- Beginners - Haskell.org

Java-Bridge, Ambiguous occurrence ‘<$’
by Desonte Jolivet 03 Nov '16

03 Nov '16

Hello All: Im interested interfacing with the JVM through Haskell and I found this neat library on Hackag name java-bridge although when I try and install with cabal I get the below error, "Ambiguous occurrence ‘<$’". Any tips on how to clear this up or how I should proceed would be very helpful, Thank you all in advance. src/Foreign/Java/IO.hs:5:7: Warning: -XOverlappingInstances is deprecated: instead use per-instance pragmas OVERLAPPING/OVERLAPPABLE/OVERLAPS[ 1 of 24] Compiling Foreign.Java.Tutorial ( src/Foreign/Java/Tutorial.hs, dist/build/Foreign/Java/Tutorial.o )[ 2 of 24] Compiling Foreign.Java.IO ( src/Foreign/Java/IO.hs, dist/build/Foreign/Java/IO.o )[ 3 of 24] Compiling Foreign.Java.Control ( src/Foreign/Java/Control.hs, dist/build/Foreign/Java/Control.o )[ 4 of 24] Compiling Foreign.Java.Bindings.ReflectHaskell ( src/Foreign/Java/Bindings/ReflectHaskell.hs, dist/build/Foreign/Java/Bindings/ReflectHaskell.o )[ 5 of 24] Compiling Foreign.Java.Bindings.Haskell2Java ( dist/build/Foreign/Java/Bindings/Haskell2Java.hs, dist/build/Foreign/Java/Bindings/Haskell2Java.o )[ 6 of 24] Compiling Foreign.Java.Bindings.HaskellTypes ( src/Foreign/Java/Bindings/HaskellTypes.hs, dist/build/Foreign/Java/Bindings/HaskellTypes.o )[ 7 of 24] Compiling Foreign.Java.Bindings.JavaSE6 ( src/Foreign/Java/Bindings/JavaSE6.hs, dist/build/Foreign/Java/Bindings/JavaSE6.o )[ 8 of 24] Compiling Foreign.Java.Utils ( src/Foreign/Java/Utils.hs, dist/build/Foreign/Java/Utils.o )[ 9 of 24] Compiling Foreign.Java.Bindings.JavaTypes ( src/Foreign/Java/Bindings/JavaTypes.hs, dist/build/Foreign/Java/Bindings/JavaTypes.o )[10 of 24] Compiling Foreign.Java.Util ( src/Foreign/Java/Util.hs, dist/build/Foreign/Java/Util.o ) src/Foreign/Java/Util.hs:5:19: Ambiguous occurrence ‘<$’ It could refer to either ‘Foreign.Java.Util.<$’, defined at src/Foreign/Java/Util.hs:24:1 or ‘Prelude.<$’, imported from ‘Prelude’ at src/Foreign/Java/Util.hs:4:8-24 (and originally defined in ‘GHC.Base’)cabal: Error: some packages failed to install:java-bridge-0.20130606.3 failed during the building phase. The exception was:ExitFailure 1 -Jo

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Re: [Haskell-beginners] Constructor classes and type classes.
by Dimitri DeFigueiredo 03 Nov '16

03 Nov '16

Hi Patrick, I am not sure I understand what you are trying to do, but here are some thoughts. - your use of the names `typeVar` and `typeCons` make total sense to me. But your name `dataCons` in the definition of SetClass2 does not. You are only talking about types here. All you are doing is talking about *type* constructors. You are not constructing a specific value of that type. Maybe one way to see the difference between the type constructors and the data constructors is to change your first line to: data SetType typeVar = SetTypeDataConstructor [typeVar] deriving Show - I also don't understand why you named your type variable `dataVariable` instead of sticking to `typeVar` in SetClass3. It is a *type* variable in the type signature of the functions in the type class. - I don't understand what you mean by a "constructor class". My simple understanding of what a type constructors and data constructors are is here https://wiki.haskell.org/Constructor#Type_constructor. - I don't see the terms super class and sub class ever used in haskell land. Although I do understand they make sense, I think they are really OO terms. I've tried to map OO concepts into haskell and there are many different ways to do it. The best description of all those ways that I found was in Oleg Kiselyov's work https://arxiv.org/pdf/cs/0509027.pdf - I think parametric polymorphism as used in haskell forces us to compare types in terms of equality constraints (this type is "the same" as that), whereas subtype polymorphism as used in OO languages forces us to think in terms of inequalities (this type is "more specific" than that). I think those force fundamentally different ways to think about and model your problems. I would encourage you to use equality as much as possible, but if you want to model subtyping in haskell, I think Oleg's work above has a few approaches that I think are better than type classes. In particular, in OO there was always an ambiguity between modelling subtyping with inheritance or with a "has a" relationship when the subclass explicitly contains an object of the superclass. You can use the latter idea in haskell, Oleg's work has a good way to do it. Hope this helps. Cheers, Dimitri > From: PATRICK BROWNE <patrick.browne(a)dit.ie> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] Constructor classes and type classes. > Message-ID: > <CAGFLrKfQVaQQJybMTn-pJksV1VKUXrmBks14Jkm==Zqt=bdy5A(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > {- > I am trying to understand constructor classes and their relationship with > ordinary type classes. > I wrote the code below to help me understand the distinction. The code is > only for explanatory purposes, set operations use a tuple syntax. > > I use the naming convention of 'typeVar', 'typeCons', and 'dataCons' for > type variables, type constructors, and data constructors respectively. > Question1: > Is my naming convention correct? > I am particularly concerned about SetClass3 where the super class seems to > use a type constructor but the subclass seems to use the same term as a > data constructor. > > Q2: > In this case constructor classes and type classes seem to provide similar > functionality. > In general what situation are each best suited? > -} > > import Data.List > data SetType typeVar = SetType [typeVar] deriving Show > > > class SetClass1 typeCons where > member1 :: Eq typeVar => (typeVar, typeCons typeVar) -> Bool > intersect1 :: (Eq typeVar,Show typeVar) => (typeCons typeVar, typeCons > typeVar) -> typeCons typeVar > > > class SetClass2 dataCons typeVar where > member2 :: (typeVar, (dataCons typeVar)) -> Bool > intersect2 :: (dataCons typeVar, dataCons typeVar) -> dataCons typeVar > > > class SetClass1 typeCons => SetClass3 typeCons dataVariable where > union3 :: (Eq dataVariable,Show dataVariable) => (typeCons dataVariable, > typeCons dataVariable) -> typeCons dataVariable > > instance SetClass1 SetType where > member1 (x ,(SetType y)) = elem x y > intersect1 ((SetType x),(SetType y)) = SetType (intersect x y) > > instance SetClass2 SetType Int where > member2 (x ,SetType y) = elem x y > intersect2 (SetType x,SetType y) = SetType (intersect x y) > > instance SetClass3 SetType Int where > union3 (SetType x,SetType y) = SetType (union x y) > > > test1a = member1 (1, (SetType [1,2])) > test1b = intersect1 ((SetType [1,3,4]),(SetType [1,2])) > test2a = member2 (1, (SetType [1::Int])) > test2b = intersect2 ((SetType [1::Int]), (SetType [(1::Int)])) > test3a = union3 ((SetType [1::Int,2::Int,3::Int]),(SetType [4::Int,5::Int])) > -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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Sum a tuple of Monoids
by Lai Boon Hui 01 Nov '16

01 Nov '16

Hi all, Is it possible to sum a tuple of monoids for example sum (Just 1, Nothing) expected result Just 1 sum (Just 1, Just 2) expected result Just 3 -- Best Regards, Boon Hui

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Re: [Haskell-beginners] Constructor classes and type classes.
by PATRICK BROWNE 31 Oct '16

31 Oct '16

My intended use of the term 'subclass' is as the Haskell report: <https://www.haskell.org/onlinereport/basic.html> "class Num of numeric types is a subclass of Eq, since all numbers may be compared for equality" Of course what I have written may not be consistent with the report. Regards, Pat On 31 October 2016 at 09:26, Imants Cekusins <imantc(a)gmail.com> wrote: > HelloPat, > > I am not sure if terms *superclass* and *subclass* are applicable to > Haskell classes. > > Haskell class is very different from Java class. It is more like an > interface. > > ;) > > -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Constructor classes and type classes.
by PATRICK BROWNE 30 Oct '16

30 Oct '16

{- I am trying to understand constructor classes and their relationship with ordinary type classes. I wrote the code below to help me understand the distinction. The code is only for explanatory purposes, set operations use a tuple syntax. I use the naming convention of 'typeVar', 'typeCons', and 'dataCons' for type variables, type constructors, and data constructors respectively. Question1: Is my naming convention correct? I am particularly concerned about SetClass3 where the super class seems to use a type constructor but the subclass seems to use the same term as a data constructor. Q2: In this case constructor classes and type classes seem to provide similar functionality. In general what situation are each best suited? -} import Data.List data SetType typeVar = SetType [typeVar] deriving Show class SetClass1 typeCons where member1 :: Eq typeVar => (typeVar, typeCons typeVar) -> Bool intersect1 :: (Eq typeVar,Show typeVar) => (typeCons typeVar, typeCons typeVar) -> typeCons typeVar class SetClass2 dataCons typeVar where member2 :: (typeVar, (dataCons typeVar)) -> Bool intersect2 :: (dataCons typeVar, dataCons typeVar) -> dataCons typeVar class SetClass1 typeCons => SetClass3 typeCons dataVariable where union3 :: (Eq dataVariable,Show dataVariable) => (typeCons dataVariable, typeCons dataVariable) -> typeCons dataVariable instance SetClass1 SetType where member1 (x ,(SetType y)) = elem x y intersect1 ((SetType x),(SetType y)) = SetType (intersect x y) instance SetClass2 SetType Int where member2 (x ,SetType y) = elem x y intersect2 (SetType x,SetType y) = SetType (intersect x y) instance SetClass3 SetType Int where union3 (SetType x,SetType y) = SetType (union x y) test1a = member1 (1, (SetType [1,2])) test1b = intersect1 ((SetType [1,3,4]),(SetType [1,2])) test2a = member2 (1, (SetType [1::Int])) test2b = intersect2 ((SetType [1::Int]), (SetType [(1::Int)])) test3a = union3 ((SetType [1::Int,2::Int,3::Int]),(SetType [4::Int,5::Int])) -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Writing huge result of `show` to file results in out of memory
by Mahdi Dibaiee 30 Oct '16

30 Oct '16

Hi, So I have a data instance which contains a few big matrices, and I want to save my instance to a file so I can `read` it back later to avoid the long computation every time (I'm training a recurrent neural network). First attempt, the simplest method: writeFile "rnn" (show dataInstance) It starts to take all of the memory and then bails out with `out-of-memory` error. So I wrote a function to write the string chunk by chunk, without buffering, here is the code: https://github.com/mdibaiee/sibe/blob/728df02fbdd6f134af107c098f5477094c61e… Copy/pasted from the link: saveRecurrent :: FilePath -> String -> Int -> IO () saveRecurrent path str chunkSize = do handle <- openFile path AppendMode hSetBuffering handle NoBuffering loop handle str hClose handle where loop _ [] = return () loop handle s = do hPutStr handle $ take chunkSize s hFlush handle loop handle $ drop chunkSize s But it doesn't work either, I still get `out-of-memory` errors. From what I understand, this should work, but it isn't. I asked on IRC and someone said "Show is not lazy enough", if that's the case, I would appreciate an explanation of that. Thanks, Mahdi

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Re: [Haskell-beginners] Beginners Digest, Vol 100, Issue 19
by Ashish Negi 30 Oct '16

30 Oct '16

Can you write your own custom function for converting the data structure to string ?

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Re: [Haskell-beginners] Beginners Digest, Vol 100, Issue 20
by Mahdi Dibaiee 29 Oct '16

29 Oct '16

That's not generally true (but I will give my guess about your assumption after this). See the example in the article I linked: ``` import Data.Char(toUpper) main = do inpStr <- readFile "input.txt" writeFile "output.txt" (map toUpper inpStr) ``` Here, `inputStr` has to be evaluated in order to run `map toUpper` on it, but it's done character by character. Character is read, `toUpper` is run on it, it's written to the file. (Though it might not be a character unless we use `NoBuffering`, it's going to be lines or blocks) Now, I guess your assumption _might_ be true in my case, because `show` might not be able to generate the string character by character (which is very likely), in this case, show will give us the whole string in one piece, leading to an out-of-memory error as the big string is getting loaded to memory in order to be used by `writeFile`. This is just a guess, I would appreciate it if someone could actually prove it. What do you think Imants? -----Original Message----- From: beginners-request <beginners-request(a)haskell.org> To: beginners <beginners(a)haskell.org> Sent: Sat, Oct 29, 2016 3:44 pm Subject: Beginners Digest, Vol 100, Issue 20 Send Beginners mailing list submissions to beginners(a)haskell.orgTo subscribe or unsubscribe via the World Wide Web, visit http://mail.haskell.org/cgi-bin/mailman/listinfo/beginnersor, via email, send a message with subject or body 'help' to beginners-request(a)haskell.orgYou can reach the person managing the list at beginners-owner(a)haskell.orgWhen replying, please edit your Subject line so it is more specificthan "Re: Contents of Beginners digest..."Today's Topics: 1. Writing huge result of `show` to file results in out of memory (Mahdi Dibaiee) 2. Re: Writing huge result of `show` to file results in out of memory (Imants Cekusins) Attached Message From Mahdi Dibaiee <mdibaiee(a)aol.com> To beginners(a)haskell.org Subject [Haskell-beginners] Writing huge result of `show` to file resultsin out of memory Date Fri, 28 Oct 2016 12:57:22 -0400 I know that there are other ways of doing it, I just want to understand where lies the problem here. I just read the realworldhaskell book by O'Reilly, in one section, Lazy I/O [0], it's explained that there should be no problem writing big strings, without consuming the whole string to files, however big they are. I would appreciate it if someone could point out the problem there. Thanks [0]: http://book.realworldhaskell.org/read/io.html#io.lazy Attached Message From Imants Cekusins <imantc(a)gmail.com> To The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> Subject Re: [Haskell-beginners] Writing huge result of `show` to fileresults in out of memory Date Fri, 28 Oct 2016 19:13:23 +0200 > there should be no problem writing big strings, without consuming the whole string lazy means evaluated when needed. However when a string is evaluated, it is evaluated fully, I guess. _______________________________________________Beginners mailing listBeginners@haskell.orghttp://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Writing huge result of `show` to file results in out of memory
by Mahdi Dibaiee 28 Oct '16

28 Oct '16

I know that there are other ways of doing it, I just want to understand where lies the problem here. I just read the realworldhaskell book by O'Reilly, in one section, Lazy I/O [0], it's explained that there should be no problem writing big strings, without consuming the whole string to files, however big they are. I would appreciate it if someone could point out the problem there. Thanks [0]: http://book.realworldhaskell.org/read/io.html#io.lazy

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The meaning of categories constructed from HASK
by Dimitri DeFigueiredo 25 Oct '16

25 Oct '16

In category theory, there are many ways one can make new categories out of an old one. In particular, given a category C one can construct: 1. The arrows category of C: arrows in C become objects and commutative squares in C become arrows 2. The slice category of C given an object A: arrows into a distinguished object A become objects in the slice commutative triangles become arrows There are also functors going from C to these new categories (and back). Are these constructed categories useful when C = `Hask` (the category of haskell types and functions)? What do they represent in programming terms? In other words, is there intuition for what the arrows category of Hask is? What about the slice category of Hask over a specific type? Do the functors between these match some programming abstractions? Any pointers are much appreciated. Thanks, Dimitri

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