July 2017 - Haskell-Cafe

Feeding a monad into itself
by Jake 21 Jul '17

21 Jul '17

I have a function f :: a -> Maybe a that I want to feed into itself with some initial input until it returns Nothing. untilNothing :: (a -> Maybe a) -> a -> a untilNothing f x = case f x of Just x' -> untilNothing f x' Nothing -> x Is there a better way to do this? I feel like there is something more general going on with Monads being fed into themselves, but maybe I'm wrong. Thoughts?

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How is this variable not in scope?
by Jeffrey Brown 21 Jul '17

21 Jul '17

The following compiles and runs perfectly: main = do (inputFile:_) <- getArgs input <- readFile inputFile let pairs = pairFilesToContents $ readSmsnLines input mapM_ f pairs where f :: (FilePath, String) -> IO () f (name, content) = writeFile name content But the following triggers the error "Not in scope: ‘root’". main' = do (root : inputFile :_) <- getArgs input <- readFile inputFile let pairs = pairFilesToContents $ readSmsnLines input mapM_ f pairs where f :: (FilePath, String) -> IO () f (name, content) = writeFile (root+name) content -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

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How to deal with last item with concatMapAccumC in Conduit.
by jun zhang 21 Jul '17

21 Jul '17

Dear cafes I use Conduit to parse a huge file. And I need merge lines by condition. I find the concatMapAccumC can do that and I write a demo as blow(with conduit-combinators-1.0.6,lts-6.18). The problem is if the last item didn’t make condition true, the data only keep in the accum but missing in stream. Any one can give me some advises? Thanks ---------------------------- import Conduit test'::Int->Int->(Int,[Int]) test' a s = case (a+s) > 5 of True -> (0,[a+s]) False -> (a+s,[]) testlog::IO [Int] testlog = runConduit $ (yieldMany [1,2,3,4,5,6,3]) $= (concatMapAccumC test' 0 ) $$ sinkList

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getChar (NoBuffering) not working on Windows
by Francesco Ariis 20 Jul '17

20 Jul '17

Hello list, I am writing a small console application. Today I was bitten by #2189 [1] ("hSetBuffering stdin NoBuffering doesn't work on Windows"). To reproduce: import System.IO main :: IO () main = hSetBuffering stdin NoBuffering >> getChar >>= putChar -- on linux you just need to press, say, 'c', on Win you -- need to press <Enter> too. Is there any sensible (cross-platform) way of working around this? [1] https://ghc.haskell.org/trac/ghc/ticket/2189

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runghc and linking in libraries
by Brian Sammon 20 Jul '17

20 Jul '17

If I want to have (and run without explicitly invoking ghc or runghc) a script that starts as follows: #!/usr/bin/runghc module Main where import MyLib I can do it just fine if MyLib is a pure-haskell MyLib.hs -- it doesn't have to be packaged or installed or anything complicated. If I want to create a MyLib that uses C code (that I wrote) through FFI, it doesn't seem to be possible to use it in a runghc script like this, unless I create a package and install it. Am I correct about this? In the course of researching this, I realized that I have a rather poor understanding of what "packages" are in ghc-land. I have a high-level-overview understanding ("It's like rubygems, but for haskell"), but I'm rather unclear about the nuts-and-bolts of it. The tight coupling between packages and the ghc core is rather different than what I've seen in other programming languages I've used (I haven't programmed in Java though) It would be useful if ghc had an argument to take a "MyLib.lib" file (or maybe a directory) and temporarily act like it's an installed package for the one invocation. Unsurprisingly, web-searching for "haskell" and "package" doesn't give me the focused result set that I want. I found the relevant section of the GHC Users Guide https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/packages.ht… but links to other reading materials would be appreciated.

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[ANN] New release of SBV (v7.0), now with incremental solving
by Levent Erkok 20 Jul '17

20 Jul '17

I'm pleased to announce v7.0 release of SBV, a library for integrating SMT solvers into Haskell. This release adds incremental solving capabilities to SBV: Users can now interact with the underlying solver using a typed API capturing the SMTLib language, issuing commands, getting model values, and asserting further constraints on-the-fly. The possibility of issuing multiple check-sat commands allows incremental assertion of facts, a key feature of modern constraint solving systems. Hackage: https://hackage.haskell.org/package/sbv Homepage: http://leventerkok.github.io/sbv/ Release notes: https://github.com/LeventErkok/sbv/blob/master/CHANGES.md SMTLib initiative: http://smtlib.cs.uiowa.edu/ Feedback and bug reports are most welcome! -Levent.

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How to statically link some external libraries?
by Kai Zhang 20 Jul '17

20 Jul '17

I want to produce a statically-linked executable. First I tried "-optl-static -optc-static -optl-pthrea", which statically links all external libraries. But when I ran this executable it gives me "abort (core dumped)" error. I suspect the statically-linked glibc library might be the cause. So I decide to statically link all other libraries and leave the glibc as a dynamic library. How to do this using Cabal? Should I write a custom "Setup.hs"?

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DeepSeq and random generators
by Dennis Raddle 19 Jul '17

19 Jul '17

I'm having trouble using Control.DeepSeq.force (necessary for parallel code) on a StdGen from System.Random. I don't know how to derive NFData. A standalone deriving statement, like this deriving instance NFData StdGen gets me the error "the data constructors of StdGen are not all in scope..." Is it actually possible to derive an NFData instance of StdGen, and how would I do that? Or if not possible, how can I work about this? Note: I'm going to see if I can use tf-random in this application. Maybe it will be easier. D

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any cloud computing services for running Haskell?
by Dennis Raddle 19 Jul '17

19 Jul '17

I'm thinking of using cloud computing services for running my CPU-intensive parallel Haskell code, in order to get results faster. Instead of buying a faster computer, I would take advantage of existing services. Note that my budget is small, so I need something that doesn't have a large minimum charge. I'm thinking $10 - 30 per month. Are there any cloud services like that that run Haskell? D

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"cabal quine"
by Albert Y. C. Lai 19 Jul '17

19 Jul '17

Today a few of us on IRC (freenode #haskell) discovered: The following cabal-install command cabal manpage outputs cabal-install's man page (and is mainly how cabal-install can come with a man page without carrying an extra source file). This inspires the following feature request in jest: Could "cabal quine" output its source code please? :) kthxbye

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