October 2014 - Beginners

Question on syntax
by Rohit Sharma 25 Oct '14

25 Oct '14

All, I started learning haskell very recently and have a question on the pattern matching style for lists. In the below snippet i.e. "(x:xs)" why do we went with round braces and not square? I know we are using cons that tells this is not a tuple but would it not make more sense to write something like [x:xs] instead of (x:xs), i thought round braces was used for pair/tuples? safeHead [] = Nothing safeHead (x:xs) = Just x Thanks, Rohit

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GHCi error msgs
by Julius Gedvilas 24 Oct '14

24 Oct '14

How difficult would it be to change format of GHCi compile-time error-messages of type "couldn't match expected type"?

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learning lazy evaluation
by Dimitri DeFigueiredo 21 Oct '14

21 Oct '14

I found an interesting situation while making recursive calls that I am trying to understand. I re-wrote the 'minimum' function from the prelude as such: minimum_bug :: (Ord a) => [a] -> a minimum_bug [x] = x minimum_bug (x:xs) | x > minimum_bug xs = minimum_bug xs | otherwise = x (I understand I should be using something like "minbug xs = foldr1 min xs" for this, but bare with me) The interesting thing is that the function has exponential time behavior. In a way, that makes sense as I am making two recursive calls. But I was expecting GHC to be smart enough to transform it into something like: minimum_bug' :: (Ord a) => [a] -> a minimum_bug' [x] = x minimum_bug' (x:xs) | x > y = y | otherwise = x where y = minimum_bug' xs (This one always works as expected) I understand that lazy evaluation implies memoization in some cases. When does GHC only use memoization to avoid this kind of behavior? Another interesting fact is that in my simple tests the exponential behavior does NOT occur when I pass the function a list in sorted order. main = do putStrLn "started - in order list" putStrLn $ show $ minimum_bug [1..30000] -- no problem putStrLn "started - out of order list" putStrLn $ show $ minimum_bug [27,26..1] -- don't do this with large numbers! putStrLn "done!" It's not clear to me how the sorted list is able to escape the exponential slow down. Cheers, Dimitri

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GhcDynamic and DYNAMIC_GHC_PROGRAMS
by Jeremy 20 Oct '14

20 Oct '14

mk/config.mk contains GhcDynamic and DYNAMIC_GHC_PROGRAMS variables. What is the difference between them? From the names and comments, it looks like DYNAMIC_GHC_PROGRAMS controls whether all the GHC binaries will be static or dynamically linked, but GhcDynamic only controls the GHC executable?

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Is working with Haskell easier on Linux than on a Mac?
by Jeffrey Brown 19 Oct '14

19 Oct '14

I have read that Haskell is easier to work with on Linux than on Windows. Is Haskell on Linux also easier than Haskell on OS X? I'm trying to do realtime OSC output from Haskell, because concurrency in Python is hard. Brandon Allbery on the haskell-cafe list, told me "chrt" and "sched_setscheduler" would be helpful. At the shell prompt I found that my system (OS X 10.9) does not recognize "chrt". I found a library on Hackage, "posix-realtime", that claims Mac compatibility and has a "sched_setscheduler" function, but my attempts to install it fail: sh-3.2# cabal install posix-realtime Resolving dependencies... Downloading unix-2.3.2.0... Configuring unix-2.3.2.0... Building unix-2.3.2.0... Failed to install unix-2.3.2.0 Last 10 lines of the build log ( /var/root/.cabal/logs/unix-2.3.2.0.log ): Building unix-2.3.2.0... Preprocessing library unix-2.3.2.0... dist/build/System/Posix/Signals.hs:124:10: fatal error: 'Signals.h' file not found #include "Signals.h" ^ 1 error generated. Even if I found a solution to this particular problem, I'm worried I'll keep running into similar ones, because I'm sure I'll keep trying new packages. Would this kind of work be substantially easier if I were using, say, Linux Mint?

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Functors and Applicatives; I'm just not getting it ...
by Frank 19 Oct '14

19 Oct '14

I've had a go at LYAH and CIS 194 and the Typeclassopedia and I just don't get get functors and applicatives. I'm simply not understanding them, what the various symbols/keywords mean, what they represent, how to think of them, etc. Nothing. Is there any kind of documented model I should be considering? Is there a "functors and applicatives for Dummies" I should read? Should I just give it up, not bother with Haskell and just stick to scheme/ruby/C++? -- P.S.: I prefer to be reached on BitMessage at BM-2D8txNiU7b84d2tgqvJQdgBog6A69oDAx6

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Trouble Understanding How to Start
by Keeley Abbott 18 Oct '14

18 Oct '14

Hi All, So, I am taking a Functional Programming class this fall, and I have struggling to understand how to build types and functions correctly. For some reason it just isn’t clicking. I have the following module I am working on, and I need some guidance. — Variable names type Name = String — An environment for looking up the value of a variable type Env = Name -> Maybe Int — An empty environment empty :: Env empty env = Nothing — Set a variable to a value in the environment if it doesn’t already exist set :: Name -> Int -> Env -> Env set x i e = \y -> if x == y then (Just i) else e y — Lookup the value of a variable in the environment get :: Name -> Env -> Int get x env = ?? (I previously had “type Env = Name -> Int”, so this was working with env x, but we had to change empty to be something other than just a runtime error, which is why I made Env = Name -> Maybe Int) —Remove a variable from the environment unset :: Name -> Env -> Env unset x e = \y -> if x == y then empty x else env y At this point I don’t know how to fix get, so I get an int to display (it keeps telling me it can’t match the type ‘Maybe Int’ with the expected type ‘Int’). And I don’t know where to go from there to create getOr, setAll, and mapEnv functions, because I am just not understanding what I am doing. For the most part I have been doing simpler things that I could just mess with the functions until they work, but even at that I’m not getting HOW they work… Any assistance or instruction on how I can GET what I am trying to do would be greatly appreciated. Thanks, Kallarhynn

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Re: [Haskell-beginners] How to override a type class instance ?
by Johannes Joachim Engels 18 Oct '14

18 Oct '14

> You usually wrap the data type in a newtype and use > GeneralizedNewtypeDeriving to derive the classes you want to keep. Then > write custom instances for the type classes you want to override. thank you very much, this worked! Best regards Johannes

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How to override a type class instance ?
by Johannes Joachim Engels 18 Oct '14

18 Oct '14

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QuickCheck Crazy
by Dimitri DeFigueiredo 17 Oct '14

17 Oct '14

Hello Everyone, I am trying to do homework #3 of Stephanie Weirich's programming class. It is about QuickCheck. http://www.seas.upenn.edu/~cis552/current/hw/hw03/index.html However, quickCheck is magically coming up with the wrong answer! I am loading a very simple file (distilled from the homework). Here it is: ~~~~ import Test.QuickCheck ------------------------------------------------------------------------------ prop_const :: Eq a => (a -> a -> a) -> a -> a -> Bool prop_const const' a b = const' a b == a const_bug :: a -> b -> b const_bug _ b = b -- Oops: this returns the *second* argument, not the first. --main = quickCheck (prop_const const) ~~~~ Now, I just wanted to check that 'prop_const' holds for the 'const' function, but fails for 'const_bug' So, I fired up GHCi, but here's what happens: econ1gw-131-21-dhcp:week3 dimitri$ ghci GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> :l weird-quickcheck.hs [1 of 1] Compiling Main ( weird-quickcheck.hs, interpreted ) weird-quickcheck.hs:22:1: Warning: The import of `Test.QuickCheck' is redundant except perhaps to import instances from `Test.QuickCheck' To import instances alone, use: import Test.QuickCheck() Ok, modules loaded: Main. *Main> const 1 2 1 *Main> const_bug 1 2 Loading package array-0.4.0.1 ... linking ... done. Loading package deepseq-1.3.0.1 ... linking ... done. Loading package old-locale-1.0.0.5 ... linking ... done. Loading package time-1.4.0.1 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package primitive-0.5.0.1 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package containers-0.5.0.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package tf-random-0.5 ... linking ... done. Loading package transformers-0.3.0.0 ... linking ... done. Loading package QuickCheck-2.7.6 ... linking ... done. 2 *Main> :t prop_const prop_const :: Eq a => (a -> a -> a) -> a -> a -> Bool *Main> quickCheck (prop_const const) +++ OK, passed 100 tests. *Main> quickCheck (prop_const const_bug) +++ OK, passed 100 tests. *Main> quickCheck (prop_const const_bug :: Char -> Char -> Bool) *** Failed! Falsifiable (after 1 test and 2 shrinks): 'a' 'b' *Main> This makes no sense to me! First, I have no idea how quickCheck is able to run any tests at all as the type of 'const' is: *Main> :t const const :: a -> b -> a In other words, it's a polymorphic type. Second, why does GHCi load so many files when I try to evaluate 'const_bug'? Finally and most puzzling, why does quickCheck give out the wrong result if I don't specify the type when testing 'const_bug'? That's really scary. It means that quickCheck can easily fool me if I forget to specify the necessary types. (GHC complains if I uncommented the '--main' line, so I can only do this in GHCi.) I am totally lost here, any pointers would be much appreciated. Thanks, Dimitri

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