- Beginners - Haskell.org

Showing Types
by Dániel Arató 09 Mar '16

09 Mar '16

How does GHCi show types? Is there a magic function (showType :: a -> String) that does that? Or is this feature buried somewhere deep in the compiler? Can I show, compare and reason about types in a Haskell program? This would be cool: ```haskell test = do let len = sum . map (const 1) when (isInfixOf "Integer" (showType len)) $ putStrLn "restrictive type inferred; maybe try turning off monomorphism restriction" ```

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Re: [Haskell-beginners] Question on terminology
by Ben Rogalski 06 Mar '16

06 Mar '16

I'm having trouble understanding this: > IIUC, one would describe fmap as "lifting" a function g into a functor f, > in the sense of > > Functor f => (g a b) -> g a -> g b > > > Is there an inverse concept (? co-lift ?) that describes > > Functor f => (g a -> g b) -> a -> b > It seems like these are type declarations for functions, with class constraints that say f must be a Functor, but then f doesn't appear anywhere else in the declaration. Why is this? Thanks, -Ben

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failing to install GHC 7.10.3 appears to have broken cabal and stack
by Jeffrey Brown 04 Mar '16

04 Mar '16

I use Ubuntu 14.04. Outside of a stack context, I have been using GHC 7.8.4. I tried to install 7.10.3. ./configure worked, but make install failed with this error: make[1]: *** No rule to make target `utils/haddock/dist/build/tmp/haddock', needed by `install_libexecs'. Stop. make: *** [install] Error 2 I tried to manually undo the install, by deleting it from /usr/local/lib, and by changing all the symlinks in /usr/local/bin back to 7.8.4. If I run ghci without stack, it works, and is still at version 7.8.4. But now cabal and stack are producing errors. "cabal update" runs, but "cabal install" gets confused: jeff@jeffLenovUbu:~/dwt_git_hask$ cabal install regex-compat Warning: cannot determine version of /usr/local/bin/hpc : "" Warning: cannot determine version of /usr/local/bin/hsc2hs : "" Resolving dependencies... Configuring regex-posix-0.95.2... Building regex-posix-0.95.2... Preprocessing library regex-posix-0.95.2... /usr/local/bin/hsc2hs: line 29: /usr/local/lib/ghc-7.10.3/bin/hsc2hs: No such file or directory Failed to install regex-posix-0.95.2 cabal: Error: some packages failed to install: regex-compat-0.95.1 depends on regex-posix-0.95.2 which failed to install. regex-posix-0.95.2 failed during the building phase. The exception was: ExitFailure 127 jeff@jeffLenovUbu:~/dwt_git_hask$ As an example of a stack problem, I downloaded Oleg Prophet's implementation of MicroKanren [1]. "stack build" ran without complaint (or visible output), but "stack ghci" fails thus: jeff@jeffLenovUbu:~/work/computer/Haskell/microKanren/featherweight$ stack ghciConfiguring GHCi with the following packages: microKanren GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help <command line>: cannot satisfy -package QuickCheck (use -v for more information) jeff@jeffLenovUbu:~/dwt_git_hask$ stack ghci even fails (with a different error) on my own project [2]: jeff@jeffLenovUbu:~/dwt_git_hask$ stack ghci Configuring GHCi with the following packages: Dwt GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help <command line>: can't load .so/.DLL for: /home/jeff/dwt_git_hask/.stack-work/install/x86_64-linux/nightly-2015-08-03/7.10.2/lib/x86_64-linux-ghc-7.10.2/Dwt-0.1.0.0-7VW1kKp7le6K6OdeGoRsDt/ libHSDwt-0.1.0.0-7VW1kKp7le6K6OdeGoRsDt-ghc7.10.2.so (/home/jeff/dwt_git_hask/.stack-work/install/x86_64-linux/nightly-2015-08-03/7.10.2/lib/x86_64-linux-ghc-7.10.2/Dwt-0.1.0.0-7VW1kKp7le6K6OdeGoRsDt/ libHSDwt-0.1.0.0-7VW1kKp7le6K6OdeGoRsDt-ghc7.10.2.so: undefined symbol: Dwtzu7VW1kKp7le6K6OdeGoRsDt_DwtziSearch_QNode_con_info) jeff@jeffLenovUbu:~/dwt_git_hask$ Thanks in advance for your help. [1] https://github.com/Oregu/featherweight [2] https://github.com/JeffreyBenjaminBrown/digraphs-with-text -- Jeffrey Benjamin Brown

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general observation about programming
by Dennis Raddle 03 Mar '16

03 Mar '16

This is more about programming in general than Haskell, although Haskellers probably know it well. I don't claim to have expert knowledge on this, but I'm gradually getting better at it. When I set out to write a program, or refactor a program, or modify a program, it helps to set out my thinking in a clear way. And how I make it clear is to document my thoughts. An outline is one good way to organize thoughts and is probably my main tool. But good English prose is also helpful. The key factor is "editing." In what sense do I mean that? Good writers do it, and the Haskell documentation does it. I mean (1) brevity and (2) good flow. To achieve brevity, you must think about the essence of each statement and trim away the unnecessary stuff. Good flow refers to how the document builds up and modifies your concepts as you read it. A document can actually mirror an effective learning process, or influence and change your process. I work with my documentation, making several editing passes. By the time I'm done, I am in a great position to write a concise and flexible program. It's interesting that not only is Haskell a concise language, but the Haskell library documentation is concise. Contrast that with the Python documentation which often wanders about into areas that are irrelevant--it could easily be cut into one third its present size. Mike

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Question on terminology
by Joel Neely 03 Mar '16

03 Mar '16

IIUC, one would describe fmap as "lifting" a function g into a functor f, in the sense of Functor f => (g a b) -> g a -> g b Is there an inverse concept (? co-lift ?) that describes Functor f => (g a -> g b) -> a -> b Similarly is there standard terminology for "distributing" and "gathering" in the sense of Functor f => f [a] -> [f a] and Functor f => [f a] -> f [a] respectively? References much appreciated! -jn- -- Beauty of style and harmony and grace and good rhythm depend on simplicity. - Plato

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problem installing persistent-postgresql together with persistent-template
by Miro Karpis 02 Mar '16

02 Mar '16

Please,..can you help me with following? I'm trying to install both persistent-postgresql together with persistent-template, but I keep getting: mac:~ miro$ cabal install persistent-template Resolving dependencies... In order, the following would be installed: aeson-0.9.0.1 (latest: 0.11.0.0) (via: persistent-template-2.1.5 persistent-2.2.4 aeson-compat-0.3.1.0) (new version) aeson-compat-0.3.1.0 (via: persistent-template-2.1.5) (new package) persistent-2.2.4 (via: persistent-template-2.1.5) (reinstall) (changes: aeson-0.11.0.0 -> 0.9.0.1) persistent-template-2.1.5 (new package) cabal: The following packages are likely to be broken by the reinstalls: persistent-postgresql-2.2.2 Use --force-reinstalls if you want to install anyway. If I continue with force-reinstall then I broke postgresql. Is there a way to force persistent-template to use 2.2.2 version (I know if I download it and change the dep. in cabal file,....but is that what I should do :) )? running on: mac:~ miro$ cabal --version cabal-install version 1.22.8.0 using version 1.22.7.0 of the Cabal library mac:~ miro$ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 cheers, m.

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Stack minimal dependency specification, or dependency tree output
by Lyndon Maydwell 27 Feb '16

27 Feb '16

Hi Beginners, I'm finally getting my hands dirty with Stack, and am using it in conjunction with Docker, but not with the built-in docker functionality. My Dockerfile is constructed so that it first installs a whole bunch of dependencies globally, like so: ... RUN stack install HUnit ... Then after that, installs the project: ... COPY . /app WORKDIR /app RUN stack install ... This means that on repeated docker builds the app build and install time should be limited to just the application itself, because the dependency builds were cached. Which is great! However, I'm currently generating the list of dependencies just by looking at the output of the stack build of the app, and this displays everything as a flat list. I'd like to see some kind of tree instead, so that when I pre-install the dependencies, I can specify a minimal list, rather than a whole slew of dependencies that would be pulled in transitively anyway. Is there an easy way to do this? Regards, - Lyndon

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Folders and sub-folders
by Mike Houghton 27 Feb '16

27 Feb '16

Hi, I’m trying to build up a list of all folders (including subfolders) from a given root folder. So I have folders :: FilePath -> IO [FilePath] folders fp = do all <- getDirectoryContents fp filterM doesDirectoryExist $ map (fp </>) all and this just gets the immediate folders within the given folder. I’m stuck on how to recursively call folders and build up the IO [FilePath] folders :: FilePath -> IO [FilePath] folders fp = do all <- getDirectoryContents fp -- z :: IO [FilePath] let z = filterM doesDirectoryExist $ map (fp </>) all — z’ :: [FilePath] z' <- z — ?? what should happen here? z : (map folders z’) Couldn't match expected type ‘[FilePath]’ with actual type ‘IO [FilePath]’ In the first argument of ‘(:)’, namely ‘z’ In a stmt of a 'do' block: z : (map folders z') etc... Thanks Mike

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The (x:xs) in function parameter is a tuple?
by Nan Xiao 26 Feb '16

26 Feb '16

Hi all, Greetings from me! I am confused about the function parameters and tuple. E.g.: occurs value [] = 0 occurs value (x:xs) = (if value == x then 1 else 0) + occurs value xs should we consider (x:xs) as a tuple? Thanks in advance! Best Regards Nan Xiao

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to avoid naming intermediate variables
by Dennis Raddle 23 Feb '16

23 Feb '16

I recently realized one important reason Haskell is so compact---it provides structures that avoid the need to name intermediate variables. I guess it's obvious, but coming from an imperative background I didn't see this at first. I am starting to recognize situations where all the existing wonderful typeclasses assist in eliminating variables. Function composition is the simplest example. I sometimes write long chains of composed functions. In an imperative language with a less compact function call syntax, that would require so many parentheses that you would probably break it over several statements, then forcing you to use more variables. Then I realized that Arrows allow a little more flexibility when you have different numbers of arguments to feed, say when you split a single argument into two, or want to construct a function that applies to one argument and ignores the other. ...So I had to write something like this. compute :: [Int] -> [Int] -> [Int] -> [(Int,Int)] func :: [Int] -> [(Int,Int)] func xs = compute xs (filter isSmall xs) (filter isLarge xs) but I could also write compute :: ([Int],([Int],[Int])) -> [(Int,Int)] func = compute . (id &&& filter isSmall &&& filter isLarge) So I had to change the inputs to 'compute' into this kind of awkward tuple form, but I eliminated four 'xs', plus eliminated the need to come up with the name 'xs'. Can I get a comment on whether this makes sense as way to do things? D

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