August 2008 - Beginners

Monads and infinite types
by Greg Best 31 Aug '08

31 Aug '08

Hello-- I'm still trying to wade into Haskell after a fair amount of experience in other (mostly, but not exclusively, C-type) languages, and I'm finding this mind numbingly difficult. Usually the way I learn a new language is by finding an application for it and pounding my way through that application. In this case I've started by trying to parse apart a simple GPS NMEA sentence. I'm sure there are prefab libraries for parsing strings, but this seemed like a good way of getting aquatinted with the Monad. I've created a new data type similar to Maybe, but which can return no value, a single value, or a list of values. It seems to behave, but I haven't ruled it out as the cause of my frustration. The NMEAParser monad will eventually take a comma separated string and return a list of fields, lopping off the checksum at the end. Once I can do that, I can start adding other intelligence such as testing the checksum, bounds checking the fields, etc. At the bottom is the little bit of code I'm working with, and the error messages I'm getting out of ghci. Both errors confuse me-- infinite type message because I'm not sure where I've suggested that it build that type, and the "expected ZeroOrMore a but got ZeroOrMore b" message because I thought 'b' and 'a' were allowed to be different types but could also be the same. Any help wrapping my head around this would be appreciated, and will almost certainly be rewarded with more dumb questions in the near future. Thanks-- Greg --------------------------------- NMEATest.hs---------------------------------- module NMEATest where data ZeroOrMore a = NoVal | SingleVal a | MultiVal [a] deriving (Eq,Ord,Show) type Sentence = String newtype NMEAParser a = NMEAParser(Sentence -> (ZeroOrMore a, Sentence)) instance Monad NMEAParser where return a = NMEAParser(\s -> (SingleVal a,s)) NMEAParser k >>= f = NMEAParser(\s0 -> let (r1, s1) = k s0 k2 = f r1 (r2, s2) = k2 s1 in (r1,s2)) ------------------------------------------------------------------------------------ ------------------ghci output-------------------------------------------- Prelude> :l NMEATest.hs [1 of 1] Compiling NMEATest ( NMEATest.hs, interpreted ) NMEATest.hs:26:45: Occurs check: cannot construct the infinite type: a = ZeroOrMore a When generalising the type(s) for `k2' In the expression: let (r1, s1) = k s0 k2 = f r1 (r2, s2) = k2 s1 in (r1, s2) In the first argument of `NMEAParser', namely `(\ s0 -> let (r1, s1) = k s0 k2 = f r1 .... in (r1, s2))' NMEATest.hs:28:42: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the type signature for `>>=' at <no location info> `a' is a rigid type variable bound by the type signature for `>>=' at <no location info> Expected type: ZeroOrMore b Inferred type: ZeroOrMore a In the expression: r1 In the expression: let (r1, s1) = k s0 k2 = f r1 (r2, s2) = k2 s1 in (r1, s2) Failed, modules loaded: none. Prelude> -------------------------------------------------------------------------------------------------

3 2

profiling in haskell
by Vlad Skvortsov 29 Aug '08

29 Aug '08

Hi! I'm trying to profile the code pasted below. Per the profiler output it takes about 30% of my program running time and I'd like to analyze it further. What I need is a breakdown on a finer level, so I inserted SCC annotations. However, they appear to have attributed zero cost. I use GHC 6.8.2 on FreeBSD, the code is compiled without -O options. What am I doing wrong? Note: I'm not trying to *optimize* this code (I intuitively know where the problems are). My intention is to learn the mechanics of profiling Haskell code. serialize :: Database -> [[String]] serialize (dmap, tmap) = [ {-# SCC "XXX1" #-} [dbFormatTag], {-# SCC "XXX2" #-} (dumpWith docToStr dmap), {-# SCC "XXX3" #-} (dumpWith termToStr tmap) ] where dumpWith f = Data.Map.foldWithKey f [] docToStr k (Doc { docName=n, docVectorLength=vl}) = (:) ("d " ++ show k ++ " " ++ n ++ " " ++ (show vl)) termToStr t il = (:) ("t " ++ t ++ " " ++ (foldl ilItemToStr "" il)) ilItemToStr acc (docid, weight) = show docid ++ ":" ++ show weight ++ " " ++ acc -- Vlad Skvortsov, vss(a)73rus.com, http://vss.73rus.com

2 1

Re: [Haskell-cafe] Is there GHC 6.8.3 on Ubuntu?
by Rafael Gustavo da Cunha Pereira Pinto 28 Aug '08

28 Aug '08

I'll take a look on how to build this package... Although I just want to use 6.8.3, I would really like to share my efforts. Now I just have to find a gap on my (and my wife's) agenda to do that! :-) Best regards On Thu, Aug 28, 2008 at 03:25, Ketil Malde <ketil(a)malde.org> wrote: > "Rafael Gustavo da Cunha Pereira Pinto" <RafaelGCPP.Linux(a)gmail.com> > writes: > > > Is there anyone packing GHC 6.8.3 for Ubuntu Hardy? > > The next Ubuntu release (8.10 Intrepid), seems to come with 6.8.2 as > well. > > If you want to pack 6.8.3, go for it! If you just wanted to use it, > I've had success using the binary snapshots on previous Ubuntus. > > -k > -- > If I haven't seen further, it is by standing in the footprints of giants > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

1 0

Functional programming concepts
by Rafael Gustavo da Cunha Pereira Pinto 28 Aug '08

28 Aug '08

Hi folks! Since I don't have a CS degree, there are some concepts I don't fully understand, and I'd like to propose a "begginers" thread to discuss some of those topics. For starters, what does a WHNF (weak-head normal form) mean? Cheers, -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

4 7

ICFP 2007
by Rafael Gustavo da Cunha Pereira Pinto 27 Aug '08

27 Aug '08

First of all, sorry asking too many questions! I am using the ICFP 2007 (http://save-endo.cs.uu.nl/) problem as a case for learning Haskell. I started with the prototype, listed below =================Main.hs======================= module Main(main) where import qualified Data.ByteString.Lazy.Char8 as L import qualified Data.ByteString.Char8 as S import Data.ByteString.Search.BoyerMoore as BM import Debug.Trace data DNA=DNA {prefix :: L.ByteString, suffix :: L.ByteString} main::IO () main=do d<-L.readFile "endo.dna" let dna=DNA (L.pack "IIIIIIIIIIICCICCICCICFFIIIIIIIIIIIIIIIICFP") d let pat=S.pack "ICFP" print $ take 1 $ matchSL pat $ L.append (prefix dna) (suffix dna) =================EOF Main.hs=================== My question is: Knowing that the searched pattern is in DNA prefix, will "suffix dna" ever be called here? I mean, will the "append" suspension ever be fully evaluated? I am thinking on using a data type like data DNA=DNA {prefix::L.ByteString, worked::L.ByteString, suffix::ByteString} The invariant is that worked should be L.empty whenever prefix is L.empty. Thanks in advance -- Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

2 1

Re: Beginners Digest, Vol 2, Issue 7
by Emmanuel 27 Aug '08

27 Aug '08

>> >> I am having troubles bulding the plugins package : >> >> manu:~/Desktop/tmp manu$ cabal install plugins >> Resolving dependencies... >> cabal: cannot configure plugins-1.2. It requires ghc >=6.8 > > That is the ghc-api package, of which it requires a version >= 6.8. > Could you verify that you have it installed (ghc-pkg list will tell > you what > you have)? it's not listed by ghc-pkg list, I have ... {ghc-6.8.1}, {ghc-6.8.2} ... where do I get it ? It's not listed on Hackage Thanks Emmanuel

2 1

findSubstring deprecated
by Rafael Gustavo da Cunha Pereira Pinto 27 Aug '08

27 Aug '08

Two quick questions: 1) Is there any replacement for the (now deprecated) findSubstring function on ByteString.Char8? 2) Does it work with ByteString.Lazy.Char8? Best Regards, Rafael Gustavo da Cunha Pereira Pinto Electronic Engineer, MSc.

3 3

ord
by Paul Johnston 26 Aug '08

26 Aug '08

Hi Using the book by Hutton and in chapter 5 it talks about 'ord' and 'chr' being in the standard prelude. paulj@opensolaris4:~$ ghci GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> ord 3 <interactive>:1:0: Not in scope: `ord' Prelude> chr 67 <interactive>:1:0: Not in scope: `chr' Prelude> They seem to do what you would expect from their perl counterparts Any ideas? Cheers Paul

2 1

Help with monad syntax?
by Arindam Roy 26 Aug '08

26 Aug '08

I'm trying to learn Haskell using Hutton's book and Hugs (Sep 06), and am trying to work the parsing examples (chapter 8, Functional parsers), but I can't understand the error I'm seeing when I try to load the program into Hugs. Following the book, I've defined a Parser type, then written the basic parsers, then a sequencing operator >>=, followed by the example p parser (bottom of page 77). However, trying to load this file in Hugs generates an error (caused by the definition of p). I've given below the entire file I've created and the error message I get on trying to load it. Any help much appreciated - Arindam ---------------------------------------- type Parser a = String -> [(a, String)] return :: a -> Parser a return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp (>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out p :: Parser (Char, Char) p = do x <- item item y <- item Main.return (x, y) ------------------------------------------ Hugs> :r ERROR file:c:\parsers.hs:23 - Type error in explicitly typed binding *** Term : p *** Type : [Char] -> [(([(Char,[Char])],[(Char,[Char])]),[Char])] *** Does not match : Parser (Char,Char) Hugs> -- Arindam Roy arind_roy(a)fast-mail.org -- http://www.fastmail.fm - IMAP accessible web-mail

2 2

How does the type of "ap = liftM2 id" work?
by Quergle Quergle 26 Aug '08

26 Aug '08

Hello, I'm a bit perplexed by what it means to do something like "ap = liftM2 id" liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r id :: a -> a liftM2 id :: (Monad m) => m (a2 -> r) -> m a2 -> m r My intuitive understanding is that liftM2 is expecting a two-argument function as its first argument, so it seems a little unexpected to pass it "id", a one-argument function. And I can't really see how the type signature of "liftM2 id" is derived from "liftM2" and "id". Any help much appreciated! -- Matt

5 5