April 2014 - Beginners

Error while using indices and elem
by Nishant 21 Apr '14

21 Apr '14

Hi, Do I need to define indices and elem for Graph a w here explicitly ? import Data.Array data Graph a w = Array a [(a,w)] -- array (1,3) [(1,[(2,3),(3,2)])] --Given a vertex list out adjacents getAdjVertices :: (Ix a, Eq a) => a -> (Graph a w) -> [a] getAdjVertices x arr | not (x `elem` (indices arr)) = error "No such Vertex" | otherwise = extractV (arr ! 2) where extractV [] = [] extractV (x:xs) = (fst x):extractV (xs) GHCI gives : Prelude> :l programs\graph.hs [1 of 1] Compiling Main ( programs\graph.hs, interpreted ) programs\graph.hs:16:47: Couldn't match expected type `Array a e0' with actual type `Graph a w' In the first argument of `indices', namely `arr' In the second argument of `elem', namely `(indices arr)' In the first argument of `not', namely `(x `elem` (indices arr))' programs\graph.hs:17:46: Couldn't match expected type `Array Integer [(a, b0)]' with actual type `Graph a w' In the first argument of `(!)', namely `arr' In the first argument of `extractV', namely `(arr ! 2)' In the expression: extractV (arr ! 2) Failed, modules loaded: none. Prelude> Regards. Nishant

3 2

Where does cabal store its junk?
by John M. Dlugosz 21 Apr '14

21 Apr '14

When I use cabal to “install” a package, where does it end up on my disk? I can't seem to find it.

2 2

type problems with my little code
by raffa f 20 Apr '14

20 Apr '14

hi everyone, i'm a complete beginner on programming and i've been solving the 99 haskell problems and i've come into a situation. just like all my problems with haskell, it's about types. i wrote a little thing that takes a list and puts all the equal characters together, like [1,1,3,2,3] will be [ [1,1], [3,3], [2] ]. here it is: pack xs = pack' xs where pack' :: [a] -> [[a]] pack' [] = [] pack' (x:y) = extra x xs ++ pack' y extra x xs = [sum' (map (remover x) (map ((==) x) xs))] remover :: a -> Bool -> [a] remover y x = if x then [y] else [] sum' :: [[a]] -> [a] sum' [] = [] sum' (x:xs) = x ++ sum' xs i know, i know, this code is probably terrible and i'm sure there are more clever ways to do this...but i wanna understand what's wrong. the "extra" function works perfectly, it takes a variable and then looks at how many times it's presented on a list and outputs a list with a list of that variable the amount of times that it was presented on the original list. however, pack does not work quite right. first of all, it'll use extra on repeated characters, creating repeated lists, but that doesn't matter because i have a function that fixes that issue and i'll use it after i figure out what's wrong with pack. now, the real problem is with types. here's what haskell says when i try to load it: noventa.hs:77:32: Couldn't match type `a' with `a1' `a' is a rigid type variable bound by the inferred type of pack :: [a] -> [[a]] at noventa.hs:73:1 `a1' is a rigid type variable bound by the type signature for pack' :: [a1] -> [[a1]] at noventa.hs:75:19 Expected type: [a1] Actual type: [a] In the second argument of `extra', namely `xs' In the first argument of `(++)', namely `extra x xs' In the expression: extra x xs ++ pack' y Failed, modules loaded: none. i don't understand this at all! if i replace [a] with String and [[a]] with [String], it works! but i want pack to work for lists of numbers too... types are so confusing. can anyone help me?

4 3

About scope range: outside of the do-block
by S. H. Aegis 20 Apr '14

20 Apr '14

I'm trying to solve problem 22 of Euler Project. ( http://projecteuler.net/problem=22) I got file contents using " names <- readFile "names.txt" " in "do-block". When I try to use 'names' outside of "do-block", I get following error message; namesScores.hs:30:35: Not in scope: `names' Failed, modules loaded: none. Is there any other ways to use variables outside of the "do-block"? Sorry for taking your time. Thank you. My code is... ------------------------------------------------------------- import Data.List import Data.List.Split import Data.Char import Data.Maybe main :: IO() main = do names <- readFile "names.txt" print $ sum $ map (\x -> getPosition x * getScore x) names getPosition :: String -> Int getPosition x = fromMaybe 0 (elemIndex x sortedNamesList) + 1 getScore :: String -> Int getScore xs = sum $ map (\x -> ord x - 64) xs sortedNamesList :: [String] sortedNamesList = sort $ nameList names nameList :: String -> [String] nameList = split (dropDelims . dropBlanks $ oneOf ",\"") Error message is... ------------------------------------------------------------------ Prelude> :l namesScores.hs [1 of 1] Compiling Main ( namesScores.hs, interpreted ) namesScores.hs:31:35: Not in scope: `names' Failed, modules loaded: none.

4 5

Platform for Mac OS 10.6
by Henry Lockyer 20 Apr '14

20 Apr '14

Hello people - trying to get back into Haskell again after previous flirtation which was long enough ago to forget almost everything, it seems. I appear to be lost in a mirror maze in trying to work out again how to bring my platform more up to date on Mac OS 10.6.8 (Snow Leopard). The macos link for download at http://www.haskell.org/platform/mac.html however does not seem to work. I do not get a working installer package just an "Unknown" 204mb file. ? Sniffing around via GHC pages I find this page: http://www.haskell.org/ghc/distribution_packages where it says "For Mac OS X we provide a native system installer, available on the download page for each release. We recommend using these installers " However the 'download page' link just takes you circularly back to http://www.haskell.org/ghc/download ? This page http://www.haskell.org/platform/index.html says the the current platform version is 2013.2.0 From my own past notes I should currently have platform 2011.4.0.0 I do definitely have GHC 7.0.4-x86_64 I also ran 'cabal update' as I dimly remembered that I should do this, and it indicated there was another version of cabal install available by doing a "cabal install cabal-install" but this complains: "$ cabal install cabal-install Resolving dependencies... cabal: cannot configure process-1.2.0.0. It requires base >=4.4 && <4.8 For the dependency on base >=4.4 && <4.8 there are these packages: . . ." I have base 4.3.1.0 (as indicated by GHC-PKG LIST). I thought refreshing the overall package level would be the most straightforward way to update to a consistent state as at least the first step. Hopefully I'm just having a bad day... I have two main motivations here (1) general refresh (2) get latest Open-GL stuff - in case that affects the answer. Can anyone recommend/remind/help with how best to approach this? Many thanks/ Henry

2 3

failed to install HTTP-4000.2.4 when executing `cabal install cabal-install`
by hengchu Zhang 19 Apr '14

19 Apr '14

Hi Friends, When I tried to update cabal-install today by doing `cabal install cabal-install`, I got the following message due to failure in compiling Network/HTTP/Auth.hs, here is a screenshot of the process I am using Mac OS 10.8.4, and I installed haskell from homebrew. Does this mean something might have went wrong when I installed haskell? I have Xcode 5.1.1 and the latest command line tools installed. Does anyone know what might be going wrong? Thank you so much for your help! Best, Hengchu

2 1

Learn You a Haskell! I have a few questions!
by Gilberto Melfe 18 Apr '14

18 Apr '14

Hi there to you all! I've been reading through the first chapter of "Learn You a Haskell" and I'd like to ask the community a few questions. Any help would be appreciated... -- ---------- ---------- ---------- ---------- ---------- All the standard Prelude functions that throw out an error when fed the []! head maximum ... Are there safe versions anywhere, or do we have to define them ourselves? -- ---------- ---------- ---------- ---------- ---------- This one is really important! I understand that for the definition of product to work as it is: product [] must be equal to 1 But what if we want to add a product to something else??? Shouldn't the result be Nothing? (I guess we would have to guard against this! But we must guard against the parameter of product being the empty list, anyway. Otherwise we risk adding 1 when there is nothing do multiply) (The same question arises with the functions and and or, and their boolean results, I think! Right?) -- ---------- ---------- ---------- ---------- ---------- -- ---------- Start Quote Names can't be enumerated. What comes after "John"? I don't know. -- ---------- End Quote "a" to "z" then "aa" to "zz" then "aaa" to "zzz" and so on! Is it to difficult or impossible to create a function that enumerates all possible strings? -- ---------- ---------- ---------- ---------- ---------- -- ---------- Start Quote To make a list with all the numbers from 20 to 1, you can't just do [20..1], you have to do [20,19..1]. -- ---------- End Quote Why is this? If the first was greater than the second it would just subtract! Right? -- ---------- ---------- ---------- ---------- ---------- -- ---------- Start Quote Watch out when using floating point numbers in ranges! Because they are not completely precise (by definition), their use in ranges can yield some pretty funky results. ghci> [0.1, 0.3 .. 1] [0.1,0.3,0.5,0.7,0.8999999999999999,1.0999999999999999] -- ---------- End Quote Can anyone explain me why it works for the first few values, and not "completely"? -- ---------- ---------- ---------- ---------- ---------- Any thoughts? Thank You Very Much in advance! Gilberto

6 6

Learning Haskell: Web Project
by Ari King 17 Apr '14

17 Apr '14

Hi, I'm just starting out with Haskell and I thought a good way to learn/practice would be to re-implement a small web app. The web app I plan on re-implementing is made up of a restful (java) api with a angularjs client. The api and client is served by jetty. I'd appreciate suggestions on frameworks and/or tools that make the most sense as I start out. I was thinking of using scotty + warp + persistence for the api/serving of static content. As for tooling, I was thinking of vim with ghc-mod & syntastic. Thanks. -Ari

2 1

recursive 'let' ?
by John M. Dlugosz 15 Apr '14

15 Apr '14

I understand that the definitions introduced by 'let' can be recursive, even mutually recursive among several names. Why would you want to do that? I saw contrived examples, and wonder why the authors never show a realistic example. let b = f a c c = f a b in ... I see that makes sense in light of lazy evaluation: b is really an alias for a (recursive) function, not a value that needs to find fixed points. Is this used for common idioms and problem-solving approaches in Haskell? --John

6 6

Why getArgs is in the IO monad
by Mike Meyer 14 Apr '14

14 Apr '14

> > > From: "John M. Dlugosz" <ngnr63q02(a)sneakemail.com> > (And that begs the question of why getArgs needs to be monadic in the > first place. It > doesn't change its value; it's a strict constant at run-time, and not > knowing it at > compile time is my problem how?) > Actually, the value of the arguments can be changed - at least on some platforms. They are writable from C, if nothing else. What should getArgs do if some ffi changes the arguments before it's called? Mike

2 1