December 2013 - Beginners

Question on : usage
by Angus Comber 23 Dec '13

23 Dec '13

I am playing with concatenating lists so I start with this: testconcat :: [[a]] -> [a] testconcat [[]] = [] testconcat [(x:xs)] = x : xs Now, testconcat [[]] and testconcat [[1,2]] works So now I want to try with a 2nd inner list so I do this: testconcat :: [[a]] -> [a] testconcat [[]] = [] testconcat [(x:xs)] = x : xs testconcat [(x:xs), (y:ys)] = x : xs : y : ys and I get load error: Couldn't match expected type `a' with actual type `[a]' `a' is a rigid type variable bound by the type signature for testconcat :: [[a]] -> [a] at prog_haskell.hs:218:15 In the first argument of `(:)', namely `xs' In the second argument of `(:)', namely `xs : y : ys' In the expression: x : xs : y : ys But this loads ok: testconcat :: [[a]] -> [a] testconcat [[]] = [] testconcat [(x:xs)] = x : xs testconcat [(x:xs), (y:ys)] = x : xs Even this line cannot be loaded: testconcat [(x:xs), (y:ys)] = x : xs : [] Couldn't match expected type `a' with actual type `[a]' `a' is a rigid type variable bound by the type signature for testconcat :: [[a]] -> [a] at prog_haskell.hs:218:15 In the first argument of `(:)', namely `xs' In the second argument of `(:)', namely `xs : []' In the expression: x : xs : [] Failed, modules loaded: none. if x : xs works why not x : xs : <something else> ??? Can someone please explain?

5 4

Is Cons in the Haskell Module Library
by Patrick Lynch 22 Dec '13

22 Dec '13

Good morning, I'm going through Graham Hutton's link on Category Theory [and yes I understand that you needn't study CT in order to use Haskell] and he uses Cons. I realize that Cons may be a synonym for ':'. But can someone tell me how to search the Haskell Library to find the module containing Cons? Thank you and Merry Christmas

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retreive a list of installed packages
by harry 22 Dec '13

22 Dec '13

Is there any simple way for a program to retrieve a list of installed packages (such as ghc-pkg list returns), or would it be easier to execute phc-pkg as an external process and catch the output?

2 1

Use of (n + 1) as parameter - is it more efficient?
by Angus Comber 20 Dec '13

20 Dec '13

I created my own drop' function like this: drop' :: Int -> [a] -> [a] drop' 0 xs = xs drop' n [] = [] drop' n (x:xs) = drop' (n - 1) xs But in the Haskell book I am reading it is written like this: drop' :: Int -> [a] -> [a] drop' 0 xs = xs drop' (n + 1) [] = [] drop' (n + 1) (x:xs) = drop' n xs My version seems to work but I am concerned about my third line (drop' n [] = []). Is that a problem? Why does this author use n + 1 as a parameter as opposed to n - 1 like I do in body? Or does it make no difference?

3 2

strip --strip-unneeded on package libraries
by robert 16 Dec '13

16 Dec '13

I've noticed that installed packages all have a large .a archive which can be shrunk by about 10% with strip --strip-unneeded. Is there any downside to stripping, and if not, why isn't it done by GHC when the package is installed? There's a (default) option for cabal-install to strip executables, but it seems that package stripping has to be done manually, each time.

1 0

Error with floats in implementation of Horner's method.
by Andrew Fleckenstein 13 Dec '13

13 Dec '13

Hi all, I have implemented Horner's method for evaluating a polynomial as follows: -- the nth horner element, given a list of coefficients and a value at which it needs to be evaluated horner_element l x n | n == (length l)-1 = last l | otherwise = (l !! n) + (horner_element l x (n+1))*x -- compute a polynomial given as a list of coefficients at the value x using horner's method. horner l x = horner_element l x 0

3 4

Cabal build dependency using a git repo
by Michael Baker 11 Dec '13

11 Dec '13

I have a library which isn't on Hackage that I would like to use as a build dependency in another project. In Rubygems you can provide a git repository in the absence of an actual Rubygems repository. Does Cabal support something like that? I would like to specify that a particular build depend can be fetched from a given git repository rather than from Hackage and have Cabal fetch it when installing the build dependencies. I'm guessing that it's not possible because I don't see a mention of this feature anywhere. If it's not possible, how do people typically handle this problem? Git submodules?

4 6

setting up haskell-mode in gnu-emacs (OSX)
by Miro Karpis 11 Dec '13

11 Dec '13

Hi, I'm having troubles to set-up haskell-mode in gnu-emacs. I'm new to emacs,... Instructions says: via package manager add: M-x package-install <RET> haskell-mode <RET> I can open package manager, but where should I paste the command? Thanks, m.

3 3

autocomplete in haskell-aquamacs interpreter
by Miro Karpis 11 Dec '13

11 Dec '13

Please, is there a shortcut for autocomplete in aquamacs (for files and haskell functions)? Cheers, m.

1 0

aquamacs - history interpreter
by Miro Karpis 11 Dec '13

11 Dec '13

Hi, please,..... I have started to use aquamacs and their haskell interpreter. Only problem is that I don't know how to access interpreter commands history. Usually it would be key UP/DOWN, but that will move the cursor UP/DOWN. So,... please how can I access interpreter/console user commands history in aquamacs? Cheers, Miro

3 4