[Haskell-cafe] Mutually recursive modules
I wonder whether it's always possible to break cycles using GHC's
.hs-boot files.
Consider the following schematic example:
module A where
import B
data A
f :: B - A
f = undefined B.g
module B where
import A
data B
g :: A - B
g = undefined A.f
A.hs-boot must give a type signature for f, and since the signature
contains 'B', it must import 'B'. Ditto for B.hs-boot — it must import
'A'.
Even if we treat all imports as {-# SOURCE #-}, there is still a cycle
between the hs-boot files.
So, am I right in understanding that these recursive modules cannot be
compiled by GHC at all?
Roman
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Re: [Haskell-cafe] Incrementation fails
Hi John, Can you tell us what your function is supposed to do? It appears to be some kind of search-and-replace for strings. What is the relationship between 'findStr', 'replaceStrList' and 'myText'? 2013/5/8 John knowledge1...@gmail.com Hello All, I'm in a big trouble with incrementation of a counter in this code. It doesn't increment. Could you please tell me where the problem ist and how can I solve it? replaceBasedIdx:: String - [String] - String - String replaceBasedIdxfindStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText 0 replaceBasedIdxSub :: String - [String] - String - Int - String replaceBasedIdxSub findStr replaceStrList myText counter = loop myText where loop [] = [] loop myText = let (prefix, rest) = splitAt n myText in if findStr == prefix-- found an occurrence? then (replaceStrList !! (counter+1)) ++ loop rest -- yes: replace it else head myText : loop (tail myText) -- no: keep looking n = length findStr Thank you very mutch! Greetings! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Incrementation fails
I stared at the code some more and deduced what I think is the intented meaning. Occurences of 'findStr' in 'myText' are replaced with the strings in 'replaceStrList'. So replaceBasedIdx X [b,d,f] aXcXeXg = abcdefg The reason your counter didn't increment is because it was defined as an argument to 'replaceBasedIdxSub'. That means its value is fixed once you evaluate that function. Its value will not change no matter how many times the inner 'loop' function is evaluated. The solution is to pass the counter as an argument to said 'loop' function. Now when 'loop' enters the recursion you can pass a different value, such as counter + 1. replaceBasedIdx :: String - [String] - String - String replaceBasedIdx findStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText replaceBasedIdxSub :: String - [String] - String - String replaceBasedIdxSub findStr replaceStrList myText = loop 0 myText where loop :: Int - String - String loop counter [] = [] loop counter myText = let (prefix, rest) = splitAt n myText in if findStr == prefix then (replaceStrList !! counter) ++ loop (counter + 1) rest else head myText : loop counter (tail myText) n :: Int n = length findStr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Mutually recursive modules
At Wed, 8 May 2013 09:46:08 +0300,
Roman Cheplyaka wrote:
I wonder whether it's always possible to break cycles using GHC's
.hs-boot files.
Consider the following schematic example:
module A where
import B
data A
f :: B - A
f = undefined B.g
module B where
import A
data B
g :: A - B
g = undefined A.f
A.hs-boot must give a type signature for f, and since the signature
contains 'B', it must import 'B'. Ditto for B.hs-boot — it must import
'A'.
Even if we treat all imports as {-# SOURCE #-}, there is still a cycle
between the hs-boot files.
So, am I right in understanding that these recursive modules cannot be
compiled by GHC at all?
This configuration works for me:
A.hs:
module A where
import B
data A
f :: B - A
f = undefined B.g
A.hs-boot:
module A where
import {-# SOURCE #-} B
data A
f :: B - A
B.hs:
module B where
import {-# SOURCE #-} A
data B
g :: A - B
g = undefined A.f
B.hs-boot:
module B where
data B
Then I can compile them:
bitonic@clay /tmp % ghc -c B.hs-boot
bitonic@clay /tmp % ghc -c A.hs-boot
bitonic@clay /tmp % ghc -c B.hs
bitonic@clay /tmp % ghc -c A.hs
Francesco
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Re: [Haskell-cafe] Incrementation fails
problem = replaceBasedIdx X [b,d,f] aXcXeXgX -- abcdefg*** Exception: Prelude.(!!): index too large This problem occurs because you never check if 'counter' is a valid index in the 'replaceStrList'. You can solve it by not using the !! operator at all. The solution is to also pass 'replaceStrList' in the recursion. That way you can check whether you have exhausted all strings in that list and as a bonus you do not need your counter anymore: replaceBasedIdx :: String - [String] - String - String replaceBasedIdx findStr replaceStrList myText = loop replaceStrList myText where loop :: [String] - String - String loop rs [] = [] -- Check for empty list of replacements loop [] text = text -- Pattern match on the list of replacements to get both the -- entire list rs, the first element r and the tail rs'. loop rs@(r:rs') text = let (prefix, rest) = splitAt n text in if findStr == prefix then r ++ loop rs' rest else head text : loop rs (tail text) n :: Int n = length findStr 2013/5/8 Roel van Dijk vandijk.r...@gmail.com I stared at the code some more and deduced what I think is the intented meaning. Occurences of 'findStr' in 'myText' are replaced with the strings in 'replaceStrList'. So replaceBasedIdx X [b,d,f] aXcXeXg = abcdefg The reason your counter didn't increment is because it was defined as an argument to 'replaceBasedIdxSub'. That means its value is fixed once you evaluate that function. Its value will not change no matter how many times the inner 'loop' function is evaluated. The solution is to pass the counter as an argument to said 'loop' function. Now when 'loop' enters the recursion you can pass a different value, such as counter + 1. replaceBasedIdx :: String - [String] - String - String replaceBasedIdx findStr replaceStrList myText = replaceBasedIdxSub findStr replaceStrList myText replaceBasedIdxSub :: String - [String] - String - String replaceBasedIdxSub findStr replaceStrList myText = loop 0 myText where loop :: Int - String - String loop counter [] = [] loop counter myText = let (prefix, rest) = splitAt n myText in if findStr == prefix then (replaceStrList !! counter) ++ loop (counter + 1) rest else head myText : loop counter (tail myText) n :: Int n = length findStr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Mutually recursive modules
Ah yes, thank you!
* Francesco Mazzoli f...@mazzo.li [2013-05-08 08:51:12+0100]
At Wed, 8 May 2013 09:46:08 +0300,
Roman Cheplyaka wrote:
I wonder whether it's always possible to break cycles using GHC's
.hs-boot files.
Consider the following schematic example:
module A where
import B
data A
f :: B - A
f = undefined B.g
module B where
import A
data B
g :: A - B
g = undefined A.f
A.hs-boot must give a type signature for f, and since the signature
contains 'B', it must import 'B'. Ditto for B.hs-boot — it must import
'A'.
Even if we treat all imports as {-# SOURCE #-}, there is still a cycle
between the hs-boot files.
So, am I right in understanding that these recursive modules cannot be
compiled by GHC at all?
This configuration works for me:
A.hs:
module A where
import B
data A
f :: B - A
f = undefined B.g
A.hs-boot:
module A where
import {-# SOURCE #-} B
data A
f :: B - A
B.hs:
module B where
import {-# SOURCE #-} A
data B
g :: A - B
g = undefined A.f
B.hs-boot:
module B where
data B
Then I can compile them:
bitonic@clay /tmp % ghc -c B.hs-boot
bitonic@clay /tmp % ghc -c A.hs-boot
bitonic@clay /tmp % ghc -c B.hs
bitonic@clay /tmp % ghc -c A.hs
Francesco
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Re: [Haskell-cafe] Categories (cont.)
Hi Wren,
Have you taken this constrained categories experiment further,
particularly for adding products? As I mentioned in a haskell-cafe note
yesterday, I tried and got a frightening proliferation of constraints when
defining method defaults and utility functions (e.g., left- or
right-associating).
-- Conal
On Fri, Dec 21, 2012 at 8:59 PM, wren ng thornton w...@freegeek.org wrote:
On 12/21/12 2:35 PM, Chris Smith wrote:
It would definitely be nice to be able to work with a partial Category
class, where for example the objects could be constrained to belong to a
class. One could then restrict a Category to a type level representation
of the natural numbers or any other desired set. Kind polymorphism should
make this easy to define, but I still don't have a good feel for whether
it
is worth the complexity.
Actually, what you really want is ConstraintKinds. The following works
just fine in GHC 7.6.1:
{-# LANGUAGE KindSignatures
, ConstraintKinds
, PolyKinds
, TypeFamilies
, MultiParamTypeClasses
, FunctionalDependencies
, FlexibleInstances
, FlexibleContexts
#-}
class Category (c :: k - k - *) where
-- | The kind of domain objects.
type DomC c x :: Constraint
-- | The kind of codomain objects.
type CodC c x :: Constraint
-- | The identity morphisms.
id :: (ObjC c x) = c x x
-- | Composition of morphisms.
(.) :: (DomC c x, ObjC c y, CodC c z) = c y z - c x y - c x z
-- | An alias for objects in the centre of a category.
type ObjC c x = (Category c, DomC c x, CodC c x)
-- | An alias for a pair of objects which could be connected by a
-- @c@-morphism.
type HomC c x y = (Category c, DomC c x, CodC c y)
Notably, we distinguish domain objects from codomain objects in order to
allow morphisms into or out of the category, which is indeed helpful in
practice.
Whether there's actually any good reason for distinguishing DomC and CodC,
per se, remains to be seen. In Conal Elliott's variation[1] he moves HomC
into the class and gets rid of DomC and CodC. Which allows constraints that
operate jointly on both the domain and codomain, whereas the above version
does not. I haven't run into the need for that yet, but I could easily
imagine it. It does add a bit of complication though since we can no longer
have ObjC be a derived thing; it'd have to move into the class as well, and
we'd have to somehow ensure that it's coherent with HomC.
The above version uses PolyKinds as well as ConstraintKinds. I haven't
needed this myself, since the constraints act as a sort of kind for the
types I'm interested in, but it'll definitely be useful if you get into
data kinds, or want an instance of functor categories, etc.
[1] https://github.com/conal/**linear-map-gadt/blob/master/**
src/Control/ConstraintKinds/**Category.hshttps://github.com/conal/linear-map-gadt/blob/master/src/Control/ConstraintKinds/Category.hs
--
Live well,
~wren
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Re: [Haskell-cafe] ANN: Robot - Simulate keyboard and mouse events under X11
Hi, I just started using your library to move my cursor. Is it possible that it ignores negative values in moveBy? In other words, I can only move the cursor into one direction. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Formal Methods/Functional programming job position at Intel
Our formal methods team at Intel has a full-time position available that I think would be a good fit for functional programming and formal methods enthusiasts. I'm including the description below. Do not hesitate to contact me if you've any questions, or just want to talk about it in general. To apply for the position, please visit: http://www.intel.com/jobs/jobsearch/index.htm, and in the advanced search area enter the job number: 709631. Thanks, -Levent. Job Description *Formal Methods Validation Architect* *-* *709631* Description If you're interested in products going into future super computer markets then the Intel® Many Integrated Core (Intel® MIC) Hardware Engineering Group is the place for you! We design and validate silicon chips with many Intel cores integrated inside being used in high performance computing architectures. In this position you will work as part of the pre-silicon formal methods, tools, and verification team to support a continued high quality of the future Intel many core processor products. You will work together with Intel's formal verification and validation community, the Formal Verification Center of Expertise (FV CoE) in a team of experts in formal methods and AV Validation. Your specific responsibilities will include defining formal verification (FV) test plans as well as Cluster Test Environment (CTE) based test plans for dynamic simulation validation (DV). The goal is to help optimize a combined use of FV and CTE based validation techniques (DV) and contribute to an increasing use of formal methods at Intel. Based on the test plans you will write properties in formal language, and prove the properties using our model checkers and theorem proving tools. You will also write CTE based test cases and coverage plans. Your area of strength may currently lie within either FV or DV with strong skills rooted in software development. But through your skills and work, you will become an expert in both formal methods and validation technologies. You will interact very closely with design teams, and other validation teams, as well as with Intel's internal RD groups that continue to improve and develop formal method and verification tools. You must be able to communicate effectively with various technical groups and coordinate activities amongst those groups. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Algebra in Haskell
Dear all, There are several libraries in haskell for abstract algebra but they don't seem to cover my use case I was wondering if other people have had similar issues and if there are any packages I am missing. What I am prinicpaly interested in is operation on algebraic stuctures (homomorphisms, kernels, quotients, colimits, etc) whereas most algebra packages seem to just give operations on the elements of an algebraic structure. So I would like to be able to write something like... g = freeGroup([x,y]) h = freeGroup([z]) phi = grpMorphismOnGens([(x,z^6) , (y,z)]) k = grpKernel phi (gens,rels) = grpPresentation (grpQuotient g k) Thank you, Robert Goss ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Robot - Simulate keyboard and mouse events under X11
On Thu, May 9, 2013 at 4:47 AM, Niklas Hambüchen m...@nh2.me wrote: Hi, I just started using your library to move my cursor. Is it possible that it ignores negative values in moveBy? In other words, I can only move the cursor into one direction. I did some research, and traced this to a bug in an old (1.6) version of xcb-proto [1]. The coordinates were declared incorrectly as Word16, instead of Int16 as they should have been. It's been fixed in xcb-proto since 1.7. I've cc'd Antoine Latter, the maintainer of XHB, about this bug. Once he uploads a new version of XHB, I'll be happy to fix it on my end. [1] http://cgit.freedesktop.org/~keithp/xcb-proto/commit/src/xtest.xml?id=f3ae971edce37ad96ef0b8a6059c1f853e88fcf3 On Tue, May 7, 2013 at 5:18 AM, Jeanne-Kamikaze jeannekamik...@gmail.com wrote: Looks like an interesting library. Will it be able to read pixels from a window at some point? Not sure -- I have no idea how screen capturing works in X11. Calling gnome-screenshot should probably cover most use cases. Chris -- Chris Wong, fixpoint conjurer e: lambda.fa...@gmail.com w: http://lfairy.github.io/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ghci: Difference in garbage collection etc. between REPL and function
I have an annoying bug in a C binding that can be triggered like this: handle - open ... prep - makePreparedStatement handle INSERT ... performGC runStatement prep close handle If I run these steps one by one in ghci, garbage ends up in my handle as expected. However, if I let main = do ... this whole block in order to pack it in a test case, it does not happen, neither in ghci nor ghc. What might be the special magic in ghci's REPL that allows me to trigger my bug so easily there? Thanks Niklas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ghci: Difference in garbage collection etc. between REPL and function
On Wed, May 8, 2013 at 6:54 PM, Niklas Hambüchen m...@nh2.me wrote: I have an annoying bug in a C binding that can be triggered like this: handle - open ... prep - makePreparedStatement handle INSERT ... performGC runStatement prep close handle If I run these steps one by one in ghci, garbage ends up in my handle as expected. However, if I let main = do ... this whole block in order to pack it in a test case, it does not happen, neither in ghci nor ghc. What might be the special magic in ghci's REPL that allows me to trigger my bug so easily there? One thing to investigate: Thread Local Storage By default ghci puts each action you run into a thread and executes it. If the underlying C code stores something (or accesses it) from thread local storage, then you will run into issues like this. Try starting ghci with -fno-ghci-sandbox. If the bad behavior goes away there, try running each step of your test case inside a forkIO from ghc with the threaded RTS. If the problem disappears in ghci but shows up with forkIO, then it's a pretty good indicator that it's related to the way the C code uses thread local storage. I hope that helps, Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ghci: Difference in garbage collection etc. between REPL and function
On Wed, May 8, 2013 at 8:12 PM, Jason Dagit dag...@gmail.com wrote: On Wed, May 8, 2013 at 6:54 PM, Niklas Hambüchen m...@nh2.me wrote: I have an annoying bug in a C binding that can be triggered like this: handle - open ... prep - makePreparedStatement handle INSERT ... performGC runStatement prep close handle If I run these steps one by one in ghci, garbage ends up in my handle as expected. However, if I let main = do ... this whole block in order to pack it in a test case, it does not happen, neither in ghci nor ghc. What might be the special magic in ghci's REPL that allows me to trigger my bug so easily there? One thing to investigate: Thread Local Storage By default ghci puts each action you run into a thread and executes it. If the underlying C code stores something (or accesses it) from thread local storage, then you will run into issues like this. My point was meant to be, it's something to try but I have no idea if it's going to be the issue here. The caveat I forgot to add is: I've never seen the thread local storage issue affect GC. In particular, ghc might be running a finalizer in the case of individual actions because it doesn't know you are about to use the handle? Now that I think about it more, I suspect that is more likely to be the case. Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Test access resources files in cabal-dev
Hi, whats the recommended way to access a resource file for a test within cabal. I have a text file containing data that I use in a test and want to be able to access it in a root path independent manner. Thanks ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Accessing resource files in tests
Hi, whats the recommended way to access a resource file for a test within cabal. I have a text file containing data that I use in a test and want to be able to access it in a root path independent manner. Thanks ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
