December 2013 - Beginners

foldM with tuple and maybe
by Miro Karpis 03 Feb '14

03 Feb '14

Hi, please can you help me with following? I have a following function: f1 :: (String, String) -> IO (Maybe String) which I would like to apply to a list of tuples.I have defined the function like this, but that gives me error (below): f2 :: [(String, String)] -> IO (Maybe String) f2 x = foldM f1 0 x error: Couldn't match expected type `IO (Maybe String)' with actual type `[b0] -> m0 (String, String)' In the return type of a call of `foldM' thanks, m.

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breaking code to several lines
by Miro Karpis 29 Jan '14

29 Jan '14

Hi please,... I have one sql insert statement which is too long to be on one line. Is there a way that I can break it several lines? Or does it have to be everything in one line? here is the query: execute_ conn "INSERT INTO ttableXY (column1, column2, column3, column4, column5, column6, column7) VALUES ('var1', 'var2', 'var3', 'var4', 'var5', 'var6', 'var7')" Thanks, Miro

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Constrained polymorphic functions as natural transformations
by Matt R 05 Jan '14

05 Jan '14

I read this excellent blog article on natural transformations[1], which helped me gain one intuition as to what they are: "functions that don't make too many assumptions about their arguments". In the article, natural transformations make "no assumptions at all" about their arguments. However, it's often the case that a Haskell function will require a type argument constrained by a type class, e.g. show :: Show a => a -> String. I wondered if it was possible to characterise these functions using category theory too. Below is my attempt, and I'm hoping people can tell me whether I'm in the right ballpark (or point me in the direction of a better ball-park...) For a particular type class T, we can form a subcategory Hask_T of Hask consisting of all the instances of the type class as objects, with the arrows just those functions that "commute" with the signature of the type class. For example, in Hask_Show, the arrows would be all the functions f: A -> B such that: show == show . f Or for Data.Monoid, all the monoid homomorphisms f: mappend (f x) (f y) == f (mappend x y) mempty == f mempty In general, for any type class, we can formulate two functors f and g and a function sig :: f a -> g a that captures the operations in the type class, and then the condition is that: fmap f . sig == sig . fmap f. Then we have that the polymorphic functions with a type class constraint of T are just the same thing as natural transformations in this subcategory Hask_T. Is the above correct / along the right lines? Thanks, -- Matt [1] "You Could Have Defined Natural Transformations", http://blog.sigfpe.com/2008/05/you-could-have-defined-natural.html

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Question on (x:xs) form
by Angus Comber 02 Jan '14

02 Jan '14

Eg for a definition of reverse: reverse' :: [a] -> [a] reverse' [] = [] reverse' (x:xs) = reverse' xs ++ [x] In the last line of the definition, x is an element in the list (the first element) and xs represents the remainder of the list. so if list was [1,2,3] then x is 1 and xs is [2,3] Why are the brackets required? And what do they signify? Eg reverse' x:xs = reverse' xs ++ [x] results in a parse error.

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How to get each list in a list of lists for filter
by Angus Comber 31 Dec '13

31 Dec '13

I am getting a list of lists from chop9. I want to somehow filter each element in the list. How do I do that. I have encoded binary data eg [1,1,0,0,1,0] etc Each 9th bit is a parity bit which is checked using this function: type Bit = Int paritychecker :: [Bit] -> Bool paritychecker xs | length xs == 9 && ((sum (init xs)) `mod` 2) == (last xs) = True | otherwise = False In the stream (the list) I use chop to retrieve each block of 9 bits as in: chop9 :: [Bit] -> [[Bit]] chop9 [] = [] chop9 bits = take 8 bits : chop9 (drop 8 bits) I have been playing with this sort of thing: filter paritychecker ??? chop9 [1,0,0,0,1,1,1,0,0,...] But doesn't work. I want to end up with all the 9 bit chunks which pass the filter. So my end type should be [[Bit]] paritychecker requires a list - eg [Bit] . So I want to run paritychecker on each element returned from chop9. How do I do that? Example encoded data stream: [1,0,0,0,1,1,1,0,0,1,1,1,0,1,1,1,0,0,1,0,1,0,0,1,1,0,0,0,1,0,0,1,1,1,0,0,0,0,1,0,1,1,1,0,0,1,0,0,1,1,1,1,0,1] Angus

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Trying to understand function types eg iterate (a -> a)
by Angus Comber 31 Dec '13

31 Dec '13

iterate' :: (a -> a) -> a -> [a] I am trying going to go ahead and write my own iterate function. But before I do I want to be clear on types. Looking at :: (a -> a) -> a -> [a] The first part is (a -> a) Now because it is in parentheses it is a function? I can call iterate like this: take 5 $ iterate (*2) 5 So (*2) is a possible function. Does the brackets mean it is a function, the left hand a is indicating a general type and the right hand a means the return type must be the same as the function type. Eg in the case of (*2) the 2 is an Int so the function returns and Int? Is my understanding correct? How could this be better explained? The last bit is easier to understand. a -> [a] meaning a singleton of general type and [a] means a list of same type.

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anonymous arguments
by harry 29 Dec '13

29 Dec '13

Is there any reason not to include Scala-like anonymous arguments in Haskell? For example, I would like to write map (\x -> foo x 3) as map (foo _ 3) The named argument in this and similar cases adds nothing but visual and namespace clutter.

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OMG - Cabal again...
by Gregory Guthrie 25 Dec '13

25 Dec '13

YACP - Yet another cabal problem... Trying to install criterion, which fails for test-framework, which fails for regex, which fails for mtl, which reports that it is fine and installed! Is there any simple recipe to restart the libraries for cabal (on windows)? C:\Users\guthrie>cabal install criterion Resolving dependencies... In order, the following will be installed: ... criterion-0.8.0.0 depends on test-framework-0.8.0.3 which failed to install. hastache-0.5.1 failed during the building phase. The exception was: ExitFailure 1 math-functions-0.1.4.0 failed during the building phase. The exception was: ExitFailure 1 monad-par-0.3.4.6 depends on test-framework-0.8.0.3 which failed to install. monad-par-extras-0.3.3 failed during the building phase. The exception was: ExitFailure 1 mwc-random-0.13.1.0 failed during the building phase. The exception was: ExitFailure 1 statistics-0.10.5.1 depends on test-framework-0.8.0.3 which failed to install. test-framework-0.8.0.3 failed during the configure step. The exception was: ExitFailure 1 test-framework-hunit-0.3.0.1 depends on test-framework-0.8.0.3 which failed to install. vector-algorithms-0.6.0.1 depends on mwc-random-0.13.1.0 which failed to install. vector-binary-instances-0.2.1.0 failed during the building phase. The exception was: ExitFailure 1 C:\Users\guthrie>cabal install test-framework Resolving dependencies... Configuring test-framework-0.8.0.3... cabal: The following installed packages are broken because other packages they depend on are missing. These broken packages must be rebuilt before they can be used. package regex-base-0.93.2 is broken due to missing package mtl-2.1.2-5337caef659244e51e2f5fb2e944d97f Failed to install test-framework-0.8.0.3 cabal: Error: some packages failed to install: test-framework-0.8.0.3 failed during the configure step. The exception was: ExitFailure 1 C:\Users\guthrie>cabal install mtl Resolving dependencies... All the requested packages are already installed: mtl-2.1.2 Use --reinstall if you want to reinstall anyway. ------------------------------------------- Dr. Gregory Guthrie Computer Science Department Dean, and Professor of Computer Science School of Computer Science & Mathematics Maharishi University of Management -------------------------------------------

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Trying to understand the foldl type?
by Angus Comber 24 Dec '13

24 Dec '13

The type is: foldl :: (a -> b -> a) -> a -> [b] -> a Eg I might do something like this: foldl (+) 0 [1..10] so taking (a -> b -> a) leftmost a is (+) b is 0 right a is what? The first value in the list? Then -> ? [b] ? Is this like the temporary recursion calculation? final -> a is the result?

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Database.SQLite.Simple: Select query with more than 10 elements tuple
by Miro Karpis 23 Dec '13

23 Dec '13

Hi, please ... I need help with following: I have a table with 11 columns. I would like to create a query where in return I should get values of all columns. Problem is that when I try with 10 elements everything works OK. With 11 I will get compile error. I found in the Database.SQLite.Simple documentation following: A collection type that can be converted from a sequence of fields. Instances are provided for tuples up to 10 elements and lists of any length. So is it possible to select more than 10 columns in one query? It must be right? working code: ------------------------- queryX :: Query queryX = "SELECT el1, el2, el3, el4, el5, el6, el7, el8, el9, el10 FROM tableX" saveBtceTickerData :: IO () saveBtceTickerData = do conn <- open "../mydb.sqlite" r <- query_ conn queryX :: IO [(Double, Int, Double, Double, Double, Double, Double, Double, Double, Double)] mapM_ print r close conn putStrLn "done" non-working code: ------------------------- someQuery :: Query someQuery = "SELECT el1, el2, el3, el4, el5, el6, el7, el8, el9, el10, el11 FROM tableX" saveBtceTickerData :: IO () saveBtceTickerData = do conn <- open "../mydb.sqlite" r <- query_ conn queryX :: IO [(Double, Int, Double, Double, Double, Double, Double, Double, Double, Double, Double)] mapM_ print r close conn putStrLn "done" ------ Error output: No instance for (FromRow (Double, Int, Double, Double, Double, Double, Double, Double, Double, Double, Double)) arising from a use of `query_' Possible fix: add an instance declaration for (FromRow (Double, Int, Double, Double, Double, Double, Double, Double, Double, Double, Double)) thanks, m.

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