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Understanding Monads: Help with 20 Intermediate Haskell Exercises
by Tushar Tyagi 18 May '16

18 May '16

Hi, In order to wrap my head around Applicatives and Monads, I started doing the exercises at http://blog.tmorris.net/posts/20-intermediate-haskell-exercises/ The main class being used for the questions is: class Misty m where banana :: (a -> m b) -> m a -> m b unicorn :: a -> m a furry' :: (a -> b) -> m a -> m b Question #13 asks to create the following function: apple :: (Misty m) => m a -> m (a -> b) -> m b After thinking for around an hour for this, I could not crack it and tried looking at other people's solutions, two of which are: apple = banana . flip furry' apple ma mab = banana (\ab -> furry' ab ma) mab I also tried hoogle to see what this function is in the real world, and the underlying implementation in the source code. Turns out, one of the applicatives uses this flip technique (monads use a different one): (<**>) = liftA2 (flip ($)) Although the code compiles with the solutions, I am still not able to wrap my head around them. For instance, from what I understand from composition, the output of one function becomes the input of the next one. But the input and outputs of these functions vary: banana :: Misty m => (a -> m b) -> m a -> m b flip furry' :: Misty m => m a -> (a -> b) -> m b banana . flip furry' :: Misty m => m a -> m (a -> b) -> m b I am thinking that the entire `(a -> b) -> m b` in flip furry' is being sent to banana as the first argument: (a -> m b), but what happens after that? In the second solution, the types pass, i.e. the lambda is basically furry' which has the type a -> ... -> m b, exactly what is needed in the call to banana. But where will it get the function ab? And how will banana work with the second argument? The definition in the class requires it to be `m a`, but here we are passing it `m a -> m b`. Sorry for such a long post, but I want to understand this wizardry . Thanks, Tushar

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Re: [Haskell-beginners] can I use "pure" all the time instead of "return" now?
by Imants Cekusins 16 May '16

16 May '16

why not use (%%) as alias for both pure and return? % - as in "investment return" % - as in "pure distilled .. " :-P

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can I use "pure" all the time instead of "return" now?
by Doug McIlroy 15 May '16

15 May '16

> the name [return] "stains" the functional semantics in Monadic code, > in my opinion Amusing. For me, the term "pure" stains monads as impure or diluted. The moral overtones of "pure", as in "purely functional language", drive out more benign interpretatations such as "unadorned". Not a felicitous coinage. Doug McIlroy

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can I use "pure" all the time instead of "return" now?
by Silent Leaf 15 May '16

15 May '16

All in the title; Since the change in GHC, 7.10 i think, where all Monad instances had to have Applicative instances too, in theory all monads must have a pure method that should be identical to return, right? My subjectively superficial reason for preferring pure (and caring about the issue in the first place) is twofold: shorter (i know, i know, still the shorter, the quicker to read and then understand in the code) and, mostly, less semantically-awkward --honestly the name "stains" the functional semantics in Monadic code, in my opinion, but that's just personal. (For those who know, it feels like the "new" operator in JS (OO class-instanciation semantical imitation)) Of course now I'm perfectly used to, have no trouble seeing it for what it is all the time, not mixing it up with some imperative-like "return" concept. Yet i don't really like it much at all, so I wanted to know to which extent it is safe to always use pure even in explicitly Monadic contexts. Thankx in advance!

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Type depending on value
by Dmitriy Matrosov 13 May '16

13 May '16

> {-# LANGUAGE DataKinds, KindSignatures, GADTs, StandaloneDeriving #-} Hi. Here is natural numbers and its singleton definition, which i take from "Part I: Dependent Types in Haskell" article by Hiromi ISHII [1]: > data Nat = Z | S Nat > deriving (Show) > > data SNat :: Nat -> * where > SZ :: SNat 'Z > SN :: SNat n -> SNat ('S n) > deriving instance Show (SNat n) But i can't figure out how may i define function returning SNat value depending on Nat value: f :: Nat -> SNat n f Z = SZ f (S n) = SN (f n) This does not typecheck, because, as i understand, ghc can't infer type n. Is it possible to do this at all? [1]: https://www.schoolofhaskell.com/user/konn/prove-your-haskell-for-great-safe…

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using HS to writing/managing a selfmade filesystem on a real partition?
by Silent Leaf 11 May '16

11 May '16

Mostly all in the title. I have a project of developing a personal filesystem, possibly at first virtual (the file(s) representing a virtual partition formatted with my filesystem, would be saved in a host filesys, eg ext4 or whatever), but probably in the end not virtual, directly working on the contents of a real partition. Can haskell do that kind of thing, aka writing data on a partition directly (without using a known filesys), etc? Is it at least more or less adapted for this task (not talking about performances, unless the consequences be a *really* slow filesys), aka doable, easily doable, relatively speaking (aka not worse than with another language)? Incidentally, if i wanted Linux to recognize the filesys, i've heard one has to write a module and put it in connection with the kernel or something. could haskell do that? if that's a "no" somewhere for one of my questions, which parts can't be written in haskell (without horrible performances or code very very hard to write), and can they be written in C (or whatever) as foreign functions? which parts would that represent for the whole program? Thanks a lot in advance! PS: just in case, tips on sources of information on how to do any of the above will be extremely appreciated! (even if it's in, say C, for that matter, providing there's a way to translate the steps into a haskell program)

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Fwd: CAF taking all the memory
by Mahdi 09 May '16

09 May '16

> Begin forwarded message: > > From: Mahdi <mdibaiee(a)aol.com> > Subject: Re: [Haskell-beginners] CAF taking all the memory > Date: May 8, 2016 at 18:06:29 GMT+4:30 > To: Ben Gamari <ben(a)smart-cactus.org> > > Wow, This is a great explanation! Thank you! > I see your point on using lazy lists (although I was not aware of the specific problems you mentioned). > So, I switched over to HMatrix [0], (the libraries you mentioned added a lot of complexity to the code) > which uses unboxed arrays for matrices. > > After refactoring my code to use HMatrix, I ran the program, and the same thing happens, > the speed hasn’t changed much either, then I tried to take another profile to see if anything has changed, > but I couldn’t as described here [1]. > > But my main question is, why would Haskell even store all this data? There are no side-effects, so > there should be no memory leak in my code. > From what I read online, Haskell is trying to avoid re-calculating the functions by caching their > input/outputs (they’re pure, so this can be done), but I hoped there would be a way to disable this, > unfortunately there doesn’t seem to be a way (or there is?). > If that’s the case, then using libraries won’t solve the problem, and that’s frustrating. > > * I also tried compiling with -O0, but no luck. > > [0] http://dis.um.es/~alberto/hmatrix/hmatrixtut.html > [1] https://github.com/albertoruiz/hmatrix/issues/190 > > Thank you Ben! > > - Mahdi > >> On May 3, 2016, at 14:36, Ben Gamari <ben(a)smart-cactus.org> wrote: >> >> Mahdi <mdibaiee(a)aol.com> writes: >> >>> Hello there, >>> >> Hi! >> >>> I’m pretty new to Haskell and I’m trying to write a Neural Network in >>> Haskell for educational purposes. >>> >>> So, I have my neural network working, it can learn XOR in 500 >>> iterations, but the thing is, if I increase the iterations to >>> something like 5 milion times, the process just drains my RAM until >>> either it’s killed or the OS drowns. Here is the code: [0] >>> >>> I searched online and tried to get information on why this is >>> happening, I profiled the memory usage and found that the memory is >>> taken by CAF, searching online, >>> >> Great! The next question you should then ask is "which CAF"? A CAF is >> simply a top-level "constant" in your program. Indeed it sounds like you >> have not defined any cost-centers, which means that GHC will attribute >> the entire cost of your program to the ambiguous cost-center "CAF" >> (which in this case really just means "the whole program"). >> >> As discussed in the users guide [1] one way to define cost-centers >> within your program is to manually annotate expressions with SCC >> pragmas. However, in this case we simply want to let GHC do this for us, >> for which we can use the `-fprof-auto -fprof-cafs` flags (which >> automatically annotate top-level definitions and CAFs with >> cost-centers), >> >> $ ghc -O examples/xor.hs -fprof-auto -fprof-cafs >> >> Now your program should give a much more useful profile (run having set >> the iteration count to 50000), >> >> $ time examples/xor +RTS -p >> [[0.99548584],[4.5138146e-3],[0.9954874],[4.513808e-3]] >> >> real 0m4.019s >> user 0m0.004s >> sys 0m4.013s >> $ cat xor.prof >> Tue May 3 11:15 2016 Time and Allocation Profiling Report (Final) >> >> xor +RTS -p -RTS >> >> total time = 1.11 secs (1107 ticks @ 1000 us, 1 processor) >> total alloc = 1,984,202,600 bytes (excludes profiling overheads) >> >> COST CENTRE MODULE %time %alloc >> >> matrixMap Utils.Math 21.4 26.8 >> matadd Utils.Math 20.8 22.7 >> matmul.\ Utils.Math 10.4 16.0 >> dot Utils.Math 7.1 13.4 >> column Utils.Math 7.0 2.9 >> dot.\ Utils.Math 6.9 1.9 >> rowPairs Utils.Math 5.8 6.5 >> sigmoid' NN 4.7 0.8 >> train.helper Main 4.0 1.3 >> sigmoid NN 3.3 0.8 >> matmul Utils.Math 2.2 2.0 >> hadamard Utils.Math 1.8 2.1 >> columns Utils.Math 1.2 1.3 >> >> individual inherited >> COST CENTRE MODULE no. entries %time %alloc %time %alloc >> >> MAIN MAIN 79 0 0.0 0.0 100.0 100.0 >> [snip] >> CAF:main3 Main 143 0 0.0 0.0 100.0 100.0 >> (...) Main 162 1 0.0 0.0 100.0 100.0 >> train Main 163 1 0.0 0.0 100.0 100.0 >> train.helper Main 164 50001 4.0 1.3 100.0 100.0 >> train.helper.hweights Main 258 50001 0.5 0.0 0.5 0.0 >> train.helper.oweights Main 235 50001 0.4 0.0 0.4 0.0 >> train.helper.oback Main 207 50000 0.3 0.1 19.0 20.9 >> backward' NN 208 50000 0.3 0.6 18.7 20.8 >> [snip] >> >> So, here we see that costs are actually spread throughout the program. >> >> Without diving any deeper into this particular program it's hard to give >> more guidance however I will say that your lazy list Matrix >> representation is very rarely the right choice for even toy linear >> algebra problems. >> >> First, consider the fact that even just a list cons constructor requires >> three words (an info table pointer, a pointer to payload, and a pointer >> to tail) plus the size of the payload (which in the case of an evaluated >> `Float` is 2 words: one info table pointer and the floating point value >> itself). So, a list of n *evaluated* `Float`s (which would require only >> 4*n bytes if packed densely) will require 40*n bytes if represented as a >> lazy list. >> >> Then, consider the fact that indexing into a lazy list is an O(n) >> operation: this means that your `Math.column` operation on an n x m >> matrix may be O(n*m). Even worse, `Math.columns`, as used by >> `Math.matmul` is O(n * m!). >> >> Finally, consider the fact that whenever you "construct" a lazy list you >> aren't actually performing any computation: you are actually >> constructing a single thunk which represents the entire result; however, >> if you then go to index into the middle of that list you will end up >> constructing n cons cells and a thunk for the payload of each. In the >> case of primitive linear algebra operations the cost of constructing >> this payload thunk can be greater than simply computing the result. >> >> For these reasons I wouldn't recommend that lazy lists are used in this >> way. If you have a dense matrix use an array (probably even unboxed; >> see, for instance, the `array`, `vector`, and `repa` libraries); if >> you have a sparse matrix then use an appropriate sparse representation >> (although sadly good sparse linear algebra tools are hard to come by in >> Haskell) Not only will the result be significantly more efficient in >> space and time but the runtime behavior of the program will be >> significantly easier to follow since you can more easily ensure that >> evaluation occurs when you expect it to. >> >> Hopefully this helps. Good luck and let us know if there are further >> issues. >> >> Cheers, >> >> - Ben >> >> >> [1] http://downloads.haskell.org/~ghc/master/users-guide//profiling.html#cost-c… >

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searching for an element from a list of tuples
by Anvika Kumar 09 May '16

09 May '16

I have a tuple with three values and i want to remove the tuple which contains a particular name. So d = (name, month, date) i take a user input of name and then search the list which has tuples of the above declared type. Which ever tuple has the name it should be deleted from the list. d is the tuple. db should be a list of tuples. remove :: (Eq) => IO String -> d print "Enter the name to be removed” a <- getLine remove x [] = [] print “Record not found” remove x db = if x == Please let me know! Thanks a lot

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Simple function gives UTF-8 decoding error.
by Vilpesh Mistry 08 May '16

08 May '16

Haskell version installed on my computer with OS Windows 7, 64 bit. GHCi, version 7.8.3 My haskell script is in a file named baby.hs with following functions, doubleMe x = x + x doubleUs x y = x*2 + y*2 removeNonUpperCase :: [Char] -> [Char] removeNonUpperCase st = [c | c <- st, c `élem` ['A'..'Z']] I get following error:- g>:l baby [1 of 1] Compiling Main ( baby.hs, interpreted ) baby.hs:6:42: lexical error (UTF-8 decoding error) Failed, modules loaded: none. I am not able to advance any further, please suggest. -- thanks Vilpesh

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Automatic differentiation (AD) with respect to list of matrices in Haskell
by Miguel A. Santos 08 May '16

08 May '16

I am trying to understand how can I use Numeric.AD (automatic differentiation) in Haskell. I defined a simple matrix type and a scalar function taking an array and two matrices as arguments. I want to use AD to get the gradient of the scoring function with respect to both matrices, but I'm running into compilation problems. Here is the code: ------------------------------- {-# LANGUAGE DeriveTraversable, DeriveFunctor, DeriveFoldable #-}import Numeric.AD.Mode.Reverse as Rimport Data.Traversable as Timport Data.Foldable as F --- Non-linear function on "vectors" logistic x = 1.0 / (1.0 + exp(-x) ) phi v = map logistic v phi' (x:xs) = x : (phi xs) --- dot product dot u v = foldr (+) 0 $ zipWith (*) u v --- simple matrix typedata Matrix a = M [[a]] deriving (Eq,Show,Functor,F.Foldable,T.Traversable) --- action of a matrix on a vector mv _ [] = [] mv (M []) _ = [] mv ( M m ) v = ( dot (head m) v ) : (mv (M (tail m)) v ) --- two matrices mbW1 = M $ [[1,0,0],[-1,5,1],[1,2,-3]] mbW2 = M $ [[0,0,0],[1,3,-1],[-2,4,6]] --- two different scoring functions sc1 v m = foldr (+) 0 $ (phi' . (mv m) ) v sc2 :: Floating a => [a] -> [Matrix a] -> a sc2 v [m1, m2] = foldr (+) 0 $ (phi' . (mv m2) . phi' . (mv m1) ) v strToInt = read :: String -> Double strLToIntL = map strToInt--- testing main = do putStrLn $ "mbW1:" ++ (show mbW1) putStrLn $ "mbW2:" ++ (show mbW2) rawInput <- readFile "/dev/stdin" let xin= strLToIntL $ lines rawInput putStrLn "sc xin mbW1" print $ sc1 xin mbW1 --- ok. = putStrLn "grad (sc1 xin) mbW1" print $ grad ( sc1 xin) mbW1 -- yields an error: expects xin [Reverse s Double] instead of [Double] putStrLn "grad (sc1 [3,5,7]) mbW1" print $ grad ( sc1 [3,5,7]) mbW1 --- ok. = putStrLn "sc2 xin [mbW1,mbW2]" print $ sc2 xin [mbW1, mbW2] putStrLn "grad (sc2 [3,5,7) [mbW1,mbW2]" print $ grad ( sc2 [3,5,7]) [mbW1, mbW2] --- Error: see text -------------------------------- The last line (grad on sc2) gives the following error: --------------------------------- Couldn't match type ‘Reverse s (Matrix Double)’ with ‘Matrix (Reverse s (Matrix Double))’ Expected type: [Reverse s (Matrix Double)] -> Reverse s (Matrix Double) Actual type: [Matrix (Reverse s (Matrix Double))] -> Reverse s (Matrix Double) In the first argument of ‘grad’, namely ‘(sc2 [3, 5, 7])’ In the second argument of ‘($)’, namely ‘grad (sc2 [3, 5, 7]) [mbW1, mbW2]’ --------------------------------- I don't understand where the "Matrix of Matrix" in the actual type seen comes from. I'm feeding the grad with a curried version of sc2, making it a function on a list of Matrix. Commenting out the two offending lines runs without problem, i.e., the first gradient works and is correctly calculated (I'm feeding [1,2,3] as input to the program): ------------------- mbW1:M [[1.0,0.0,0.0],[-1.0,5.0,1.0],[1.0,2.0,-3.0]] mbW2:M [[0.0,0.0,0.0],[1.0,3.0,-1.0],[-2.0,4.0,6.0]] sc1 xin mbW11232.0179800657874893 grad (sc1 [3,5,7]) mbW1 M [[3.0,5.0,7.0],[7.630996942126885e-13,1.2718328236878141e-12,1.7805659531629398e-12],[1.0057130122694228e-3,1.6761883537823711e-3,2.3466636952953197e-3]] sc2 xin [mbW1,mbW2]1.8733609463863194 ------------------- Both errors are an issue. I want to take the gradient of any such sc2 scoring function, depending on an array of matrices, evaluated at any given "point" xin. Clearly, I'm not yet understanding the AD library well enough. Any help would be appreciated. -- Public key ID: E8FE60D7 Public key server: see, e.g., hkp://keys.gnupg.net

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