July 2011 - Beginners

cabal check complains about missing Setup.hs
by Ovidiu Deac 28 Jul '11

28 Jul '11

When I run cabal check I get the following message: $ cabal check The following errors will cause portability problems on other environments: * The package is missing a Setup.hs or Setup.lhs script. Hackage would reject this package. ...but except this the package builds fine so far and the executable runs fine. So what does this exactly mean? ovidiu

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lazyness and tail recursion
by Ovidiu Deac 28 Jul '11

28 Jul '11

I've read the paragraph below from Real World Haskell (http://book.realworldhaskell.org/read/efficient-file-processing-regular-exp… section "An important aside: writing lazy functions") I can get a feeling why lazyness compensates for the lack of tail recursion but I don't really understand why it would work in general. My understanding is that that a function which returns a list with non-tail recursion would only work this way with certain usage patterns. In this case the caller of the funtion will have to use the string in a stream-like fashion. Can someone explain this better? Thanks, Ovidiu PS: Here is the paragraph that I'm talking about: "Looking at the globToRegex' function, we can see that it is not tail recursive. To see why, examine its final clause again (several of its other clauses are structured similarly). No comments -- file: ch08/GlobRegex.hs globToRegex' (c:cs) = escape c ++ globToRegex' cs 1 comment It applies itself recursively, and the result of the recursive application is used as a parameter to the (++) function. Since the recursive application isn't the last thing the function does, globToRegex' is not tail recursive. No comments Why is our definition of this function not tail recursive? The answer lies with Haskell's non-strict evaluation strategy. Before we start talking about that, let's quickly talk about why, in a traditional language, we'd try to avoid this kind of recursive definition. Here is a simpler definition, of the (++) operator. It is recursivem, but not tail recursive. 8 comments -- file: ch08/append.hs (++) :: [a] -> [a] -> [a] (x:xs) ++ ys = x : (xs ++ ys) [] ++ ys = ys No comments In a strict language, if we evaluate "foo" ++ "bar", the entire list is constructed, then returned. Non-strict evaluation defers much of the work until it is needed. No comments If we demand an element of the expression "foo" ++ "bar", the first pattern of the function's definition matches, and we return the expression x : (xs ++ ys). Because the (:) constructor is non-strict, the evaluation of xs ++ ys can be deferred: we generate more elements of the result at whatever rate they are demanded. When we generate more of the result, we will no longer be using x, so the garbage collector can reclaim it. Since we generate elements of the result on demand, and do not hold onto parts that we are done with, the compiler can evaluate our code in constant space."

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[Code Review] csv file to sqlite3
by Shakthi Kannan 27 Jul '11

27 Jul '11

Hi, I am trying to write a simple Haskell program to read from a .csv file and write to a .sqlite3 database. Can anyone please review the following code? === BEGIN === import Text.ParserCombinators.Parsec import Database.HDBC import Database.HDBC.Sqlite3 import Control.Monad(when) -- | Initialize DB and return database Connection connect :: FilePath -> IO Connection connect fp = do dbh <- connectSqlite3 fp prepDB dbh return dbh prepDB :: IConnection conn => conn -> IO () prepDB dbh = do tables <- getTables dbh when (not ("entries" `elem` tables)) $ do run dbh "CREATE TABLE entries ( \ \id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, \ \first TEXT, \ \second TEXT, \ \third TEXT)" [] return () commit dbh csvFile = endBy line eol line = sepBy cell (char ';') cell = quotedCell <|> many (noneOf ";\n\r") quotedCell = do char '"' content <- many quotedChar char '"' <?> "quote at end of cell" return content quotedChar = noneOf "\"" <|> try (string "\"\"" >> return '"') eol = try (string "\n\r") <|> try (string "\r\n") <|> string "\n" <|> string "\r" <?> "end of line" process :: [String] -> IO () process r = do conn <- connectSqlite3 "simple1.db" run conn "INSERT INTO entries (first, second, third) VALUES (?, ?, ?)" (map toSql r) commit conn disconnect conn main = do dbh <- connect "simple1.db" do c <- getContents case parse csvFile "(stdin)" c of Left e -> do putStrLn "Error parsing input:" print e Right r -> do mapM_ process r === END === Appreciate any suggestions, improvements in this regard. Thanks! SK -- Shakthi Kannan http://www.shakthimaan.com

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code critique
by Bryce Verdier 27 Jul '11

27 Jul '11

Hi all, I'm new to haskell, and I'm trying to get better with it. Recently I completed one of the challenges from Programming Praxis and I was wondering if people would be willing to spend some time and tell me how I could improve my code. Thanks in advance, Bryce Here is a link to the programming praxis: http://programmingpraxis.com/2011/07/19/sum-of-two-integers/ And here is my code: import Data.List import Data.Maybe sumCheck :: Int -> [Int] -> [Int] -> Maybe (Int, Int) sumCheck _ [] _ = Nothing sumCheck total (x:xs) ys = if total' == Nothing then sumCheck total xs ys else return (x, (ys !! ( fromJust total'))) where total' = (total - x) `elemIndex` ys

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minimal Haskell concepts subset
by Davi Santos 27 Jul '11

27 Jul '11

Hello, which would be the minimal Haskell concepts subset equivalent to a Turing-complete high level language? I bet there would be no monads and type classes in the subset. Please, see that I am not an idealist nor want to use the acclaimed very powerful abstraction capabilities of Haskell. I just need to know how much is strictly needed, even if a real Haskeller or other person couldn't understand my code. Davi

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Code-review of "toy SQL" parser using Parsec
by Andy Elvey 26 Jul '11

26 Jul '11

Hi all - I'm poking around with Parsec and looking at getting this "toy SQL" parser up and running. So, I'm keen to get feedback on whether I'm on the right track (or off on a siding to nowhere..... ;) ) This is my first Parsec grammar, so I hope you'll be gentle...... :) ( Note - I've put the whereStmt in parens as I want that to be an optional statement. I'm not sure how to denote that in Parsec. ) Many thanks in advance - ** Start of code ** -- toysql.hs -- This code is aimed at parsing phrases like the -- following - -- select * from mytable where city = "Sydney"; -- select foo, bar from mytable where var1 > 10; -- select baz from mytable; -- This code is released to the public domain. -- "Share and enjoy....." ;) module Main where import Text.ParserCombinators.Parsec sqlStmt = do{ createStmt ; selectStmt ; fromStmt ; (whereStmt) ; semicolon } createStmt = do{ CREATE ; tablename ; AS } tablename = identifier selectStmt = do{ SELECT ; varStmt } varStmt = star <|> singlevar <|> varlist star = '*' singlevar = identifier varlist = sepBy singlevar (char ',') fromStmt = do{ FROM ; tableStmt } tableStmt = singleTablestmt <|> multiTablestmt singleTablestmt = identifier multiTablestmt = sepBy identifier (char ',') whereStmt = do{ WHERE ; condStmt } condStmt = multiCondstmt <|> singleCondstmt singleCondstmt = singleCondstmtnoparens <|> singleCondstmtwithparens singleCondstmtnoparens = do{ ; singlevar ; OP ; value } singleCondstmtwithparens = do( ; char '(' ; singleCondstmtnoparens ; char ')' multiCondstmt = sepBy singleCondstmt ( AND <|> OR ) OP = ( '=' <|> '<' <|> '>' <|> "<=" <|> ">=" <|> "!=" ) semicolon = char ';' parseSQL :: String -> Either ParseError [[String]] parseSQL input = parse sqlFile "(unknown)" input ** end of code **

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Re: [Haskell-beginners] a problem with maps
by David Place 25 Jul '11

25 Jul '11

On Jul 25, 2011, at 6:38 PM, beginners-request(a)haskell.org wrote: > It is at this point that the special > connection between politics and the debasement of language becomes > clear. Ouch! All because I used "perspicuously?" In the future, I'll keep my responses monosyllabic. Ugh! I can't even say "monosyllabic" monosyllabically! :-)

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a problem with maps
by Dennis Raddle 25 Jul '11

25 Jul '11

In my application I find an issue coming up frequently. I have a map Ord k => Map k [a] That is, a list of things may be stored at each key. I sometimes want to convert it to [(k,a)] where each value is paired with its key, and some keys may repeat in that form of the list. No particular order is needed. so I have the following code. Would appreciate hearing if there are any tricks I'm missing import qualified Map as M listOutMap :: Ord k => Map k [a] -> [(k,a)] listOutMap m = concatMap (\(k,a) -> zip (repeat k) a) (M.toList m) mapOutList :: Ord k => [(k,a)] -> Mpa k [a] mapOutList list = M.fromList $ map (second (: [])) list

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Problem with space leak in GHC compiled code using fix
by Will Ness 25 Jul '11

25 Jul '11

Hi, could someone please help me understand this, thanks. In reformulation of a code with no space leak, the leak reappeares. It takes near constant space to get at the n-th elt in the produced list here (http://ideone.com/wxmR5) {-# OPTIONS_GHC -O2 -fno-cse #-} primes = 2 : ([3,5..] `minus` foldi (\(x:xs) -> (x:) . union xs) [[x*x, x*x+2*x..] | x<- ys]) where ys = 3 : ([5,7..] `minus` foldi (\(x:xs) -> (x:) . union xs) [[x*x, x*x+2*x..] | x<- ys]) foldi f (x:xs) = f x (foldi f (pairs f xs)) pairs f (x:y:t) = f x y : pairs f t Here the memory usage grows linearly at least (https://ideone.com/qpnqe) {-# OPTIONS_GHC -O2 #-} primes = 2 : g (fix g) where g = (3:) . ([5,7..] `minus`) . foldi (\(x:xs) -> (x:) . union xs) . map (\x-> [x*x, x*x+2*x..]) I expected the 2nd to be equivalent to the 1st. This also impacts its performance: the 2nd code runs at empirical complexity of O(n^1.24) instead of O(n^1.20) of the 1st one (in ''n'' primes produced). In Hugs (Nov 2002) the reported stats and run times are very similar for the both versions. Is this a GHC thing, or a language thing? Can anything be done to have the shorter 2nd variant without a space leak? Thanks,

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Urgent help required, life threatening...
by Sean Charles 25 Jul '11

25 Jul '11

Does anybody know any way at all that I can de-program my mind to go back to working with OO, specifically PHP, ever again? I've seen the Haskell light and it is good and pure and PHP is brown, dirty and slimy. Please boss, don't make me code more communications protocols in PHP, please. Please help.

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