- Beginners - Haskell.org

What does "(# #)" mean?
by John M. Dlugosz 06 Apr '14

06 Apr '14

The sources I've learned about thus far are not helping me with this one. What's "(# #)"? Haskell is search-hostile.

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iterated function value sequence
by John M. Dlugosz 06 Apr '14

06 Apr '14

What's a good way to produce an iterated function value sequence? That is, f(x), f(f(x)), f(f(f(x))), etc. ref: https://www.youtube.com/watch?v=09JslnY7W_k

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Trying to grasp Monads - IO and delayed actions
by John M. Dlugosz 06 Apr '14

06 Apr '14

A spiral approach to learning: you understand, then you learn more, and then you are more confused than ever. I recall that a function in the IO Monad just combines actions to make a big action list, and doesn't actually do the side-effect-laden "work" until it is later triggered, normally because the program is defined to evaluate main. Depending on the time of day and which example I pick, I can sometimes follow what's "really happening" in the definition of >>=. But here are some specific questions: In <http://www.haskell.org/haskellwiki/Monads_as_computation> > What is a for-each loop really? It's something which performs some action based on each > element of a list. So we might imagine a function with the type: > > forM :: (Monad m) => [a] -> (a -> m b) -> m [b] > > (as an added bonus, we'll have it collect the results of each iteration). > > We can write this with sequence and map: > > forM xs f = sequence (map f xs) > > we apply the function to each element of the list to construct the action for that > iteration, and then sequence the actions together into a single computation. > So map by itself produces a [m b]. Why does it need to be turned into m [b]? What does the 'sequence' accomplish, other than restructuring the results that already exist? The reason I asked about (#⋯#) is because I wanted to see what IO was really doing, to see what the difference was between using >>= initially and then somehow "cranking" it later. <http://hackage.haskell.org/package/base-4.6.0.1/docs/src/GHC-Base.html#%3E%…> lists > instance Monad IO where > {-# INLINE return #-} > {-# INLINE (>>) #-} > {-# INLINE (>>=) #-} > m >> k = m >>= \ _ -> k > return = returnIO > (>>=) = bindIO > fail s = failIO s > > returnIO :: a -> IO a > returnIO x = IO $ \ s -> (# s, x #) > > bindIO :: IO a -> (a -> IO b) -> IO b > bindIO (IO m) k = IO $ \ s -> case m s of (# new_s, a #) -> unIO (k a) new_s > > thenIO :: IO a -> IO b -> IO b > thenIO (IO m) k = IO $ \ s -> case m s of (# new_s, _ #) -> unIO k new_s > > unIO :: IO a -> (State# RealWorld -> (# State# RealWorld, a #)) > unIO (IO a) = a where bindIO is the function of interest. In a chain of commands, getLine might be the 'k' argument to bindIO. Somewhere there's a real machine function called to do the reading from the file, right?

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How can I fix this 'where' error ?
by S. H. Aegis 05 Apr '14

05 Apr '14

Hi. I'm new to Haskell. I try to solve Euler Project for studying Haskell through Googling, reading books. There is a error in where sentence, but I can't fix this. maxEleList :: String -> String -> [Int] maxEleList xs ys = maxOfTwo upperList lowerList where upperList = map turpleSum $ zip (toIntList xs) (toIntList ys) lowerList = map turpleSum $ zip (toIntList xs) (tail $ toIntList ys) Error message is ~/maxPathSum.hs: line 4, column 75: Parse error Error message: Parse error: = Code: maxEleList xs ys = maxOfTwo upperList lowerList where upperList = map turpleSum $ zip (toIntList xs) (toIntList ys) > lowerList = map turpleSum $ zip (toIntList xs) (tail $ toIntList ys) How can I fix this? Thank you for your kind help. S. Chang ---------------- This code is for Euler project: problem 18, and whole code is like this... (Not yet finished...) maxEleList :: String -> String -> [Int] maxEleList xs ys = maxOfTwo upperList lowerList where upperList = map turpleSum $ zip (toIntList xs) (toIntList ys) lowerList = map turpleSum $ zip (toIntList xs) (tail $ toIntList ys) maxOfTwo :: [Int] -> [Int] -> [Int] maxOfTwo [] [] = [] maxOfTwo (x:xs) (y:ys) = max x y : maxOfTwo xs ys turpleSum :: (Int, Int) -> Int turpleSum (x, y) = x + y toIntList :: String -> [Int] toIntList = map (\x -> read x :: Int) . words rowData :: [[Int]] --rowData = reverse $ map (map (\x -> read x :: Int) . words) [r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14, r15] rowData = map (map (\x -> read x :: Int) . words) [r1, r2, r3, r4, r5, r6, r7, r8, r9, r10, r11, r12, r13, r14, r15] r1 = "75" r2 = "95 64" r3 = "17 47 82" r4 = "18 35 87 10" r5 = "20 04 82 47 65" r6 = "19 01 23 75 03 34" r7 = "88 02 77 73 07 63 67" r8 = "99 65 04 28 06 16 70 92" r9 = "41 41 26 56 83 40 80 70 33" r10 = "41 48 72 33 47 32 37 16 94 29" r11 = "53 71 44 65 25 43 91 52 97 51 14" r12 = "70 11 33 28 77 73 17 78 39 68 17 57" r13 = "91 71 52 38 17 14 91 43 58 50 27 29 48" r14 = "63 66 04 68 89 53 67 30 73 16 69 87 40 31" r15 = "04 62 98 27 23 09 70 98 73 93 38 53 60 04 23"

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Definition of last: why complicate it?
by Dimitri DeFigueiredo 05 Apr '14

05 Apr '14

I was looking at the definition of last in the prelude and saw this code (from http://hackage.haskell.org/package/base-4.2.0.1/docs/src/GHC-List.html) -- | Extract the last element of a list, which must be finite and non-empty. last :: [a] -> a #ifdef USE_REPORT_PRELUDE last [x] = x last (_:xs) = last xs last [] = errorEmptyList "last" #else -- eliminate repeated cases last [] = errorEmptyList "last" last (x:xs) = last' x xs where last' y [] = y last' _ (y:ys) = last' y ys #endif Question: What does the second "eliminate repeated cases" definition of last gain compared to the first (simpler) one? Thanks Dimitri

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Data type definition for a list of elements of alternating types?
by Jacek Dudek 04 Apr '14

04 Apr '14

-- As an exercise I wanted to define a datatype that is an alternating list of elements of two different types. The best that I could do are the type definitions below: module BiList ( BiList (..) , AList (EmptyA) , BList (EmptyB) ) where data BiList a b = Empty | BA (AList a b) | BB (BList a b) data AList a b = EmptyA | AL a (BList a b) data BList a b = EmptyB | BL b (AList a b) (<#) :: a -> (BList a b) -> (AList a b) a <# bs = AL a bs (<@) :: b -> (AList a b) -> (BList a b) b <@ as = BL b as infixr 5 <# infixr 5 <@ example :: BiList Int Char example = BA $ 1 <# 'a' <@ 2 <# 'b' <@ 3 <# 'c' <@ 4 <# 'd' <@ EmptyA -- There are two things that I don't like about this implementation. -- (1) The BA and BB constructors that must be applied on top of instances of (AList a b) and (BList a b) to lift them to be of type (BiList a b). -- (2) Having three different constructors for an empty list: Empty, EmptyA, EmptyB, where ideally I would just have one. -- Is it possible to get around either of these annoyances with some type theory gymnastics? Maybe something like the function fromIntegral (the mechanics of which I don't really understand at this point)?

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How do you pronounce these operators?
by John M. Dlugosz 04 Apr '14

04 Apr '14

I'm having some difficult reading because I don't have "names" for many of the operators, such as <*>. I know that >>= is pronounced “bind”, but what about the others? Is there some common consensus, a list somewhere, or at least the proper mathematical names to serve as a starting point? I'd hate to have to make something up (see < http://www.muppetlabs.com/~breadbox/intercal-man/tonsila.html >) —John

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Help installing Esqueleto (for Yesod)!
by Gilberto Melfe 03 Apr '14

03 Apr '14

Hello community! I'm trying to install Yesod on a freshly setup openSUSE 13.1 virtual machine! I installed the Haskell Platform and three other (ghc) packages required (for yesod-bin), from the standard online repositories! Then i issued: cabal update cabal install cabal-install (because cabal told me so) export PATH=$HOME/.cabal/bin:$PATH cabal install yesod-platform After churning for a while, that last command stated that some packages, more precisely the Esqueleto package, failed to install. I tried to install it alone and got these error messages: -- Begin Command Output Resolving dependencies... Configuring esqueleto-1.3.5... Building esqueleto-1.3.5... Preprocessing library esqueleto-1.3.5... [1 of 4] Compiling Database.Esqueleto.Internal.PersistentImport ( src/Database/Esqueleto/Internal/PersistentImport.hs, dist/build/Database/Esqueleto/Internal/PersistentImport.o ) [2 of 4] Compiling Database.Esqueleto.Internal.Language ( src/Database/Esqueleto/Internal/Language.hs, dist/build/Database/Esqueleto/Internal/Language.o ) [3 of 4] Compiling Database.Esqueleto.Internal.Sql ( src/Database/Esqueleto/Internal/Sql.hs, dist/build/Database/Esqueleto/Internal/Sql.o ) src/Database/Esqueleto/Internal/Sql.hs:531:46: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:559:42: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:619:31: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:634:24: Not in scope: type constructor or class `C.ResourceT' src/Database/Esqueleto/Internal/Sql.hs:636:17: Not in scope: `C.runResourceT' Failed to install esqueleto-1.3.5 cabal: Error: some packages failed to install: esqueleto-1.3.5 failed during the building phase. The exception was: ExitFailure 1 -- End Command Output Can someone figure out what might be wrong? (I guess it's not a dependencies issue...) Thank You Very Much, in advance, for your time! Gilberto PS: Should I have included the command output in an attached file? Which "method" is preferred?

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Functor on Tree data constructor
by Nishant 03 Apr '14

03 Apr '14

Hi, Can some explain me the error and a possibly a fix : data Tree a = NULL | Node (Tree a) a (Tree a) deriving Show instance Functor Tree where ............fmap f NULL = NULL ............fmap f (Node left x right) | (left == NULL) && (right == NULL) = Node left (f x) right ................................................... | (left /= NULL) = Node (fmap f left) x right ................................................... | (right /= NULL) = Node left x (fmap f right) ghci gives: Prelude> :load programs\tree.hs [1 of 1] Compiling Main ( programs\tree.hs, interpreted ) programs\tree.hs:32:78: Couldn't match type `a' with `b' `a' is a rigid type variable bound by the type signature for fmap :: (a -> b) -> Tree a -> Tree b at programs\tree.hs:31:7 `b' is a rigid type variable bound by the type signature for fmap :: (a -> b) -> Tree a -> Tree b at programs\tree.hs:31:7 Expected type: Tree b Actual type: Tree a In the first argument of `Node', namely `left' In the expression: Node left (f x) right In an equation for `fmap': fmap f (Node left x right) | (left == NULL) && (right == NULL) = Node left (f x) right | (left /= NULL) = Node (fmap f left) x right | (right /= NULL) = Node left x (fmap f right) Failed, modules loaded: none. Regards. Nishant

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Mapping inside State
by martin 01 Apr '14

01 Apr '14

Hello all, I found myselft writing this ugly piece of code silenceAll :: TI -> State (M.Map CH StringState,[DtzEvent]) TI silenceAll t = silence t 0 >> silence t 1 >> silence t 2 >> silence t 3 >> silence t 4 >> silence t 5 >> silence t 6 >> return t because I couldn't figure out how to map (silence t) over [0..6]. What is the standard way of doing this?

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