitl?ir <[email protected]>
Cc: [email protected]
Message-ID:
<cakjsnqgix-emad9o7xlxpewwpiynf6cyr01tdpapi2zjs5e...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Fri, Nov 4, 2011 at 7:51 AM, Amy de Buitl?ir <[email protected]> wrote:
> Antoine Latter <aslatter <at> gmail.com> writes:
>
>> A really simple and common way to do this would be using a sum-type:
>
> Here's what I'm trying to accomplish. I want to write a daemon that will cycle
> through the files, load each one in turn, and invoke the doSomething method. I
> would like to be able to allow people to use my daemon with any assortment of
> new
> types that they create, as long as that type implements the doSomething,
> readThing, and writeThing methods.
>
> I could make the sum-type approach work, but then users would have to modify
> the
> type. And there might be dozens of different types in use, some provided by
> me,
> and some provided by the user, so it would get messy. But I might have to go
> with that approach.
>
An executable, once compiled, cannot really learn about new types
without dynamically loading new object code. Which is possible but
often tricky.
You'd have this problem in other languages, too, except Java and .Net
have spent a lot of time working on the dynamic loading of new object
code into a running process (but you would still need to get the
.class file or .dll into a folder where the executable can read it).
Maybe I'm mis-understanding your requirements, though.
Antoine
------------------------------
Message: 5
Date: Fri, 4 Nov 2011 08:52:36 -0500
From: aditya siram <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: Antoine Latter <[email protected]>
Cc: Amy de Buitl?ir <[email protected]>, [email protected]
Message-ID:
<CAJrReyirWkF7K4=1gvttnipocgahljgmduggwfuxz2pwo1w...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
Perhaps this is what you're looking for:
{-# LANGUAGE ExistentialQuantification #-}
import Data.Binary
import Data.ByteString.Lazy as B ( readFile, writeFile )
import Codec.Compression.GZip ( compress, decompress )
data Thing = forall a. (Binary a, Show a, Eq a) => Thing a
instance Binary Thing where
get = get
put (Thing a) = put a
instance Show Thing where
show (Thing a) = show a
readThing :: FilePath -> IO Thing
readThing f = return . decode . decompress =<< B.readFile
f
writeThing :: FilePath -> Thing -> IO ()
writeThing f = B.writeFile f . compress . encode
doSomething :: Thing -> m Thing
doSomething = undefined
main = do
a <- readThing "file1.txt"
a' <- doSomething a
writeThing "file2.txt" a'
It compiles on my machine (GHC 7.2.1) but I haven't tested it. It uses the
existential quantification extension to constrain a datatype to the
typeclasses you mention. The caveat is that the only functions you can run
on "a" or "a'" are those defined by the Eq, Show and Binary typeclasses.
-deech
On Fri, Nov 4, 2011 at 8:26 AM, Antoine Latter <[email protected]> wrote:
> On Fri, Nov 4, 2011 at 7:51 AM, Amy de Buitl?ir <[email protected]>
> wrote:
> > Antoine Latter <aslatter <at> gmail.com> writes:
> >
> >> A really simple and common way to do this would be using a sum-type:
> >
> > Here's what I'm trying to accomplish. I want to write a daemon that will
> cycle
> > through the files, load each one in turn, and invoke the doSomething
> method. I
> > would like to be able to allow people to use my daemon with any
> assortment of new
> > types that they create, as long as that type implements the doSomething,
> > readThing, and writeThing methods.
> >
> > I could make the sum-type approach work, but then users would have to
> modify the
> > type. And there might be dozens of different types in use, some provided
> by me,
> > and some provided by the user, so it would get messy. But I might have
> to go
> > with that approach.
> >
>
> An executable, once compiled, cannot really learn about new types
> without dynamically loading new object code. Which is possible but
> often tricky.
>
> You'd have this problem in other languages, too, except Java and .Net
> have spent a lot of time working on the dynamic loading of new object
> code into a running process (but you would still need to get the
> .class file or .dll into a folder where the executable can read it).
>
> Maybe I'm mis-understanding your requirements, though.
>
> Antoine
>
> _______________________________________________
> Beginners mailing list
> [email protected]
> http://www.haskell.org/mailman/listinfo/beginners
>
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------------------------------
Message: 6
Date: Fri, 4 Nov 2011 13:56:12 +0000 (UTC)
From: Amy de Buitl?ir <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
Antoine Latter <aslatter <at> gmail.com> writes:
> An executable, once compiled, cannot really learn about new types
> without dynamically loading new object code. Which is possible but
> often tricky.
I don't mind if the user has to re-compile the code to add in their new types.
In addition to the daemon, I'm providing a library of functions that they can
use when implementing new types, so they'd have to re-compile anyway.
------------------------------
Message: 7
Date: Fri, 4 Nov 2011 13:58:35 +0000 (UTC)
From: Amy de Buitl?ir <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
aditya siram <aditya.siram <at> gmail.com> writes:
> Perhaps this is what you're looking for:
Thank you! I think that's what I'm looking for.
------------------------------
Message: 8
Date: Fri, 4 Nov 2011 12:22:23 -0400
From: Brent Yorgey <[email protected]>
Subject: Re: [Haskell-beginners] Haskell wants the type, but I only
know the class.
To: [email protected]
Message-ID: <[email protected]>
Content-Type: text/plain; charset=us-ascii
On Fri, Nov 04, 2011 at 08:52:36AM -0500, aditya siram wrote:
> Perhaps this is what you're looking for:
> {-# LANGUAGE ExistentialQuantification #-}
> import Data.Binary
> import Data.ByteString.Lazy as B ( readFile, writeFile )
> import Codec.Compression.GZip ( compress, decompress )
>
> data Thing = forall a. (Binary a, Show a, Eq a) => Thing a
>
> instance Binary Thing where
> get = get
> put (Thing a) = put a
>
> instance Show Thing where
> show (Thing a) = show a
>
> readThing :: FilePath -> IO Thing
> readThing f = return . decode . decompress =<< B.readFile
> f
>
> writeThing :: FilePath -> Thing -> IO ()
> writeThing f = B.writeFile f . compress . encode
>
> doSomething :: Thing -> m Thing
> doSomething = undefined
>
> main = do
> a <- readThing "file1.txt"
> a' <- doSomething a
> writeThing "file2.txt" a'
>
> It compiles on my machine (GHC 7.2.1) but I haven't tested it. It
> uses the
This will not work. The problem is that once you have a Thing you
cannot do anything with it, because you have no information about what
type is inside. In other words you cannot implement 'doSomething' to
do anything interesting at all. I am actually surprised that
'readThing' type checks -- I am not sure what type it thinks the read
thing has, or how it can guarantee that it satisfies the given
constraints.
I tried adding a Typeable constraint to Thing and using 'cast' to
recover the type, but that doesn't really work either. You would
really have to do something like changing the Binary instance for
Thing so that it also serializes/deserializes a TypeRep along with the
value, and then does some sort of unsafe cast after reading.
You may want to take a look at how xmonad handles this problem -- it
allows arbitrary user-extensible state and layouts, which it needs to
serialize and deserialize when restarting itself.
-Brent
------------------------------
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