[Haskell-cafe] Deriving instances with GADTs
Hi all,
I've been writing a DSL to describe securities orders, and after a lot
of help from the kind folk of this list and #haskell have come up with
the following implementation, using generalised algebraic data types:
https://gist.github.com/1124621
Elsewhere in my application, I make use of the order type defined
therein in the following newtype declaration:
newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]}
deriving (Eq, Show)
However, the 'deriving' clause here fails:
src/Simulation/OrderList.hs:9:82:
No instance for (Eq (Order s Limit))
arising from the 'deriving' clause of a data type declaration
at src/Simulation/OrderList.hs:9:82-83
src/Simulation/OrderList.hs:9:86:
No instance for (Show (Order s Limit))
arising from the 'deriving' clause of a data type declaration
at src/Simulation/OrderList.hs:9:86-89
I don't fully understand this - the error is correct that there is no
instance of either Eq or Show for (Order s Limit), however, instances
are defined for Order Buy Limit and Order Sell Limit, and since these
are the only possible types that a value can be constructed with (the
type constructor is 'closed' over these types in some sense I guess),
it seems to me that this should provide enough information to derive
the Eq and Show instances. Am I making unreasonable expectations of
ghci's instance-deriving mechanism here, or missing something obvious?
Many thanks in advance,
Tim
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[Haskell-cafe] library-profiling default
Hi Is there a good reason that the default for library-profiling in .cabal/config is set to False? It seems a lot of people hit the problem of trying to profile for the first time, finding it doesn't work because profiling libraries haven't been installed, then they have to walk the dependencies reinstalling everything. Is there a major cost or problem with just defaulting this to True? Apologies if this is answered elsewhere, I saw various discussions on why it is difficult to automatically build required libs with profiling on demand, but nothing that discussed changing the default so that they are always built. Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Deriving instances with GADTs
Hi Tim,
On Thu, Aug 4, 2011 at 08:57, Tim Cowlishaw t...@timcowlishaw.co.uk wrote:
Hi all,
I've been writing a DSL to describe securities orders, and after a lot
of help from the kind folk of this list and #haskell have come up with
the following implementation, using generalised algebraic data types:
https://gist.github.com/1124621
Elsewhere in my application, I make use of the order type defined
therein in the following newtype declaration:
newtype OrderListLevel s = OrderListLevel {orders :: [Order s Limit]}
deriving (Eq, Show)
However, the 'deriving' clause here fails:
src/Simulation/OrderList.hs:9:82:
No instance for (Eq (Order s Limit))
arising from the 'deriving' clause of a data type declaration
at src/Simulation/OrderList.hs:9:82-83
src/Simulation/OrderList.hs:9:86:
No instance for (Show (Order s Limit))
arising from the 'deriving' clause of a data type declaration
at src/Simulation/OrderList.hs:9:86-89
I don't fully understand this - the error is correct that there is no
instance of either Eq or Show for (Order s Limit), however, instances
are defined for Order Buy Limit and Order Sell Limit, and since these
are the only possible types that a value can be constructed with (the
type constructor is 'closed' over these types in some sense I guess),
it seems to me that this should provide enough information to derive
the Eq and Show instances. Am I making unreasonable expectations of
ghci's instance-deriving mechanism here, or missing something obvious?
Here you seem to be using newtype deriving in particular, which behaves
differently from standard deriving. Compiling with -ddump-deriv will show
you the instances GHC is generating, which can help in debugging.
Note however that deriving instances for GADTs is not trivial, in general.
In particular, you should not assume that GHC knows that `s` can only be
instantiated with `Buy` and `Sell` since (because we lack a proper kind
system) nothing prevents you from later using, say, `Order Int Limit`
somewhere.
I describe the issue in more detail in the paper:
José Pedro Magalhães and Johan Jeuring. Generic Programming for Indexed
Datatypes.
Color pdf: http://dreixel.net/research/pdf/gpid.pdf
Greyscale pdf: http://dreixel.net/research/pdf/gpid_nocolor.pdf
Cheers,
Pedro
Many thanks in advance,
Tim
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Re: [Haskell-cafe] library-profiling default
On 4 August 2011 17:06, Tom Doris tomdo...@gmail.com wrote: Hi Is there a good reason that the default for library-profiling in .cabal/config is set to False? It seems a lot of people hit the problem of trying to profile for the first time, finding it doesn't work because profiling libraries haven't been installed, then they have to walk the dependencies reinstalling everything. Is there a major cost or problem with just defaulting this to True? Apologies if this is answered elsewhere, I saw various discussions on why it is difficult to automatically build required libs with profiling on demand, but nothing that discussed changing the default so that they are always built. My guess is that this way, it reduces the build-time for users that are just using cabal-install to get Haskell programs (darcs, pandoc, etc.) or using Haskell for assignments but with no need/interest in profiling. -- Ivan Lazar Miljenovic ivan.miljeno...@gmail.com IvanMiljenovic.wordpress.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Deriving instances with GADTs
2011/8/4 José Pedro Magalhães j...@cs.uu.nl: Here you seem to be using newtype deriving in particular, which behaves differently from standard deriving. Compiling with -ddump-deriv will show you the instances GHC is generating, which can help in debugging. Aah - this is very useful, thanks! Note however that deriving instances for GADTs is not trivial, in general. In particular, you should not assume that GHC knows that `s` can only be instantiated with `Buy` and `Sell` since (because we lack a proper kind system) nothing prevents you from later using, say, `Order Int Limit` somewhere. Aah - this is something like what I suspected (The type signature for OrderListLevel doesn't preclude eg OrderListLevel Int which would need an instance of (Eq|Show) for Order Int Limit, which does not exist. I describe the issue in more detail in the paper: José Pedro Magalhães and Johan Jeuring. Generic Programming for Indexed Datatypes. Color pdf: http://dreixel.net/research/pdf/gpid.pdf Greyscale pdf: http://dreixel.net/research/pdf/gpid_nocolor.pdf Oh, brilliant, thank you! I'll take a look now. Thanks, Tim ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] One place available at CamHac
Hi Folks, There is a place available at CamHac next Friday-Sunday (12-14 August). First come first served! For instructions on how to register, see the wiki page: http://www.haskell.org/haskellwiki/CamHac#Registration Also a reminder, if you have already registered but won't be able to make it, please let us know so that we can reallocate your place. Cheers, Simon ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] weird type signature in Arrow Notation
On Tue, Aug 02, 2011 at 05:08:33PM -0400, bob zhang wrote: hi, all testB :: (ArrowChoice t1, Num a1, Num a) = (a - a1 - t2) - t1 a t3 - t1 a1 t3 - t1 (a, a1) t testB f g h = proc (x,y) - do if (f x y)then g - x + 1 else h - y + 2 it's very strange that the type of _f_ is (a-a1-t2) which I thought should be a - a1 - Bool, btw, is there any way to get the output of preprocessing using -XArrow extensions, Thanks a lot best, bob Congratulations, you have definitely found a GHC bug! Note there are actually two things wrong with testB's type signature: first, t2 ought to be Bool, as you note. But even worse, notice that the return type of the result arrow, t, has nothing to do with any of the other types! This means that we can use testB along with the (-) instance for Arrow to construct elements of arbitrary types: ghci let anythingYouWant = testB (\x y - False) (const 3) (const 2) (2,2) ghci :t anythingYouWant anythingYouWant :: t ghci anythingYouWant :: Integer 2 ghci anythingYouWant :: Int 2 ghci anythingYouWant :: Double 1.0e-323 ghci anythingYouWant :: Char '\STX' ghci (anythingYouWant :: (Double - Double) - [Double]) sqrt [ [1]17391 segmentation fault ghci whoops! I'm using GHC 7.0.3, but Daniel Wagner and I also tried it (with the same results) on GHC 7.2.0rc1 and GHC HEAD. I wasn't able to find a ticket for this on the GHC bug tracker, I guess we should file one! I tried to find a way to get the output of preprocessing using -XArrow but wasn't able to find one (other than -ddump-ds which gives you the unoptimized *GHC core* output, which is quite hard to read). -Brent ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] weird type signature in Arrow Notation
here is a reduced program that still segfaults:
{-# LANGUAGE Arrows #-}
import Control.Arrow
main :: IO ()
main = print segfault
segfault :: [()]
segfault = anythingYouWant ()
anythingYouWant :: a
anythingYouWant = testB False (const ()) ()
testB :: ArrowChoice arrow
= bool - arrow () () - arrow () anything
testB bool arrow =
proc () -
do if bool then arrow - ()
else arrow - ()
Sebastian
On Fri, Aug 5, 2011 at 6:20 AM, Brent Yorgey byor...@seas.upenn.edu wrote:
On Tue, Aug 02, 2011 at 05:08:33PM -0400, bob zhang wrote:
hi, all
testB :: (ArrowChoice t1, Num a1, Num a) = (a - a1 - t2) - t1 a t3
- t1 a1 t3 - t1 (a, a1) t
testB f g h = proc (x,y) - do
if (f x y)then g - x + 1 else h - y + 2
it's very strange that the type of _f_ is (a-a1-t2) which I thought
should be a - a1 - Bool,
btw, is there any way to get the output of preprocessing using -XArrow
extensions,
Thanks a lot
best, bob
Congratulations, you have definitely found a GHC bug! Note there are
actually two things wrong with testB's type signature: first, t2 ought
to be Bool, as you note. But even worse, notice that the return type
of the result arrow, t, has nothing to do with any of the other types!
This means that we can use testB along with the (-) instance for
Arrow to construct elements of arbitrary types:
ghci let anythingYouWant = testB (\x y - False) (const 3) (const 2)
(2,2)
ghci :t anythingYouWant
anythingYouWant :: t
ghci anythingYouWant :: Integer
2
ghci anythingYouWant :: Int
2
ghci anythingYouWant :: Double
1.0e-323
ghci anythingYouWant :: Char
'\STX'
ghci (anythingYouWant :: (Double - Double) - [Double]) sqrt
[
[1]17391 segmentation fault ghci
whoops!
I'm using GHC 7.0.3, but Daniel Wagner and I also tried it (with the
same results) on GHC 7.2.0rc1 and GHC HEAD.
I wasn't able to find a ticket for this on the GHC bug tracker, I
guess we should file one!
I tried to find a way to get the output of preprocessing using -XArrow
but wasn't able to find one (other than -ddump-ds which gives you the
unoptimized *GHC core* output, which is quite hard to read).
-Brent
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Re: [Haskell-cafe] weird type signature in Arrow Notation
I created a ticket with a slightly further simplified program:
http://hackage.haskell.org/trac/ghc/ticket/5380
On Fri, Aug 5, 2011 at 10:10 AM, Sebastian Fischer fisc...@nii.ac.jpwrote:
here is a reduced program that still segfaults:
{-# LANGUAGE Arrows #-}
import Control.Arrow
main :: IO ()
main = print segfault
segfault :: [()]
segfault = anythingYouWant ()
anythingYouWant :: a
anythingYouWant = testB False (const ()) ()
testB :: ArrowChoice arrow
= bool - arrow () () - arrow () anything
testB bool arrow =
proc () -
do if bool then arrow - ()
else arrow - ()
Sebastian
On Fri, Aug 5, 2011 at 6:20 AM, Brent Yorgey byor...@seas.upenn.eduwrote:
On Tue, Aug 02, 2011 at 05:08:33PM -0400, bob zhang wrote:
hi, all
testB :: (ArrowChoice t1, Num a1, Num a) = (a - a1 - t2) - t1 a t3
- t1 a1 t3 - t1 (a, a1) t
testB f g h = proc (x,y) - do
if (f x y)then g - x + 1 else h - y + 2
it's very strange that the type of _f_ is (a-a1-t2) which I thought
should be a - a1 - Bool,
btw, is there any way to get the output of preprocessing using -XArrow
extensions,
Thanks a lot
best, bob
Congratulations, you have definitely found a GHC bug! Note there are
actually two things wrong with testB's type signature: first, t2 ought
to be Bool, as you note. But even worse, notice that the return type
of the result arrow, t, has nothing to do with any of the other types!
This means that we can use testB along with the (-) instance for
Arrow to construct elements of arbitrary types:
ghci let anythingYouWant = testB (\x y - False) (const 3) (const 2)
(2,2)
ghci :t anythingYouWant
anythingYouWant :: t
ghci anythingYouWant :: Integer
2
ghci anythingYouWant :: Int
2
ghci anythingYouWant :: Double
1.0e-323
ghci anythingYouWant :: Char
'\STX'
ghci (anythingYouWant :: (Double - Double) - [Double]) sqrt
[
[1]17391 segmentation fault ghci
whoops!
I'm using GHC 7.0.3, but Daniel Wagner and I also tried it (with the
same results) on GHC 7.2.0rc1 and GHC HEAD.
I wasn't able to find a ticket for this on the GHC bug tracker, I
guess we should file one!
I tried to find a way to get the output of preprocessing using -XArrow
but wasn't able to find one (other than -ddump-ds which gives you the
unoptimized *GHC core* output, which is quite hard to read).
-Brent
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