June 2011 - Beginners

nub... almost
by Christopher Howard 04 Jun '11

04 Jun '11

Weird request: For a certain application, the "nub" function from Data.List is exactly what I need... almost. Nub removes duplicates, keeping the /first/ occurrence of each element. However, I need a function that removes duplicates, keeping the /second/ occurrence of each element. There wouldn't happen to be a function already that does this, would there? I'm working with a custom type which is a member of Eq, but some of the data in the type is not used in the comparison. So it is important which of the two "duplicates" are actually saved. -- frigidcode.com theologia.indicium.us

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Data declarations
by Christopher Howard 03 Jun '11

03 Jun '11

I'm trying to understand the Haskell type system, because it is portrayed as a rather powerful tool when properly harnessed. There are a few things, however, I could use some clarification on. For starters, I'm a bit confused about the "data" declarations: I understand (correctly, I hope) that the purpose of a "data" declaration is to create a new type. So this makes sense: data KeyValuePair = KeyValuePair String String However, this also compiles: data KeyValuePair a b = KeyValuePair String String As well as does this: data KeyValuePair a b = KeyValuePair String And this: data KeyValuePair a b = KeyValuePair So you can see how I'm getting a bit confused? -- frigidcode.com theologia.indicium.us

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Text.Regex.TDFA docs?
by Mike Meyer 03 Jun '11

03 Jun '11

I'm running ghc 7.0.3 on FreeBSD, and trying to figure out Text.Regex - pretty much any flavor, actually. Based on this http://www.haskell.org/haskellwiki/Regular_expressions, it appears that TDFA is probably the right library to use, though Posix seems to be popular with the tutorials. There's a couple of documents around: http://www.serpentine.com/blog/2007/02/27/a-haskell-regular-expression-tuto… http://book.realworldhaskell.org/read/efficient-file-processing-regular-exp… But it appears to be dated, as parts of them just don't work, using either TDFA or Posix. For instance, both imply that [String] ought to give me a list of matches, but that doesn't work in either Posix or TDFA. Prelude Text.Regex.TDFA> "this foo is the foo" =~ "foo" :: [String] <interactive>:1:23: No instance for (RegexContext Regex [Char] [String]) arising from a use of `=~' Possible fix: add an instance declaration for (RegexContext Regex [Char] [String]) In the expression: "this foo is the foo" =~ "foo" :: [String] In an equation for `it': it = "this foo is the foo" =~ "foo" :: [String] Prelude Text.Regex.TDFA> :m -Text.Regex.TDFA Prelude> :m +Text.Regex.Posix Prelude Text.Regex.Posix> "this foo is the foo" =~ "foo" :: [String] <interactive>:1:23: No instance for (RegexContext Regex [Char] [String]) arising from a use of `=~' Possible fix: add an instance declaration for (RegexContext Regex [Char] [String]) In the expression: "this foo is the foo" =~ "foo" :: [String] In an equation for `it': it = "this foo is the foo" =~ "foo" :: [String] Prelude Text.Regex.Posix> Come to think of it, this seems to be the case for most of the [<type>] types - except that [[String]] seems to work. I.e. - (Int, Int) works, but [(Int, Int)] doesn't; (MatchOffset, MatchLength) works, but [(MatchOffset, MatchLength)] doesn't. The docs here http://hackage.haskell.org/packages/archive/regex-base/0.93.2/doc/html/Text… Seem to imply that there are types not mentioned in the tutorials, but trying to use most of those (actually, it looks like anything using All*) generates errors because the resulting type isn't an instance of Show: Prelude Text.Regex.Posix> "this foo is the foo" =~ "foo" :: AllTextSubmatches [] String <interactive>:1:1: No instance for (Show (AllTextSubmatches [] String)) arising from a use of `print' Possible fix: add an instance declaration for (Show (AllTextSubmatches [] String)) In a stmt of an interactive GHCi command: print it So - how do I go about getting a list of all matches (the tutorials [String] behavior) or a list of offset/length tuples (the tutorials [(MatchOffset, MatchLength)] behavior? thanks, <mike -- Mike Meyer <mwm(a)mired.org> http://www.mired.org/ Independent Software developer/SCM consultant, email for more information. O< ascii ribbon campaign - stop html mail - www.asciiribbon.org

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question regarding standard libraries
by Jake Penton 03 Jun '11

03 Jun '11

I have a couple of questions about "standard libraries" as referred to in the Haskell 98 and 2010 reports. First, is there anything to infer from the use of the term "libraries"? What I see documented in the reports is a set of modules. Is this just a loose usage of the term libraries to mean a set of modules implemented somehow or other? Second, although I have only perused the 98 and 2010 reports briefly, what "standard" means in this context does not stand out for me. Part II of the 2010 report jumps right into the description of Control.Monad with no mention of the relationship of Part II to the rest of the report. Are the documented modules part of the language? Third, as I am learning Haskell by studying Real World Haskell I note that several of the modules discussed therein are not listed in the Haskell 2010 report. A notable example is Data.Map. What is the status of such modules? Are they just (maybe) available in particular implementations, or in privately donated packages? Finally, is there a good way to track changes in the implementation of various modules, whether "standard" or otherwise? For example I discover that Data.Map.lookup has changed (in ghc anyway) since RWH was written. Is this change agreed upon somewhere, or is it just dependent on whatever happens to be delivered with the Haskell implementation that I use? Thank heaven for a beginner's mailing list where I can ask dumb questions. And thanks in advance once again for your kind efforts to help me out. - J -

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Different ByteStrings, what now?
by Raphael Päbst 02 Jun '11

02 Jun '11

Hello once again! My forays into Haskell are once again stopped by my weak Haskell Fu. I am working with ProtocolBuffers and so far everything worked fine, but now I have encountered a problem. I am trying to serialize a message that contains, among other things, a blob of binary data. My problem now is to get that binary data into the message. The message requires the data type Text.ProtocolBuffers.Header.ByteString, whereas my data is of the type Data.ByteString. How can I turn one into the other or get my data into the right format in te first place? I tried to find a function to solve this, but I couldn't find it in the ProtoBuff documentation. Thanks Raf

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How to access non-exported function from Data.Set?
by Christoph Bussler 02 Jun '11

02 Jun '11

Hi, In Data.Set there is a function splitLookup splitLookup :: Ord a => a -> Set a -> (Set a,Maybe a,Set a) that is not exported, but would precisely give me the functionality that I need. Not sure why it is not exported, but is there a way to use it? Thanks, Christoph -------------------------------------------------------------------------- Christoph Bussler ChBussler(a)aol.com www.real-programmer.com www.linkedin.com/in/chbussler www.google.com/search?hl=en&q=Christoph+Bussler www.google.com/search?hl=en&q=Christoph%20Bussler&btnI=I%27m+Feeling+Lucky --------------------------------------------------------------------------

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Install on "foreign" server problem
by Lawrence Bottorff 01 Jun '11

01 Jun '11

I'm trying to install the binary of 7.0.3 on a Bluehost.com server where I have an account and ssh shell. I want to eventually run Yesod or happstack or some other sort of haskell on web. I've talked with their tech support and they're cool with it as long as it's in-place. Good. I try the ./configure and it errors saying things like: checking for path to top of build tree... utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.9' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) utils/ghc-pwd/dist/build/tmp/ghc-pwd: /lib64/libc.so.6: version `GLIBC_2.7' not found (required by utils/ghc-pwd/dist/build/tmp/ghc-pwd) configure: error: cannot determine current directory I'm not (nor ever was) much of a sysadmin type. Any advice? Actually, I'm just trying to do Web with Haskell on a server/cloud other than my own home box. I have a Web client and I'd like to get us off PHP. Any Haskell-friendly clouds/hosts to suggest? Olwe

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Problems with the Network package
by Raphael Päbst 01 Jun '11

01 Jun '11

Hey everyone! I'm still pretty inexperienced with Haskell and am now facing problems with the Network package. I tried to install it but this always fails, no matter if I try to install Network-2.3.0.3 or a previous version. The cabal install always fails with the error compiling dist\build\Network\Socket_hsc_make.c failed (exit code 1). Now I'm wondering, where I can find out what the problem is, what I can do about it and which additional information I need to solve it. I hope this is not too stupid a question, but my searches on the net haven't turned up anything useful so far and I'm pretty much stuck now. Raf

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Question on Real World Haskell ch.13 SymbolicManip
by Jake Penton 31 May '11

31 May '11

Greetings. I post here from time to time as I work my way through Real World Haskell. I believe that I have understood most of what I have studied. However, today I ran into something that seems like magic. I hope it's ok to refer to RWH here, as I believe that the book is very commonly used for learning the language. I'll include the code that is puzzling me here anyway, though. On p.310 of RWH we have: data Op = Plus | Minus | Mul | Div | Pow deriving (Eq, Show) data SymbolicManip a = Number a | Arith Op (SymbolicManip a) (SymbolicManip a) deriving (Eq, Show) instance Num a => Num (SymbolicManip a) where a + b = Arith Plus a b a - b = Arith Minus a b a * b = Arith Mul a b negate a = Arith Mul (Number (-1)) a abs a = error "abs is unimplemented" signum _ = error "signum is unimplemented" fromInteger i = Number (fromInteger i) Ok, so far so good. But the following example seems quite astounding to me: ghci> (5 * 10 + 2)::SymbolicManip Int Arith Plus (Arith Mul (Number 5) (Number 10)) (Number 2) The book says to notice that haskell "converted" 5 * 10 + 2 into a SymbolicManip. Indeed! My understanding breaks down at this point. I suspect that it may be my weak grasp of the implications of lazy evaluation, and typeclasses generally. My (incorrect) intuition makes me think that the stuff inside the parentheses should be evaluated first, and then typed as a SymbolicManip. This would give the result Number 52. My question may be too vague to answer easily, but what I am hoping for is a bit of a narrative regarding how haskell knows that the pieces inside the parentheses are each of type SymbolicManip Int. Or something like that.... Thanks in advance. - Jake -

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