- Beginners - Haskell.org

prefix instead of infix
by John Moore 10 Jan '10

10 Jan '10

Hi, Can anyone explain how to turn the code from infix to Prefix. I want to include a let statement below. I not sure of how prefix works. expr :: Parser Expr expr = buildExpressionParser table factor <?> "expression" table :: [[Operator Char st Expr]] table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft] ,[op "+" Add AssocLeft, op "-" Sub AssocLeft] ] where op s f assoc = Infix (do{ string s; return f}) assoc factor :: Parser Expr factor = do{ char '(' ; x <- expr ; char ')' ; return x } <|> number <?> "simple expression" number :: Parser Expr number = do{ ds <- many1 alphaNum ; return (Val $ read ds) } John

3 2

Re: [Haskell-beginners] prefix instead of infix
by Daniel Fischer 10 Jan '10

10 Jan '10

Am Sunday 10 January 2010 21:24:40 schrieben Sie: > Hi Daniel, > I want to add a let expression to the program so as I can > enter let x = y in z or something to that effect. I was going along the > lines of using prefix as I cant use infix so I thought I would make it > easier by using prefix notation. Probably a bad idea. > > John If you want only one binding per let, I think letExpr = do keyword "let" -- implementation of keyword is left as an exercise bind <- variable -- binding patterns like (x,y) is left for later -- maybe check for "=" ? rhs <- expression keyword "in" use <- expression return (Let bind rhs use) expression = letExpr <|> expr factor = between (char '(') (char ')') expression <|> atom atom = variable <|> number would work (untested). > > On Sun, Jan 10, 2010 at 8:14 PM, Daniel Fischer <daniel.is.fischer(a)web.de>wrote: > > Am Sonntag 10 Januar 2010 20:33:40 schrieb John Moore: > > > Hi, > > > Can anyone explain how to turn the code from infix to Prefix. I > > > want to include a let statement below. I not sure of how prefix > > > works. > > > > Can you elaborate? I don't understand what it is you want to do. > > Do you want to parse expressions in prefix notation (aka Polish > > notation), such as +(3,4) (or, more Lispy, (+ 3 4))? > > That'd be simple, but you wouldn't use buldExpressionParser for that > > (in Polish notation, neither associativity nor precedence have a > > meaning). Or do you want to add a parser for let-expressions to expr? > > > > > expr :: Parser Expr > > > expr = buildExpressionParser table factor > > > <?> "expression" > > > table :: [[Operator Char st Expr]] > > > table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft] > > > ,[op "+" Add AssocLeft, op "-" Sub AssocLeft] > > > ] > > > where > > > op s f assoc > > > = Infix (do{ string s; return f}) assoc > > > factor :: Parser Expr > > > factor = do{ char '(' > > > ; x <- expr > > > ; char ')' > > > ; return x > > > } > > > <|> number > > > <?> "simple expression" > > > number :: Parser Expr > > > number = do{ ds <- many1 alphaNum > > > ; return (Val $ read ds) > > > } > > > > > > John

1 0

variable parser
by John Moore 09 Jan '10

09 Jan '10

Hi, I just want to add a new parser for variables but this doesn't seem to work can anybody tell me what I'm doing wrong? module ExprSyn where import Text.ParserCombinators.Parsec import Text.ParserCombinators.Parsec.Expr runExpr :: String -> IO () runExpr str = case runParser expr () " " str of Left err -> putStrLn "Error:" >> print err Right val -> print val demo1 = runExpr "2+5" data Expr = Mul Expr Expr | Div Expr Expr | Add Expr Expr | Sub Expr Expr | Val Float |Let String Expr Expr | Var String deriving (Eq,Show) expr :: Parser Expr expr = variable <|> buildExpressionParser table factor <?> "expression" table :: [[Operator Char st Expr]] table = [[op "*" Mul AssocLeft, op "/" Div AssocLeft] ,[op "+" Add AssocLeft, op "-" Sub AssocLeft] ] where op s f assoc = Infix (do{ string s; return f}) assoc factor :: Parser Expr factor = do{ char '(' ; x <- expr ; char ')' ; return x } <|> number <|> variable <?> "simple expression" variable :: Parser Expr variable = do { b <- letter ; do { cs <- word ;return (c:cs) <|> return [c] <|> number } number :: Parser Expr number = do{ ds <- many1 digit ; return (Val $ read ds) } John

2 1

Help! I can't compile an example program
by Tim Perry 09 Jan '10

09 Jan '10

Hi all, I found an example program that computes grades here: http://donsbot.wordpress.com/?s=grades I copied it to my computer and tried to compile it and if failed. Strangely enough, it works on the command line using 'runhaskell'. This makes me think I didn't set up the libraries correctly. So I downloaded and re-installed the Haskell Platform from: http://hackage.haskell.org/platform/2009.2.0.2/HaskellPlatform-2009.2.0.2-s… The problem remains. I included the errors, the source and the results of runhaskell below. I hope someone can help me sort out what went wrong. Thanks, Tim C:\Documents and Settings\perry\hs>ghc -O Grades.hs Grades.o:fake:(.text+0x2f7): undefined reference to `containerszm0zi2zi0zi1_DataziMap_Tip_closure' Grades.o:fake:(.text+0x930): undefined reference to `containerszm0zi2zi0zi1_DataziMap_lvl47_closure' Grades.o:fake:(.text+0xa8b): undefined reference to `__stginit_containerszm0zi2zi0zi1_DataziMap_' Grades.o:fake:(.text+0x219): undefined reference to `containerszm0zi2zi0zi1_DataziMap_insertWithKey_info' Grades.o:fake:(.text+0x504): undefined reference to `containerszm0zi2zi0zi1_DataziMap_lookup_info' Grades.o:fake:(.text+0x90e): undefined reference to `containerszm0zi2zi0zi1_DataziMap_polyzugo1_info' Grades.o:fake:(.text+0x93f): undefined reference to `containerszm0zi2zi0zi1_DataziMap_foldr_info' Grades.o:fake:(.data+0x50): undefined reference to `containerszm0zi2zi0zi1_DataziMap_insertWithKey_closure' collect2: ld returned 1 exit status C:\Documents and Settings\perry\hs>cat Grades.hs import Data.Char import Data.Maybe import Data.List import Data.Map hiding (map) import Text.Printf main = do src <- readFile "grades.txt" let pairs = map (split.words) (lines src) grades = foldr insert empty pairs mapM_ (draw grades) (sort (keys grades)) where insert (s, g) = insertWith (++) s [g] split [name,mark] = (name, read mark) draw g s = printf "%s\t%s\tAverage: %f\n" s (show marks) avg where marks = findWithDefault (error "No such student") s g avg = sum marks / fromIntegral (length marks) :: Double C:\Documents and Settings\perry\hs>runhaskell Grades.hs Alonzo [70.0,90.0,95.0] Average: 85.0 Bob [80.0] Average: 80.0 Eugenio [69.0] Average: 69.0 Henk [79.0] Average: 79.0 Oleg [77.0] Average: 77.0 Philip [73.0] Average: 73.0 Simon [94.0,99.0,93.0] Average: 95.33333333333333 C:\Documents and Settings\perry\hs>

2 1

[luca_ciciriello@hotmail.com: Re: [Haskell-beginners] reflection]
by Brent Yorgey 05 Jan '10

05 Jan '10

Well, there are a few things I could suggest for you to look at: * the hint package [1] for runtime Haskell interpretation * look at xmonad [2] for an example of a program which recompiles and reloads itself on the fly * the haskell-src-exts library [3] for a full-featured Haskell parser/pretty printer There is no way to actually inspect and modify the structure of a running Haskell program in memory; it is simply not stored in a format which would let you do this (it is highly optimized and compiled). However, using the above tools I imagine you should be able to accomplish what you want. -Brent [1] http://hackage.haskell.org/package/hint [2] http://xmonad.org/ [3] http://hackage.haskell.org/package/haskell-src-exts ----- Forwarded message from Luca Ciciriello <luca_ciciriello(a)hotmail.com> ----- From: Luca Ciciriello <luca_ciciriello(a)hotmail.com> Date: Tue, 5 Jan 2010 18:20:13 +0100 To: Brent Yorgey <byorgey(a)seas.upenn.edu> Subject: Re: [Haskell-beginners] reflection My need is to have a program that at run-time should be able to read itself and eventually modify itself. I've used this reflection in Common LISP. I don't know if this is possible in Haskell using GHC or GHCi (perhaps there are more chances inside the eval-loop of GHCi). Thanks anyway for the answer. Luca. On Jan 5, 2010, at 5:58 PM, Brent Yorgey wrote: > On Tue, Jan 05, 2010 at 09:43:21AM +0100, Luca Ciciriello wrote: >> Hi. >> Someone has performed or know how to use reflection in Haskell? >> I've found something on google, but that stuff seems to me very confusing. > > It depends what you mean by "reflection". Can you give an example of > the sorts of things you'd like to do? Generally speaking, Haskell > doesn't support "reflection" in the same way as many other languages > do, but there are usually idiomatic ways to accomplish the same things > you would use reflection for. > > -Brent > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners > ----- End forwarded message -----

3 2

reflection
by Luca Ciciriello 05 Jan '10

05 Jan '10

Hi. Someone has performed or know how to use reflection in Haskell? I've found something on google, but that stuff seems to me very confusing. Thanks in advance for any answer? Luca.

2 1

Unexpected Space Leak (despite using external-sort)
by Peter Green 04 Jan '10

04 Jan '10

I am using the external-sort package to sort my output in the program below. I made this choice because my output dataset [[Int]] can be large (e.g. >3M items, each [Int]). What my program does: (1) Reads in a file containing 'compressed lists' which look like so: 8+12,11,7+13,10 1+2+3,1+9,3+6,4 . . One compressed list per line. These compressed lists are parsed to become [[[Int]]] [[[8,12],[11],[7,13],[10]], [[1,2,3],[1,9],[3,6],[4]], . . ] Generally files of compressed lists have lengths of ~10,000 lines. (2) Compressed lists are exploded to [[int]] via concatMap Cartesian Product over [[[Int]]], so we end up with [[Int]] [[8, 11, 7, 10], [8, 11, 13, 10], [12, 11, 7, 10], [12, 11, 13, 10], [1, 1, 3, 4], . . [3, 9, 6, 4]] These 'exploded lists are' *much* longer than the input lists. It's common for them to have >100K [Int] members, or even >1M [Int] members. (3) This [[Int]] data must be sorted in to lexicographic order and output as CSV data: i.e. output should be: 1,1,3,4 3,9,6,4 8,11,7,10 . . 12,11,13,10 There is no way to avoid the necessity of sorting the final product. I can *not* take advantage of any structure in the input data to avoid this sort stage. e.g. sorting the (smaller) input compressed lists *will not* obviate the need to lex sort the (larger) final output. The program listing below works correctly, but *is not* space- efficient. In fact it is *less* space-efficient than the equivalent program using Prelude sort! e.g. with one data set which explodes to ~3.7M I am seeing 639MB of total memory use. I am sure externalSort is not at fault because: (a) I'm the Newbie. (b) ExternalSort.bin is 'only' 228MB in size. (Expected to be not small because sorting a [[Int]] with 3.7M [Int] members.) I'm sure it must be something I'm doing wrong elsewhere which is causing the entire output data list to be read into memory. What I really want to happen is: (1) Data lazily read in from stdin and lazily parsed (2) Data lazily sucked out to disk and strictly sorted by externalSort (but no big deal since this happening on disk) (3) Data lazily sucked back out of externalSort, lazily formatted for output, and lazily written to stdout. So memory usage should hopefully never go over (say 10MB). But in fact I'm using 638MB. So somewhere in the code below I must be doing something very wrong. I hope someone can tell me what I am doing wrong here. TIA! import Algorithms.ExternalSort import Data.List.Split (splitOn) import System.IO -- Cartesian Product over a List of Lists -- Http://www.cs.nott.ac.uk/~gmh/sudoku.lhs -- cp [[1,2],[3],[4,5,6]] --> [[1,3,4],[1,3,5],[1,3,6],[2,3,4],[2,3,5], [2,3,6]] cp :: [[a]] -> [[a]] cp [] = [[]] cp (xs:xss) = [y:ys | y <- xs, ys <- cp xss] -- fromCSV ["8+12,11,7+13,10", "1+2+3,1+9,3+6,4"] --> -- [[[8,12],[11],[7,13],[10]],[[1,2,3],[1,9],[3,6],[4]]] fromCSV :: [String] -> [[[Int]]] fromCSV = map parseOneLine where parseOneLine = map parseGroup . splitOn "," where parseGroup = map read . splitOn "+" -- explode [[[1,2],[3],[4,5,6]], [[1, 2], [14,15], [16]]] --> [[1,3,4], [1,3,5], -- [1,3,6],[2,3,4],[2,3,5],[2,3,6],[1,14,16],[1,15,16],[2,14,16], [2,15,16]] explode :: [[[a]]] -> [[a]] explode = concatMap cp -- toSingles "8+12,11,7+13,10\n1+2+3,1+9,3+6,4" --> -- [[8,11,7,10],[8,11,13,10],[12,11,7,10],[12,11,13,10],[1,1,3,4], [1,1,6,4], -- [1,9,3,4],[1,9,6,4],[2,1,3,4],[2,1,6,4],[2,9,3,4],[2,9,6,4], [3,1,3,4], -- [3,1,6,4],[3,9,3,4],[3,9,6,4]] toSingles :: String -> [[Int]] toSingles = explode . fromCSV . lines -- toCSV [8,11,7,10,12] --> "8,11,7,10,12" toCSV :: (Show a) => [a] -> String toCSV = tail . init . show main = do getContents >>= externalSort . toSingles >>= mapM_ (putStrLn . toCSV)

3 4

in which monad am I?
by Francesco Guerrieri 03 Jan '10

03 Jan '10

Hello, this is my first post to the list. I am learning Haskell and I think I understand monads (well, that doesn't mean that I actually understand them :-) ) Nevertheless, I would like to check something with you. My question is: how do you know in which monad you are computing at a given time? What I mean is that, when dealing with more than a monad at the same time (Maybe, List, IO come to my mind) if I understand correctly you have to lift values back and forth from a monad to the other. How do you "keep" track of the monad you are in a given time? I think that the type system will help to resolve to the relevant function (eg, thinking to liftM signature, liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r But doesn't ever happen that poor liftM be confused? and do poor haskell programmers never feel the need to explicitly state to which monad they "wish to lift" ? I hope it's not a too silly question. If so, please point me to the relevant resouces (Are monad transformers relevant to my issue?) Thanks in advance, Francesco

4 8

writing many1Till combinator for Parsec
by Vlad Skvortsov 01 Jan '10

01 Jan '10

Hi, I'm writing a parser where I find myself in need to use manyTill combinator with an additional constraint that there should be at least one meaningful element before the trailer (say, a word ended with a period: 'abc.'). Here is what I have: many1Till :: Parser a -> Parser end -> Parser [a] many1Till p end = do p1 <- p ps <- manyTill p end return (p1:ps) The problem here is that I want to catch and report a case when 'p1' could be actually parsed by 'end' ('..' is not a word ended by a period). Generally 'p' and 'end' can parse the same subset of strings. Another version a had was: many1Till :: Parser a -> Parser end -> Parser [a] many1Till p end = do try (end >> (unexpected "sequence terminator")) <|> (do { p1 <- p; ps <- manyTill p end; return (p1:ps) }) Here there are two disadvantages: 1) I don't like hardcoding "sequence terminator" here; 2) the error output should say that only 'p' parser is expected, while it says (technically correct) that either 'p' or 'end' is expected: Prelude Main Text.ParserCombinators.Parsec> parseTest (many1Till letter (char '.')) "1" parse error at (line 1, column 1): unexpected "1" expecting "." or letter (What I want here is to say "expecting letter") Any suggestions? Thanks! -- Vlad Skvortsov, vss(a)73rus.com, http://vss.73rus.com

2 4

Problem installing graphalyze with Cabal
by John Velman 01 Jan '10

01 Jan '10

I have Haskell Platform downloaded and installed today. When I tried to install graphalyze with cabal install, it had problems with pandoc. To make a long story fairly short, I downloaded pandoc-1.3 from hackage, and tried building it "by hand". It says it had missing dependencies of utf8-string >= 0.3, and zip-archive >=0.1.1. Tried installing them with cabal, and this worked OK. Still, pandoc setup says: jrv:pandoc-1.3 jr$ runhaskell Setup configure Configuring pandoc-1.3... Setup: At least the following dependencies are missing: utf8-string >=0.3, zip-archive >=0.1.1 jrv:pandoc-1.3 jr$ cabal install zip-archive-0.1.1.1 I _finally_ checked ghc-pkg list, and I have, among other things: /Users/jr/.ghc/i386-darwin-6.10.4/package.conf: {HOC-1.0}, {HOC-Foundation-1.0}, binary-0.5.0.1, binary-0.5.0.2, bktrees-0.2.2, darcs-2.3.0, digest-0.0.0.8, graphviz-2999.6.0.0, hashed-storage-0.3.7, haskeline-0.6.1.6, mmap-0.4.1, pandoc-1.3, polyparse-1.4, terminfo-0.3.0.2, utf8-string-0.3.5, utf8-string-0.3.6, zip-archive-0.1.1.1, zip-archive-0.1.1.4, zlib-0.5.2.0 As you see, I've ended up with two copies each (because of my messing around before I checked ghc-pkg, I guess). Well, all of them are >= the dependency requirements, If I understand correctly. What gives? How do I get rid of the extra copies? (By the way, the HOC entries are left over from an earlier time, and I'd like to get rid of those also). Thanks, John Velman

2 2