December 2014 - Beginners

Why does Haskell PVP have two values for the major version? "A.B..." and a couple other questions
by Zach Moazeni 15 Dec '14

15 Dec '14

Hello, Forgive me if this is a frequently asked question, I've searched the web and can't find an answer. What is the history that led up to the PVP <https://www.haskell.org/haskellwiki/Package_versioning_policy> specifying two values as the major version? I come from other tools that primarily use http://semver.org/ (or a variant) and I'm confused in what cases I should bump "A" but not "B" (or bump "B" but not "A")? A concrete example: If I make backwards incompatible changes to a package whose latest version is 1.0.x, should the next version be 2.0.x or 1.1.x? What sorts of things should I consider for choosing 2.0 over 1.1 and vice versa? Another question, by far most packages I have encountered either lead with a 0 or a 1 for "A". Does that have some bearing on the long term stability that package users should expect in the future? Finally, if "1.0.x.." is meant to convey a level of long term support, does "B" get rarely used? Since "C" version bumps to include backwards compatible additions, and "D"+ for backwards compatible bug fixes. (I know "D" isn't technically a PVP category, I'm just relating it to the patch version of semver). Thanks for your help! -Zach

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exercises/examples for streams
by Pascal Knodel 15 Dec '14

15 Dec '14

Hello, I'm preparing for an exam (university level introductory course). Functional programming in Haskell is part of it. That includes the concept of streams (theoretically infinite list computations). I have difficulties in solving exercises that ask me to define streams for a given problem. Do you know good examples, the difficulty should be maybe a little harder than Fibonacci or intersections, but solvable in about 5 to 10 minutes. If there are examples from the book "Haskell - the craft of functional programming", please don't spoiler, I'm reading it parallel to this conversation. Don't give solutions, just problems (5 to 10 more will do). Examples, other than integer sequences are welcome. Pascal

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Creating Indexes
by martin 15 Dec '14

15 Dec '14

Hello all, I recently wrote a Haskell program, which accesses Lists extensivly. That was okay to verify the overall idea, but performance degraded rapidly when I made the lists bigger. There was a quadratic effect and this was not surprizing, given the nature of the code. I am somewhat aware of Haskell types which offer faster access. But they all seem to assume that I know the way data is accessed when I write the type. A new access path may force me to rededign the type. What I am looking for is something which behaves like indexes in a RDBMS, i.e. to define a list-like type on an innocent data type (like a tuple) and then add indexes as needed. Is there such a thing?

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Haskell type conversion for postgresql array types
by Riaan 15 Dec '14

15 Dec '14

This is a question about database query libraries. I am trying to do some analytics on a Postgresql database and up to now have been using Database.HDBC for most of my querying. One of the columns on the database is a two dimensional float8 array. I tried using Database.PostgreSQL.Simple but got stuck on this array as I could not figure out how to extend the built in conversion functions to cater for something like this. So went back to HDBC. But now my queries are starting to get fairly long and I have been looking at libraries like Persistent with Esqualeto, HaskellDB and Groundhog to make my queries a little more composable and type safe. However I have not been able to find any examples of any of these libraries using postgres arrays and given my postgresql-simple experience I worried that I might similarly be unable, because of my low level of haskell ability, to extend the conversion functions. So to my question. Does anyone have experience with one of these libraries in dealing with postgresql arrays and could someone perhaps send me a simple example that I could use as a basis. Failing that, can anyone advise me on which of these libraries have the most haskell newbie (LYAH trained, but still struggling with RWH) friendly approach to writing new conversion functions? Thanks

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Output of `ghc-mod check` and `runhaskell`
by Dontdie YCH 14 Dec '14

14 Dec '14

Hello, My current setup for haskell. Arch Linux nix package manager - only for haskell stuff nix-shell vim syntastic + ghc-mod guard - `cabal run` on change of '*.hs' I would like to improve readability of ghc-mod's output. Compare to output of runhaskell, It's quite hard to understand. ~~~ % ghc-mod check LogAnalysis.hs LogAnalysis.hs:90:25:Couldn't match expected type ‘[ghc-prim:GHC.Types.Char]’ with actual type ‘Log.LogMessage -> [ghc-prim:GHC.Types.Char]’In the second argument of ‘(ghc-prim:GHC.Classes.==)’, namely ‘upcase GHC.Base.. LogAnalysis.getMsg’In the expression: str' ghc-prim:GHC.Classes.== upcase GHC.Base.. LogAnalysis.getMsgIn the expression: str' ghc-prim:GHC.Classes.== upcase GHC.Base.. LogAnalysis.getMsg GHC.Base.$ x LogAnalysis.hs:90:17:Couldn't match expected type ‘s -> t’ with actual type ‘ghc-prim:GHC.Types.Bool’Relevant bindings include x :: s (bound at LogAnalysis.hs:90:13) p :: s -> t (bound at LogAnalysis.hs:90:11)The first argument of ($) takes one argument,but its type ‘ghc-prim:GHC.Types.Bool’ has noneIn the expression: str' ghc-prim:GHC.Classes.== upcase GHC.Base.. LogAnalysis.getMsg GHC.Base.$ xIn an equation for ‘p’: p x = str' ghc-prim:GHC.Classes.== upcase GHC.Base.. LogAnalysis.getMsg GHC.Base.$ x % runhaskell LogAnalysis.hs LogAnalysis.hs:90:17: Couldn't match expected type ‘s -> t’ with actual type ‘Bool’ Relevant bindings include x :: s (bound at LogAnalysis.hs:90:13) p :: s -> t (bound at LogAnalysis.hs:90:11) The first argument of ($) takes one argument, but its type ‘Bool’ has none In the expression: str' == upcase . getMsg $ x In an equation for ‘p’: p x = str' == upcase . getMsg $ x LogAnalysis.hs:90:25: Couldn't match expected type ‘[Char]’ with actual type ‘LogMessage -> [Char]’ In the second argument of ‘(==)’, namely ‘upcase . getMsg’ In the expression: str' == upcase . getMsg In the expression: str' == upcase . getMsg $ x ~~~ How I can do either - change ghc-mod's output like runhaskell or - use runhaskell for vim syntastic Thanks.

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Buggy behavior of "withFile"
by Zoran Plesivčak 13 Dec '14

13 Dec '14

I've encountered unexplainable of "withFile" function. Consider "example.hs" program: import System.IO main = do cnts <- withFile "example.hs" ReadMode $ (\h -> do res <- hGetContents h --putStr res return res) putStr cnts When commented-out line is like that, program doesn't write out anything to the STDOUT. If "--" (commend characters) are removed, program writes out contents of "example.hs" two times. Is this expected behavior? I asked on #haskell (freenode) and one fellow there found it equally confusing... ------------------------------------------- GHC version: The Glorious Glasgow Haskell Compilation System, version 7.6.3 OS: Linux Thanks, Zoran Plesivčak

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Doubts regarding the "read" function.
by Venu Chakravorty 13 Dec '14

13 Dec '14

Hello everyone, I am new to Haskell and this might seem very naive, please bear with me. ======================================= Prelude> read "4" + 4 8 Prelude> (read "4" :: Int) + 4 8 Prelude> read "hello " ++ "world" "*** Exception: Prelude.read: no parse Prelude> (read "hello" :: String) ++ " world" "*** Exception: Prelude.read: no parse ======================================= Could someone please explain why the last two statements don't work? My understanding is that "read" has a type of "read :: (Read a) => String -> a". So, "read "hello" " should give me an instance of type "Read" to which I am appending the string "world" (just like the first 2 cases where I get an instance of "Read" ("Int" in this case) to which I am adding another "Int" and I get a "Num" which is then displayed). I expected to see "hello world" as the output. Is it that the type "String" is not an instance of type class "Read"? Please tell me what I am missing here. Regards, Venu Chakravorty.

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Solution to problem 17 (99 Haskell problems)‏
by jean lopes 08 Dec '14

08 Dec '14

Hello, I am wondering, is this solution acceptable to the question 17 ? ( https://www.haskell.org/haskellwiki/99_questions/11_to_20) code: split :: [a] -> Int -> ([a], [a]) split xs y = fst (mapAccumL (\a b -> (let f = fst a s = snd a l = length f in if l < y then (f ++ [b], s) else (f, s ++ [b]), b)) ([],[]) xs) Also, any inputs are welcome (indentation, misuse of something) Best regards, Jean Lopes

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Support for PostgreSQL array types
by Riaan 08 Dec '14

08 Dec '14

This is a question about database query libraries. I am trying to do some analytics on a Postgresql database and up to now have been using Database.HDBC for most of my querying. One of the columns on the database is a two dimensional float8 array. I tried using Database.PostgreSQL.Simple but got stuck on this array as I could not figure out how to extend the built in conversion functions to cater for something like this. So went back to HDBC. But now my queries are starting to get fairly long and I have been looking at libraries like Persistent with Esqualeto, HaskellDB and Groundhog to make my queries a little more composable and type safe. However I have not been able to find any examples of any of these libraries using postgres arrays and given my postgresql-simple experience I worried that I might similarly be unable, because of my low level of haskell ability, to extend the conversion functions. So to my question. Does anyone have experience with one of these libraries in dealing with postgresql arrays and could someone perhaps send me a simple example that I could use as a basis. Failing that, can anyone advise me on which of these libraries have the most haskell newbie (LYAH trained, but still struggling with RWH) friendly approach to writing new conversion functions? Thanks Riaan

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Record types with multiple constructors
by Derek McLoughlin 07 Dec '14

07 Dec '14

Hi, Record types usually have a single constructor. I've even seen blog posts that suggest that they must have a single constructor. However, multiple constructors are allowed: data Employee = RegularEmployee { name :: String } | Supervisor { name :: String, salesTarget :: Double } Manager { name :: String, salesTarget :: Double budget :: Double } I don't see this used much in Haskell code - either in explanatory books/tutorials or in code I've examined on GitHub. Are there drawbacks to using multiple constructors in this way? Derek.

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