- Beginners - Haskell.org

Pattern Matching
by Yugesh Kothari 27 Apr '19

27 Apr '19

This is probably a stupid question but I can't seem to understand the use of @ in haskell pattern matching. Ex - compress (x:ys@(y:_)) | x==y = compress us | otherwise = x : compress us compress us = us Thanks!

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IO Control Structures
by Leonhard Applis 23 Apr '19

23 Apr '19

Hey there, I am writing a simple game and the only missing part is IO (Sadly, i need that to play). I've got most things pure, e.g. makeMove :: GameState -> Move -> GameState, initialBoard:: GameState and lost :: GameState -> Bool work pure and as intended. I also got a function gameLoop :: GameState -> IO GameState which asks for Input and applies it. I don`t know how to do my main :: IO () which should welcome the player, start the gameloop, and print godbyes the game if it's either won or lost Here i am missing a control structure, to demonstrate: show hello board = initialBoard while (not end board) board = gameLoop board show winner I could also make my Gameloop recursive, while i think that i get this working, it seems awfully complex (and ugly) to me. Best regards Leonhard

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multidimensional data
by Barry DeZonia 23 Apr '19

23 Apr '19

I've played with Haskell on and off for a number of years. I have a question that isn't directly related to Haskell only but to functional programming. Sorry if this is off topic. In Haskell, how would you represent a 5 dimensional data structure of numeric values? Imagine a dataset with axes x, y, z, channel, and time. Is there an easy way to represent this in the data that doesn't run into all kinds of memory limits? How would you map the data to a new state such that the new state had pixel (1000, 2000, 100, 1, 20) set to 53? Thanks for any info.

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Newtype, Data.Text and instance declarations...
by Guy-Laurent Subri 11 Apr '19

11 Apr '19

Hi all, There are a few things that I don't understand involving Data.Text and instance declarations... I'm building a small web app to practise Haskell and am using the SQLite.Simple library. I have defined a few types and functions like so: ''' data BookmarkDB = BookmarkDB Connection Table newtype Table = Table Text instance ToField Table where toField = SQLText initDB :: FilePath -> Table -> IO BookmarkDB initDB fp table = do let query = Query ("CREATE TABLE IF NOT EXISTS :table " <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp executeNamed conn query [":table" := table] return $ BookmarkDB conn table ''' 1. My first question is about the instance declaration. Why would ''' instance ToField Table where toField = SQLText ''' return this error ? ''' Couldn't match type ‘Table’ with ‘Text’ Expected type: Table -> SQLData Actual type: Text -> SQLData • In the expression: SQLText In an equation for ‘toField’: toField = SQLText In the instance declaration for ‘ToField Table’ ''' I've noticed that if I write ''' instance ToField Table where toField (Table t) = SQLText t ''' I have no error and it compiles. I don't get it, what is 'toField' supposed to receive except a Table? 2. My second question is concerning the initDB function. If I write it that way, the code compiles, but I get an error during execution: bkmrk: SQLite3 returned ErrorError while attempting to perform prepare "CREATE TABLE IF NOT EXISTS :table (url TEXT, tags TEXT, date INTEGER)": near ":table": syntax error I don't understand what is wrong with the code and I doubt that my query is wrong because everything works with the last code snippet I wrote (see question #3). 3. Trying to get around the error I've noticed a few things. If I try to write this instead: ''' initDB :: FilePath -> Table -> IO BookmarkDB initDB fp table = do let query = Query ("CREATE TABLE IF NOT EXISTS " <> table <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp execute_ conn query return $ BookmarkDB conn table ''' it doesn't compile because table is of type Table and not Data.Text.Internal.Text, which I get, but I don't know what is the Haskell way of resolving this. I've also noticed that this will help: ''' initDB :: FilePath -> Table -> IO BookmarkDB initDB fp t@(Table table) = do let query = Query ("CREATE TABLE IF NOT EXISTS " <> table <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp execute_ conn query return $ BookmarkDB conn t ''' But I *really* don't get why! The most confusing part for me is that the last initDB function will do exactly what I want it to do, but I am unable to understand why it works...Why am I able to concatenate the 'table' now and not before and why is 't' of a different type than 'table'? All of this is very confusing and I find myself unable to reason about it. Sorry if this is a big/messy question. I hope everything is clear enough to answer... Thank you all!

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Re: [Haskell-beginners] Beginners Digest, Vol 130, Issue 3
by stevix＠zoho.com 03 Apr '19

03 Apr '19

Thank you, Simon. That worked Steve On Apr 3, 2019, 05:50 -0700, beginners-request(a)haskell.org, wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. How to use earlier ghc version with stack (stevix(a)zoho.com) > 2. Re: How to use earlier ghc version with stack (Simon Jakobi) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Tue, 2 Apr 2019 12:56:57 -0700 > From: stevix(a)zoho.com > To: beginners(a)haskell.org > Subject: [Haskell-beginners] How to use earlier ghc version with stack > Message-ID: <0f587e54-1e09-441c-8f08-fb2fbed284df@Spark> > Content-Type: text/plain; charset="utf-8" > > I'm going through a book on Haskell that is based on a ghc version between 7.8 and 8.0 > > I created a new project using the default settings and have a much more recent version of ghc installed > > How to I fall back to an earlier version? > > Using haskell stack on manjaro linux > > Steve >

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How to use earlier ghc version with stack
by stevix＠zoho.com 02 Apr '19

02 Apr '19

I'm going through a book on Haskell that is based on a ghc version between 7.8 and 8.0 I created a new project using the default settings and have a much more recent version of ghc installed How to I fall back to an earlier version? Using haskell stack on manjaro linux Steve

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ANN: Haskell Beginners PASHpost Community: "Haskell: The Haskell-Beginners Community"
by Benjamin L. Russell 02 Apr '19

02 Apr '19

Since the Google+ social network service is scheduled to be "sunset" (i.e., put out of service) on April 2, 2019, I have created a new Haskell beginners PASHpost community, "Haskell: The Haskell-Beginners Community" (see https://pashpost.com/group/haskell-the-haskell-beginners-community) and migrated a substantial portion of the original posts thereto (sans posts with missing links or irrelevant content). Feel free to join and contribute! -- Benjamin L. Russell -- Benjamin L. Russell / DekuDekuplex at Yahoo dot com http://dekudekuplex.wordpress.com/ Computer Science Document Translator/Editor "Furuike ya, kawazu tobikomu mizu no oto." -- Matsuo Basho^

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Re: [Haskell-beginners] Integrating external dependencies with stack
by Noel Kwan 01 Apr '19

01 Apr '19

I am trying to use stack to build a web scraper with scalpel. However, it requires curl-1.8.0 as a dependency and I have no clue how to integrate this dependency. Any insight would be greatly appreciated.

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IO String and String using readFile
by Yugesh Kothari 27 Mar '19

27 Mar '19

Hi, I am trying to read data from a file. When I do- ghci> contents <- readFile "file.txt" ghci> words contents Then everything works. The same thing when done in a .hs file gives an IO String vs [String] error. Why is that so?

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A sort of fold for bits
by Mario Lang 26 Mar '19

26 Mar '19

Hi. I have written the following function for "iterating" over all set bits in a Bits instance: foldBits :: (Bits bits, Num bits) => (a -> Int -> a) -> a -> bits -> a foldBits _ a 0 = a foldBits f a n = foldBits f (f a lsb) (n `clearBit` lsb) where lsb = countTrailingZeros n The type signature mimmicks foldl. However, composing several calls to foldBits ends up pretty messy: foldBits f (foldBits f (foldBits f [] value1) value2) value3 So I was thinking, maybe I should just flip the final arguments to make composition easier: fooBits :: (Bits bits, Num bits) => (a -> Int -> a) -> bits -> a -> a fooBits _ 0 a = a foldBits f n a = foldBits f (n `clearBit` lsb) (f a lsb) where lsb = countTrailingZeros n fooBits f value3 . fooBits f value2 . fooBits f value1 $ [] This looks more tidy. However, now that I am no longer mimmicking foldl, I am sort of at a loss how to name this beast. iterBits comes to bind, but this isn't really like iterate. I know, this question might look silly, but good naming conventions seem to make it easier for me to reason about the code. Any suggestions? P.S.: Mimmicking foldl is probably wrong as well, since this isn't really a fold, is it? -- CYa, ⡍⠁⠗⠊⠕

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