March 2014 - Beginners

Question about define my own typeclass
by ke dou 28 Mar '14

28 Mar '14

Hi, Sorry for the spam. I am new to Haskell. I want to define my own typeclass which can convert from my own types like MyBool, MyInt, and MyString to according Haskell types. Here is my code: module Conversion where import qualified Prelude class Conversion a where conversion :: a -> b data MyBool = MyTrue | MyFalse instance Conversion MyBool where conversion MyTrue = Prelude.True conversion MyFalse =Prelude.False Here is the error message: Couldn't match expected type `b' with actual type `Prelude.Bool' `b' is a rigid type variable bound by the type signature for conversion :: MyBool -> b at Conversion.hs:11:5 Does anyone know what's wrong with my code, and how to fix it? Any hints will be appreciated !! Best, --Ke

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What to do next?
by bruce li 27 Mar '14

27 Mar '14

Hi, there, I'm relatively new to Haskell...well... I mean I haven't done anything I believe truely in Haskell. I have gone through materials like Learn You a Haskell for Real Good, Real World Haskell, most chapters in Haskell Wikibook, Write Yourself Scheme in 48 Hours, Algorithms: A Functional Approach and other materials in Haskell Wiki. *But... what I feel is that I'm not confident while writing Haskell code.*Having gone through all those materials with "magic", I always feel I'm writing stupid code and there must be more elegant way... And... what's worse, I feel guilty while using IO monad but I simply cannot avoid it, like when I try to write code generator for a toy compiler, I need to keep state of the registers, which I need IORef... Then I feel its not pure anymore but I don't know how to get around it. I'm wondering if anyone else shares this kind of feeling and what should I do next? Could anyone suggest any project to get hands on experience with Haskell? Another question is that the deeper digging into functional algorithms design (reading the book Pearls of Functional Algorithm Design), the more ignorant I feel. So how do I make up the basics like fold law, list induction etc. Any suggested reading materials? Well.. I think that's a lot question. Thanks for your patience and your kind help.

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Unicode Fanatic — How to use?
by John M. Dlugosz 27 Mar '14

27 Mar '14

I love using non-ASCII characters! Even my name, Długosz, needs Latin-2. My wife's name, 涛, doesn't even use Latin characters. I've inserted thousands of em and en dashes in Wikipedia and can easily type at least three kinds of blank space. When I was in a college class on programming theory, we studied Backus’s FP System and I corresponded with John Backus to ask a question the prof couldn't handle, and he sent me the TeX definitions he used, so my homework really did match the appearance of the textbook. But that's another story. I managed to get my name in the Unicode 3 hardcopy book. So, I relish the ability to use proper math symbols in Haskell. Here is an example of what I've tried: {-# LANGUAGE UnicodeSyntax #-} import Prelude.Unicode f x y = x∨y result = f ∘ id -- problem = ¬ True Is the UnicodeSyntax pragma a different mechanism than the modules? Do I need to state both, or do they clash, or work together, or what? Do I need to list all the individual modules (up to 12 of them) as needed (see http://hackage.haskell.org/package/base-unicode-symbols) or can they be brought in as one easy chunk, as the package itself is one thing? The compiler does not like the ¬ symbol, with or without the pragma. http://hackage.haskell.org/package/base-unicode-symbols-0.2.2.4/docs/Prelud… clearly shows it (the first one!) and it appears in the module source so presumably it didn't choke there. I recall seeing that it is one of the "problematic" symbols but GHC has some extensions. OTOH, I read that λ can’t be made to work, which is sad. Maybe ⅄ (which is a symbol) can be used instead?

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Cabal gives error
by John M. Dlugosz 27 Mar '14

27 Mar '14

When running cabal, it tells me Note: there is a new version of cabal-install available. To upgrade, run: cabal install cabal-install And when I do that, it does a bunch of stuff and then [68 of 70] Compiling Distribution.Simple.Bench ( E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18.1.3\Distribution\Simple\Bench.hs, E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18 .1.3\dist\setup\Distribution\Simple\Bench.o ) [69 of 70] Compiling Distribution.Simple ( E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18.1.3\Distribution\Simple.hs, E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18.1.3\dist\setup\Distribution\Simple.o ) [70 of 70] Compiling Main ( E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18.1.3\Setup.hs, E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18.1.3\dist\setup\Main.o ) Linking E:\Temp\Cabal-1.18.1.3-7348\Cabal-1.18.1.3\dist\setup\setup.exe ... Configuring Cabal-1.18.1.3... setup.exe: does not exist Failed to install Cabal-1.18.1.3 cabal: Error: some packages failed to install: Cabal-1.18.1.3 failed during the configure step. The exception was: ExitFailure 1 cabal-install-1.18.0.3 depends on Cabal-1.18.1.3 which failed to install. [C:\Program Files (x86)\Haskell-Platform] This is, as I recall, where I got stuck trying to get a newer Leksah installed. Upon finishing, it deletes the temp directory tree, so I can't see if there's anything obviously strange with the file it produced. How do I troubleshoot this?

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What does the "!n" mean?
by John M. Dlugosz 27 Mar '14

27 Mar '14

I'm reading http://www.haskell.org/haskellwiki/IO_inside#inlinePerformIO and it shows a passage of code: write :: Int -> (Ptr Word8 -> IO ()) -> Put () write !n body = Put $ \c buf@(Buffer fp o u l) -> if n <= l then write' c fp o u l else write' (flushOld c n fp o u) (newBuffer c n) 0 0 0 where {-# NOINLINE write' #-} write' c !fp !o !u !l = -- warning: this is a tad hardcore inlinePerformIO (withForeignPtr fp (\p -> body $! (p `plusPtr` (o+u)))) `seq` c () (Buffer fp o (u+n) (l-n)) I got as far as the second line, looking things up in this index <http://hackage.haskell.org/package/base-4.6.0.1/docs/doc-index.html> But “write !n body = ⋯” I understand write is defined to be a function taking an Int and another function, but what does !n mean? I went through the 2010 Report (BTW, the PDF is useless for searching for the ! character so I used the HTML version page-by-page) and found it used as a modifier for named record fields (it says "strict" but I think it's describing non-optional). Then I went through the GHC users guide for language extensions, and found a reference in §7.2.1 without explanation but as an example where the difference between f x = let (Foo a b, w) = ..rhs.. in ..body.. and f x = let !(Foo a b, w) = ..rhs.. in ..body.. is "you must make any such pattern-match strict". Strict pattern matching is not mentioned elsewhere nor is it in the Haskell Report, so what am I missing? I suspect that the usage I'm asking about is related. The GHC users guide also mentions it again in §7.4.6 which I think is a reference to the same feature, “You can use strictness annotations, in the obvious places in the constructor type” but that's for use with a completely different extension (GADT types) I also recognize the “@” mark as naming the entire variable rather than just the parts of the pattern matched, but “buf” is not actually used anywhere, so does it mean something different, or has other effects, or what? Thanks in advance, —John

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Haskell Weekly News
by harry 27 Mar '14

27 Mar '14

The last edition of HWN was in December. Is it defunct, or just hibernating? (This probably belongs on haskell-cafe, but for some reason it's not accepting my email.)

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Hello World
by John M. Dlugosz 27 Mar '14

27 Mar '14

I'm learning Haskell as a background activity, and just wanted to introduce myself and get the list subscription stuff sorted out. My primary machine runs Windows 7. I have Haskell Platform working OK including GCHi-Win, but could not get Lekseh to work. However, trying that showed me the world of Cabel, and I have yet to find an introduction/tutorial on that. The Haskell Platform comes with some libraries, but doesn't have a Cabel tool or anything I'd reconize as a repository, like Lekseh was setting up. It's said that the best feature of Perl is CPAN. I suppose Cabel is Haskell's equivalent of that, or aims to be. So, what's up with that? Starting from a blank prompt (that can run ghc successfully) now what? —John

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rolling my own split
by abraham polishchuk 27 Mar '14

27 Mar '14

I am relatively new to Haskell, having spent the last half a decade or so developing in Ruby. I am attempting to (for the sake of exercise) implement a split function from the ground up in Haskell, and for the life of me I can not figure out why it doesn't work. Without further ado: module MyAwesomeModule where import qualified Data.Text as T outputSplit :: String -> [String] -> IO () outputSplit s tokens = print $ splitRecursive tokens s splitRecursive :: [String] -> String -> [String] splitRecursive tokens s = splitOneOf tokens s splitOneOf :: [String] -> String -> [String] splitOneOf [] s = [] splitOneOf (t:tokens) s = map (splitOneOf tokens)(map (T.unpack) (T.splitOn (T.pack t) (T.pack s))) ++ (splitOneOf tokens s) which errors out with: Couldn't match type `[Char]' with `Char' Expected type: String -> String Actual type: String -> [String] In the return type of a call of `splitOneOf' In the first argument of `map', namely `(splitOneOf tokens)' In the first argument of `(++)', namely `map (splitOneOf tokens) (map (T.unpack) (T.splitOn (T.pack t) (T.pack s)))' Failed, modules loaded: none. If anyone has any ideas, that would be awesome! And yes, I'm aware of Data.List.Split, like I said, trying to roll my own....

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emacs + ghc-mod + cabal repl
by Miguel Negrão 23 Mar '14

23 Mar '14

Hi Can anyone tell me how switch from using ghci to cabal repl in emacs with ghc-mod ? I would like to use emacs with cabal projects with sandboxes, where ghci will not load the right packages. best, -- Miguel Negrão http://www.friendlyvirus.org/miguelnegrao

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Implement semantic editor combinators
by Javran Cheng 23 Mar '14

23 Mar '14

Hi, I read Conal Elliott's "Semantic editor combinators" and write myself a tutorial to answer how it works and give some observations about it, here I want to share it with you: http://javran.github.io/posts/2014-03-21-implement-semantic-editor-combinat… Hope you enjoy it and please feel free to give me comments and indicate my misconceptions. Thanks, Javran

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