October 2016 - Beginners

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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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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Iterate in terms of scanl
by Atrudyjane 24 Oct '16

24 Oct '16

Hello All, Sorry about my previous post, email was hosed. I was working on a homework problem where the task is to write the iterate function in terms of scanl. Came up with this: myIterate f x = scanl (const.f) x (repeat x) I went looking around for other solutions to check my work and found the following solution on the Haskell Wiki: iterate f x = scanl f x (repeat x) myIterate seems to work checked against the Prelude iterate but I don't know if it's a good solution or not, because the iterate solution on the Haskell Wiki throws a type error. Types of Prelude iterate and scanl are different, so I can see why, or am I missing something? Here is some REPL output: Prelude iterate: λ> take 10 (iterate (+1) 1) [1,2,3,4,5,6,7,8,9,10] myIterate: λ> take 10 (myIterate (+1) 1) [1,2,3,4,5,6,7,8,9,10] Haskell Wiki solution: λ> take 10 (iterate' (+1) 1) error: • Occurs check: cannot construct the infinite type: a ~ a -> a Expected type: a -> a -> a Actual type: (a -> a) -> a -> a • In the first argument of ‘iterate'’, namely ‘(+ 1)’ In the second argument of ‘take’, namely ‘(iterate' (+ 1) 1)’ In the expression: take 10 (iterate' (+ 1) 1) • Relevant bindings include it :: [a] (bound at <interactive>:63:1) It will work if you pass it a function that takes two parameters though: λ> take 10 (iterate' (+) 1) [1,2,3,4,5,6,7,8,9,10] Any thoughts would be much appreciated! Andrea

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Iterate in terms of scanlIterate in
by Andrea ML 23 Oct '16

23 Oct '16

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