[Haskell-cafe] is this an arrow?
Dear Cafe, I have the following types: type Computer a b = ( a - IO ( Maybe b ) ) type Transformer a b c d = Computer a ( b, c - d ) For example, a SAT solver: minisat :: Computer CNF Assignment, and when I use it to solve an application problem via transformation to SAT, I need t :: Transformer Problem SAT Assignment Solution Now - what is the proper abstraction? Is this an arrow somehow? And is there already a type and a library that would contain useful combinators like some of these: http://autolat.imn.htwk-leipzig.de/gitweb/?p=box;a=blob;f=src/exotic/Strategy.hs;h=238b5f55fabf356e0ce484d0f6b882a06d6e6cfc;hb=HEAD Thanks - J.W. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] is this an arrow?
On Fri, Apr 13, 2012 at 7:49 AM, Johannes Waldmann waldm...@imn.htwk-leipzig.de wrote: type Computer a b = ( a - IO ( Maybe b ) ) type Transformer a b c d = Computer a ( b, c - d ) Computer looks like Kleisli (MaybeT IO), which would be a valid instance of Arrow. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trouble defining `instance Arrow SF'
In trying to follow along with `Programming with Arrows' by John
Hughes, I'm entering the following code:
1 -- Taken from `Programming with Arrows'.
2
3 module SF where
4
5 import Control.Arrow
6
7 newtype SF a b = SF {runSF :: [a] - [b]}
8
9 instance Arrow SF where
10 arr f = SF (map f)
11 SF f SF g = SF (f g)
and getting the following error from GHCI:
GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude :load SF
[1 of 1] Compiling SF ( SF.hs, interpreted )
SF.hs:11:10: `' is not a (visible) method of class `Arrow'
Failed, modules loaded: none.
Can anyone help?
Thanks,
-db
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Re: [Haskell-cafe] Trouble defining `instance Arrow SF'
-BEGIN PGP SIGNED MESSAGE-
Hash: SHA1
SF.hs:11:10: `' is not a (visible) method of class `Arrow'
Failed, modules loaded: none.
In the base package Arrows are defined a bit different from what you
some times see in the literature. I also stumbled over this once. The
() operator is just a synonym for (.) from Control.Category. Note
that (.) is not the same (.) as in Prelude.
class Category cat where
Control.Category.id :: cat a a
(Control.Category..) :: cat b c - cat a b - cat a c
class Category a = Arrow a where
arr :: (b - c) - a b c
first :: a b c - a (b, d) (c, d)
f g = g . f
your implementation would probably look something like this.
module Main where
import Control.Arrow
import Control.Category as Cat
newtype SF a b = SF {runSF :: [a] - [b]}
instance Category SF where
id = SF Cat.id
SF f . SF g = SF (f Cat.. g)
instance Arrow SF where
arr f = SF (map f)
first (SF f) = SF (unzip first f uncurry zip)
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Re: [Haskell-cafe] Trouble defining `instance Arrow SF'
On Thu, Oct 27, 2011 at 5:55 AM, Captain Freako capn.fre...@gmail.comwrote: SF.hs:11:10: `' is not a (visible) method of class `Arrow' Failed, modules loaded: none. import Prelude hiding ((.),id) import Control.Category you'll also need to define `instance Category SF`, since that is a requirement for Arrows. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Trouble defining `instance Arrow SF'
Thanks for the reply, David.
I tried implementing your suggestions, such that my code now looks like this:
1 -- Taken from `Programming with Arrows'.
2
3 module SF where
4
5 import Prelude hiding ((.),id)
6 import Control.Category
7 import Control.Arrow
8
9 newtype SF a b = SF {runSF :: [a] - [b]}
10
11 instance Category SF
12
13 instance Arrow SF where
14 arr f = SF (map f)
15 SF f SF g = SF (f g)
but I'm getting the same error as before:
GHCi, version 7.0.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude :load SF
[1 of 1] Compiling SF ( SF.hs, interpreted )
SF.hs:15:10: `' is not a (visible) method of class `Arrow'
Failed, modules loaded: none.
Any thoughts?
Thanks,
-db
On Thu, Oct 27, 2011 at 8:29 AM, David Barbour dmbarb...@gmail.com wrote:
On Thu, Oct 27, 2011 at 5:55 AM, Captain Freako capn.fre...@gmail.com
wrote:
SF.hs:11:10: `' is not a (visible) method of class `Arrow'
Failed, modules loaded: none.
import Prelude hiding ((.),id)
import Control.Category
you'll also need to define `instance Category SF`, since that is a
requirement for Arrows.
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Re: [Haskell-cafe] Trouble defining `instance Arrow SF'
On Thu, Oct 27, 2011 at 7:07 PM, Captain Freako capn.fre...@gmail.comwrote: Thanks for the reply, David. I tried implementing your suggestions, such that my code now looks like this: While Ross Patterson followed Hughes closely, it was recognized that Category could be separated from Arrow. For `Category`, you must define (.) and id. There is a definition in the Control.Category module: () = flip (.) Thus, once you instance Category, you you get () for free. For Arrow, you must define `arr` and `first`. Regards, Dave ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: Subcomputations in arrow notation
Paterson, Ross r.pater...@city.ac.uk wrote: See the GHC Arrow notation documentation for more about the banana brackets, which let you use user-defined control structures like event. I have reread the documentation now. It seems that there are a lot of syntactic constructs, which I've missed. Particularly the banana bracket notation does exactly what I want. Thanks a lot! Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife = sex) http://ertes.de/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Proposal: Subcomputations in arrow notation
Hello fellow Haskellers, this is a proposal to extend the arrow notation (-XArrows). I find myself writing the following very often: system :: Wire IO () String system = proc _ - do botAddPeriod - succ ^ noise - () botAddSpeed - noise1 - () botAddStart - noise1 - () botMsg - event addBot - (botAddPeriod, botAddSpeed, botAddStart) bots - manager - ((), maybe MgrNop id botMsg) let botStr = concatMap (printf %8.2) . M.elems $ bots :: String identity - printf Bot positions: %s botStr where addBot :: Wire IO (Double, Double, Double) (MgrMsg Int IO () Double) addBot = proc (addPeriod, addSpeed, addStart) - do periodically - addPeriod botId - identifier - () identity - MgrAdd botId (constant addSpeed integral addStart) The relevant part is the first paragraph of the first arrow computation: botAddPeriod - succ ^ noise - () botAddSpeed - noise1 - () botAddStart - noise1 - () botMsg - event addBot - (botAddPeriod, botAddSpeed, botAddStart) This line should generate a message for the bot manager at random intervals. The actual event generator is in the second arrow computation 'addBot'. I would like to be able to write this more in line with the rest of the code. The following is possible: system :: Wire IO () String system = proc _ - do botAddPeriod - succ ^ noise - () botAddSpeed - noise1 - () botAddStart - noise1 - () botMsg - event (proc (addPeriod, addSpeed, addStart) - do periodically - addPeriod botId - identifier - () identity - MgrAdd botId (constant addSpeed integral addStart)) - (botAddPeriod, botAddSpeed, botAddStart) bots - manager - ((), maybe MgrNop id botMsg) let botStr = concatMap (printf %8.2) . M.elems $ bots :: String identity - printf Bot positions: %s botStr This is probably not a big improvement. It's more concise, but also harder to understand. My proposal is to add syntax to allow the following notation: system :: Wire IO () String system = proc _ - do botAddPeriod - succ ^ noise - () botAddSpeed - noise1 - () botAddStart - noise1 - () botMsg - event $ do periodically - addPeriod botId - identifier - () identity - MgrAdd botId (constant addSpeed integral addStart) bots - manager - ((), maybe MgrNop id botMsg) let botStr = concatMap (printf %8.2) . M.elems $ bots :: String identity - printf Bot positions: %s botStr Again the relevant part is the event generator in the middle. In this hypothetical syntax, the compiler would figure out from the inner computation, which variables from the outer scope are used and pass them automatically in an appropriate tuple. You wouldn't need any explicit passing anymore. If others like the idea, too, and there is nobody to implement it, then I would be willing to get in touch with the GHC code and implement this as a patch myself. What do you think? Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife = sex) http://ertes.de/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: Subcomputations in arrow notation
I think this proposal makes so much sense that I'm surprised it didn't already work this way. - Jake ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Proposal: Subcomputations in arrow notation
Ertugrul Soeylemez writes: I find myself writing the following very often: system :: Wire IO () String system = proc _ - do botAddPeriod - succ ^ noise - () botAddSpeed - noise1 - () botAddStart - noise1 - () botMsg - event addBot - (botAddPeriod, botAddSpeed, botAddStart) bots - manager - ((), maybe MgrNop id botMsg) let botStr = concatMap (printf %8.2) . M.elems $ bots :: String identity - printf Bot positions: %s botStr where addBot :: Wire IO (Double, Double, Double) (MgrMsg Int IO () Double) addBot = proc (addPeriod, addSpeed, addStart) - do periodically - addPeriod botId - identifier - () identity - MgrAdd botId (constant addSpeed integral addStart) If addPeriod is supposed to be the same as botAddPeriod, etc, this should be equivalent: system :: Wire IO () String system = proc _ - do addPeriod - succ ^ noise - () addSpeed - noise1 - () addStart - noise1 - () botMsg - (|event (do periodically - addPeriod botId - identifier - () identity - MgrAdd botId (constant addSpeed integral addStart))|) bots - manager - ((), maybe MgrNop id botMsg) let botStr = concatMap (printf %8.2) . M.elems $ bots :: String identity - printf Bot positions: %s botStr See the GHC Arrow notation documentation for more about the banana brackets, which let you use user-defined control structures like event. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Weird behavior with arrow commands
El vie, 23-07-2010 a las 23:27 -0400, Ronald Guida escribió:
I am trying to figure out how to use GHC's arrow commands, and I found
some extremely weird behavior.
CC'ed to ghc-users, because this may be a ghc bug.
In GHC's manual, there is a description of arrow commands, which I
don't really understand.
http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html#id667303
(Primitive Constructs)
I have two questions:
1. What are arrow commands supposed to do?
2. What is this code supposed to do?
-- start of code --
{-# LANGUAGE Arrows #-}
module Main where
import Control.Arrow
foo :: (b - String) - b, Int), Float), Double) - String) - (b -
String)
foo f g b = f b ++ ++ g (((b, 8), 1.0), 6.0)
bar :: (t - String) - ((Double, Int) - String) - t - String
bar f g = proc x - do
(f - x) `foo` \n m - g - (n)
main = do
putStrLn $ foo show show 17
putStrLn $ bar show show 17
putStrLn $ foo show show 42
putStrLn $ bar show show 42
-- end of code --
Output from GHCi:
17 (((17,8),1.0),6.0)
17 (6.730326920298707e-306,0)
42 (((42,8),1.0),6.0)
42 (6.730326920298707e-306,0)
Output after compiling with GHC:
17 (((17,8),1.0),6.0)
17 (5.858736684536801e-270,0)
42 (((42,8),1.0),6.0)
42 (5.858736684536801e-270,0)
GHC Version:
The Glorious Glasgow Haskell Compilation System, version 6.12.3
This seems to be a bug in ghc. First, let's fix bar to give the full
three arguments (Int, Float, Double) to g:
bar f g = proc x - do
(f - x) `foo` (\n m k - g - (n,m,k))
ghc infers the type:
bar :: (t - String) - ((Double, Float, Int) - String) - t - String
and we see that the argument order in the second argument to bar is
reversed. But the arguments are still given to bar in the order (Int,
Float, Double). For example, the 6.0 in foo is interpreted as an Int and
outputs a 0 (the first 32 bits in such a small double are zeros).
When one varies the numbers in foo, one can see the effects in bar. Can
someone from GHC HQ confirm my understanding, or is this just not
supposed to work with multiple arguments?
Jürgen
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Re: [Haskell-cafe] Weird behavior with arrow commands
On Sat, Jul 24, 2010 at 01:10:56PM +0200, Jürgen Doser wrote: This seems to be a bug in ghc. First, let's fix bar to give the full three arguments (Int, Float, Double) to g: bar f g = proc x - do (f - x) `foo` (\n m k - g - (n,m,k)) ghc infers the type: bar :: (t - String) - ((Double, Float, Int) - String) - t - String and we see that the argument order in the second argument to bar is reversed. But the arguments are still given to bar in the order (Int, Float, Double). For example, the 6.0 in foo is interpreted as an Int and outputs a 0 (the first 32 bits in such a small double are zeros). When one varies the numbers in foo, one can see the effects in bar. Can someone from GHC HQ confirm my understanding, or is this just not supposed to work with multiple arguments? You're right: it's getting the argument order wrong, so -dcore-lint fails on this example and all bets are off. Definitely a bug. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Weird behavior with arrow commands
I am trying to figure out how to use GHC's arrow commands, and I found
some extremely weird behavior.
In GHC's manual, there is a description of arrow commands, which I
don't really understand.
http://www.haskell.org/ghc/docs/latest/html/users_guide/arrow-notation.html#id667303
(Primitive Constructs)
I have two questions:
1. What are arrow commands supposed to do?
2. What is this code supposed to do?
-- start of code --
{-# LANGUAGE Arrows #-}
module Main where
import Control.Arrow
foo :: (b - String) - b, Int), Float), Double) - String) - (b - String)
foo f g b = f b ++ ++ g (((b, 8), 1.0), 6.0)
bar :: (t - String) - ((Double, Int) - String) - t - String
bar f g = proc x - do
(f - x) `foo` \n m - g - (n)
main = do
putStrLn $ foo show show 17
putStrLn $ bar show show 17
putStrLn $ foo show show 42
putStrLn $ bar show show 42
-- end of code --
Output from GHCi:
17 (((17,8),1.0),6.0)
17 (6.730326920298707e-306,0)
42 (((42,8),1.0),6.0)
42 (6.730326920298707e-306,0)
Output after compiling with GHC:
17 (((17,8),1.0),6.0)
17 (5.858736684536801e-270,0)
42 (((42,8),1.0),6.0)
42 (5.858736684536801e-270,0)
GHC Version:
The Glorious Glasgow Haskell Compilation System, version 6.12.3
Thank you
-- Ron
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[Haskell-cafe] ArrowChoice for simulation arrow (Sim)
Hi, In John Hughes\' paper \Programming with arrows\ [1] Section 5, the simulation arrow is defined (Sim). All the examples went well. However, when I want to use it beyond circuit simulation and I added a \if-then-else\ line, GHC is asking to define ArrowChoice for Sim. Does anybody have the ArrowChoice definition for Sim ? Thanks, Steve [1] http://www.cs.chalmers.se/~rjmh/afp-arrows.pdf ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
