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type error in sub-function
by James Toll 16 Feb '14

16 Feb '14

Hi, I am trying to write a simple function to determine the divisors of an integer. In its simplest form the type signature should be something like: divisors :: Int -> [Int] divisors x = 1 : lower ++ upper ++ x : [] where lower = filter (\y -> mod x y == 0) [2..(ceiling . sqrt) x] upper = sort $ map (div x) lower Although, I think my type signature isn’t complete as it ignores a lot of what’s going on in the function. Regardless, the function throws an error when evaluated. I don’t know if I just need a more accurate type signature, or if there is a bigger problem. From what I can tell, my problem is in the lower function, but when I deconstruct it into its parts, they work individually and as a whole, but not as a stand-alone function. For example, using x = 36: Prelude> [2..(ceiling . sqrt) 36] [2,3,4,5,6] Prelude> filter (\y -> mod 36 y == 0) it [2,3,4,6] Or as a whole: Prelude> filter (\y -> mod 36 y == 0) [2..(ceiling . sqrt) 36] [2,3,4,6] But when I define this as a function, it throws an error when evaluated. Prelude> let lower x = filter (\y -> mod x y == 0) [2..(ceiling . sqrt) x] Prelude> lower 36 <interactive>:6:1: No instance for (RealFrac b0) arising from a use of `lower' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance RealFrac Double -- Defined in `GHC.Float' instance RealFrac Float -- Defined in `GHC.Float' instance Integral a => RealFrac (GHC.Real.Ratio a) -- Defined in `GHC.Real' In the expression: lower 36 In an equation for `it': it = lower 36 <interactive>:6:7: No instance for (Num b0) arising from the literal `36' The type variable `b0' is ambiguous Possible fix: add a type signature that fixes these type variable(s) Note: there are several potential instances: instance Num Double -- Defined in `GHC.Float' instance Num Float -- Defined in `GHC.Float' instance Integral a => Num (GHC.Real.Ratio a) -- Defined in `GHC.Real' ...plus three others In the first argument of `lower', namely `36' In the expression: lower 36 In an equation for `it': it = lower 36 At this point, I’m not sure what I need to do to get his working properly. It appears to be a type error, but I’m apparently not understanding the error message enough to fix the problem. Any suggestions would be appreciated. Thanks, James

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filter by max length a list of lists with equivalent values
by James Toll 11 Feb '14

11 Feb '14

Hi, I have what’s proving to be a tricky little problem for me to solve. I’m trying to take a list of a list of Int’s and return the longest list for each list with elements of equivalent value. That’s not a great explanation, so let me try to give an example: If I have a list of lists grouped by element value. ghci> group [1,1,1,1,2,2,2,2,3,3,2,2,2,5,6,2,2,7] [[1,1,1,1],[2,2,2,2],[3,3],[2,2,2],[5],[6],[2,2],[7]] I would like to take the subset of the outer list containing only the longest of the inner lists for any particular element. So for this particular example, the desired output would be: [[1,1,1,1],[2,2,2,2],[3,3],[5],[6],[7]] The lists [2,2,2] and [2,2] would be removed because they’re shorter than [2,2,2,2]. Any thoughts on how to do this would be appreciated. Thanks and best regards, James

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problems loading zeromq
by fa-ml 10 Feb '14

10 Feb '14

Hello list, I am trying to cabal install hbro. After a bit of fiddling I stumbled on this: Resolving dependencies... Configuring hbro-1.1.2.1... Building hbro-1.1.2.1... Preprocessing library hbro-1.1.2.1... [ 1 of 19] Compiling Paths_hbro ( dist/build/autogen/Paths_hbro.hs, dist/build/Paths_hbro.o ) [ 2 of 19] Compiling Hbro.Keys ( Hbro/Keys.hs, dist/build/Hbro/Keys.o ) Loading package ghc-prim ... linking ... done. [omissis] Loading package semigroups-0.12.2 ... linking ... done. Loading package zeromq3-haskell-0.5.1 ... linking ... ghc: /home/user/.cabal/lib/zeromq3-haskell-0.5.1/ghc-7.4.1/HSzeromq3-haskell-0.5.1.o: unknown symbol `zmq_ctx_new' ghc: unable to load package `zeromq3-haskell-0.5.1' cabal: Error: some packages failed to install: hbro-1.1.2.1 failed during the building phase. The exception was: ExitFailure 1 I am puzzled because I cabal installed zeromq3-haskell *and* downloaded/installed 0mq3 (./configure; make; su; make install). 0mq version is 3.2.5 and I am running debian/stable(i386). Any idea on what is happening here? Thanks in advance -F

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Problem with GADTs and Parsec
by Lorenzo Tabacchini 09 Feb '14

09 Feb '14

Hello, I am developing a very simple programming language for didactic purposes. Expressions can be composed of different types, like Int, Bool, String etc... The AST is compiled to JavaScript and the result is shown in the browser. The code is more or less like this: {-# LANGUAGE GADTs #-} data Expr a where Int :: Int -> Expr Int Bool :: Bool -> Expr Bool Text :: String -> Expr String -- Int operations Add :: Expr Int -> Expr Int -> Expr Int -- Bool operations And :: Expr Bool -> Expr Bool -> Expr Bool -- etc. etc. -- converts to JavaScript compile :: Expr a -> String compile (Int n) = show n compile (Text t) = show t compile (Add x y) = compile x ++ "+" ++ compile y -- etc. etc. The compilation works perfectly, but I am stuck with the parser. Since I don't know the types in advance, I would like to parse the whole expression at once. If I do: exprParser :: Parsec String u (Expr a) exprParser = parens exprParser <|> (reserved "true" >> return (Bool True)) <|> (reserved "false" >> return (Bool False)) <|> (stringLiteral >>= return . Text) The compiler says: "Couldn't match type `Bool' with `[Char]'" Where am I wrong?

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Getting GHC 7.8.1
by Mathew Phillips 08 Feb '14

08 Feb '14

Is there a recommended way to install a newer version of ghc when you already have the haskell platform installed? I've been reading the documentation and I can't seem to find anything that shows how to actually get it other than compiling from source, and I don't want to brick my current setup if both cant coexist. Matt P.

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coming to grips with hackage
by tam＠hiddenrock.com 08 Feb '14

08 Feb '14

Hi there -- I've groveled through the Hackage site, I've read a bunch of cabal documentation, and I've read far more blog entries, mailing list posts, and other things turned up by Google than I can count. Please forgive me if my confusions have been addressed elsewhere; if they have, I would appreciate a pointer. I'm having trouble wrapping my head around how to work with Hackage in the context of a Linux distribution that already has its own package management situation. Between distribution-provided packages containing Haskell programs and/or libraries, system-wide cabal installs, user-specific cabal installs, and cabal sandboxes, I can't seem to work out how the hackage/cabal architects intend people (ie, me) to juggle things. Here's a potential scenario: I want to use pandoc, for which my distro provides packages, so I install it along with its various and sundry dependencies using the package manager. Then I want to use another program for which my distro *doesn't* provide a package, so I use cabal-install to install it along with its various and sundry dependencies in ~/.cabal. Then I have a crazy idea I want to play around with in code. Initially, I've got all the libraries I need already installed, but eventually I discover there's a feature in foo-1.1.3.4 that my idea just can't live without. Unfortunately, foo-1.0.5.12 is required by pandoc. At this point, it would seem like cabalizing my little idea and letting it play by itself in a cabal sandbox is the preferred method. This seems reasonable, I say to myself, and proceed to recompile every single library upon which my little idea (recursively) depends. But then pandoc gets updated, as well as many of the libraries upon which it depends, which results in a wholesale upgrade of most of the system-wide libraries and now the stuff I've installed in my home directory has all manner of broken dependencies. There's probably a cabal-install command I can run at this point to clean things up (though presumably I'd have to run it for every source tree that uses a library that has been upgraded) but that's beside the point. My confusion is how this scales. Enough little ideas that depend on non-trivial libraries and I'm going to have eleventy-billion extremely similar copies of Data.Text littered about my home directory. Is this really the intention? Did I miss something? Are there better ``best practices'' than what I've described? Any guidance is most appreciated! Thank you. pete PS. Bonus question, which seems intimately related: if A depends on B and a new version of B appears, why does A necessarily need to be recompiled? Isn't one of the big wins of shared objects avoiding this very necessity? Presumably this is a consequence of how the GHC runtime does its linking, but I couldn't find helpful documentation on this count.

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Convert numeric IP to Network.Socket.HostAddress
by harry 06 Feb '14

06 Feb '14

I need to convert a numeric IP (such as 1.2.3.4) to a HostAddress for consumption by SockAddrInet. The only function I can find to do this is inet_addr, which is impure (can do DNS lookups). Is there a pure conversion?

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foldM with tuple and maybe
by Miro Karpis 03 Feb '14

03 Feb '14

Hi, please can you help me with following? I have a following function: f1 :: (String, String) -> IO (Maybe String) which I would like to apply to a list of tuples.I have defined the function like this, but that gives me error (below): f2 :: [(String, String)] -> IO (Maybe String) f2 x = foldM f1 0 x error: Couldn't match expected type `IO (Maybe String)' with actual type `[b0] -> m0 (String, String)' In the return type of a call of `foldM' thanks, m.

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tree structure for resolving nested references
by Renah Scarowsky 30 Jan '14

30 Jan '14

Hi, I'm doing some xml processing where elements can reference each other in one particular way. Such a reference is resolved by copying the content of the referenced element to the content of the referencing element. For example, <A ref="B">foo</A>, bar would resolve to <A>bar</A>bar. The references can also be nested, for example: <A ref="B">foo</A>, bar<C>baz</C> would resolve to <A>baz</A>baz<C>baz</C>. I'm looking to collect all the elements with such references in some sort of (tree?) container for efficient resolution. The container should serve two purposes: 1) I need to check whether there are any cyclic references (for example, A references B which references C which references A) - which is invalid. 2) I need to traverse the tree upwards in order to copy the content from one element to another - beginning with the "leaves" (elements with non-nested references), working up the "branches" (elements with nested references) to the "roots" (elements with references that are not referenced by other elements). Obviously there can be many roots, as well as many leaves that are roots themselves. I'm not familiar with trees in Haskell at all. Can anyone provide some guidance on whether there are any existing containers that would be appropriate for checking cyclic dependencies and for reverse traversal? Thanks, Renah Scarowsky Suite Solutions Create>Manage>Deploy <http://www.suite-sol.com/> http://www.suite-sol.com

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breaking code to several lines
by Miro Karpis 29 Jan '14

29 Jan '14

Hi please,... I have one sql insert statement which is too long to be on one line. Is there a way that I can break it several lines? Or does it have to be everything in one line? here is the query: execute_ conn "INSERT INTO ttableXY (column1, column2, column3, column4, column5, column6, column7) VALUES ('var1', 'var2', 'var3', 'var4', 'var5', 'var6', 'var7')" Thanks, Miro

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