March 2016 - Haskell-Cafe

Prelude.head: empty list
by Han Joosten 17 Mar '16

17 Mar '16

In a rather large program I made some changes, and now I get the runtime error: ampersand.exe: Prelude.head: empty list Of course I know that head is partial, and should be used with care. It is used many times (we have 100+ modules in the program). Is there an elegant way to get some information about where the specific call to head is being made? Thanks!

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Re: [Haskell-cafe] Link error
by Alexander Altman 17 Mar '16

17 Mar '16

This is actually nothing at all to do with formatting; instead it’s what’s known as Z-encoding: see https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/SymbolNames for details. > > > EmbedTree_zdfLiftEntryzuzdclift_closure > > > I've seen something like this before -- I think the 'zd' and 'zu' in there > are supposed to be formatting tokens, %zd and %zu, for architecture-aware > data sizing. I ran into this while building the Haskell GTK bindings on > Windows, and IIRC it was caused by GHC 7.8 bundling an older GCC. >

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Re: [Haskell-cafe] Link error
by Theodore Lief Gannon 17 Mar '16

17 Mar '16

>EmbedTree_zdfLiftEntryzuzdclift_closure I've seen something like this before -- I think the 'zd' and 'zu' in there are supposed to be formatting tokens, %zd and %zu, for architecture-aware data sizing. I ran into this while building the Haskell GTK bindings on Windows, and IIRC it was caused by GHC 7.8 bundling an older GCC.

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Link error
by fr33domlover 17 Mar '16

17 Mar '16

Hello, I'm getting this when trying to build the tests of the 'ssh' package: -------------------------------------------------------------------- ssh-0.3.2: build (lib + test) Preprocessing library ssh-0.3.2... In-place registering ssh-0.3.2... Preprocessing test suite 'ssh-test' for ssh-0.3.2... [2 of 2] Compiling Main ( test/test.hs, .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/ssh-test/ssh-test-tmp/Main.o ) /usr/bin/ld: .stack-work/dist/x86_64-linux/Cabal-1.22.5.0/build/ssh-test/ssh-test-tmp/EmbedTree.dyn_o: relocation R_X86_64_PC32 against symbol `EmbedTree_zdfLiftEntryzuzdclift_closure' can not be used when making a shared object; recompile with -fPIC /usr/bin/ld: final link failed: Bad value collect2: error: ld returned 1 exit status Progress: 1/2 -- While building package ssh-0.3.2 using: /home/fr33domlover/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.22.5.0-ghc-7.10.3 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.22.5.0 build lib:ssh test:ssh-test --ghc-options " -ddump-hi -ddump-to-file" Process exited with code: ExitFailure 1 -------------------------------------------------------------------- Other people don't get this error. What does it mean, how do I start to debug and fix it? Thanks in advance for any clue! --fr33

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GHCi on Pi Zero
by alex 17 Mar '16

17 Mar '16

Hi all, I'm struggling with getting a working ghci on the Raspberry Pi Zero. I managed to get a ghc with ghci installed via jessie-backports, but when I run ghci from the commandline I simply get a "Illegal instruction" message. Has anyone had more luck, or does anyone have ideas / tips? Best wishes alex -- http://yaxu.org/

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Zoom instance for Coroutine
by Konstantin Saveljev 15 Mar '16

15 Mar '16

Hello, I am using monad-coroutine <https://hackage.haskell.org/package/monad-coroutine> package and have something like Coroutine (Await Int) (State MyState) Int (simplified version, for example purpose). MyState is deeply nested and I do a lot of modifications in it. Previously (with different data types) I was able to use zoom from lens <https://hackage.haskell.org/package/lens>in order to simplify the typing/view of code which operates on some subtree of the state, e.g.: zoom (company.assets) $ do something.a = someComputation anotherthing.b = False this.that = "someString" The problem I am having is that I cannot come up with instance Zoom for the Coroutine and was wondering if anyone is able to help me with that? Best regards, Konstantin

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Foldable/Traversable and Applicative/Monoid?
by David Banas 15 Mar '16

15 Mar '16

Hi all, I don't understand why Foldable is a necessary super-class of Traversable, and I suspect that the Applicative/Monoid duality, which I've just begun discovering in the literature, has something to do with why that is so. Can anyone give me a hint, without giving me the answer? Thanks! -db

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ANN: katip - a new structured logging framework for Haskell
by Ozgun Ataman 15 Mar '16

15 Mar '16

Fellow Haskellers, We're delighted to release a new logging framework today called "katip", centered around the idea of making it easy to include contextual data in individual log items in the form of structured fields. Katip has become, rather quickly, our go-to logging library at Soostone, included in everything from simple ad-hoc scripts to production systems under load. You can find katip on both Hackage[1] and Github[2]. The core library has also been added to Stackage nightly, though the ElasticSearch backend is waiting on a dependency conflict on aeson. We hope you find it useful and look forward to any contributions or feedback! [1] http://hackage.haskell.org/package/katip [2] https://github.com/Soostone/katip As a little summary, here's the features checklist from the README: * *Structured:* Logs are structured, meaning they can be individually tagged with key value data (JSON Objects). This helps you add critical details to log messages before you need them so that when you do, they are available. Katip exposes a typeclass for log payloads so that you can use rich, domain-specific Haskell types to add context that will be automatically merged in with existing log context. * *Easy to Integration:* Katip was designed to be easily integrated into existing monads. By using typeclasses for logging facilities, individual subsystems and even libraries can easily add their own namespacing and context without having any knowledge of their logging environment. * *Practical Use:* Katip comes with a set of convenience facilities built-in, so it can be used without much headache even in small projects. * A `Handle` backend for logging to files in simple settings. * A `AnyLogPayload` key-value type that makes it easy to log structured columns on the fly without having to define new data types. * A `Monadic` interface where logging namespace can be obtained from the monad context. * Multiple variants of the fundamental logging functions for optionally including fields and line-number information. * *Extensible:* Can be easily extended (even at runtime) to output to multiple backends at once (known as scribes). See `katip-elasticsearch` as an example. Backends for other forms of storage are trivial to write, including both hosted database systems and SaaS logging providers. * *Debug-Friendly:* Critical details for monitoring production systems such as host, PID, thread id, module and line location are automatically captured. User-specified attributes such as environment (e.g. Production, Test, Dev) and system name are also captured. * *Configurable:* Can be adjusted on a per-scribe basis both with verbosity and severity. * *Verbosity* dictates how much of the log structure should actually get logged. In code you can capture highly detailed metadata and decide how much of that gets emitted to each backend. * *Severity* AKA "log level" is specified with each message and individual scribes can decide whether or not to record that severity. It is even possible to at runtime swap out and replace loggers, allowing for swapping in verbose debug logging at runtime if you want. * *Battle-Tested:* Katip has been integrated into several production systems since 2015 and has logged hundreds of millions of messages to files and ElasticSearch.

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ANNOUNCE: Frown, Ralf Hinze's LALR(k) parser generator for Haskell, now slightly upkept ☺
by M Farkas-Dyck 14 Mar '16

14 Mar '16

Some years ago, Ralf Hinze wrote this parser generator [0], and i have since been using it, so i cleaned it up and uploaded it to Hackage as "frown" [1]. My repo is here: https://github.com/strake/frown [0] http://www.cs.ox.ac.uk/ralf.hinze/frown/ [1] https://hackage.haskell.org/package/frown

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Module name to package name index
by Harendra Kumar 14 Mar '16

14 Mar '16

Is there something available which can answer questions like this - what all packages on Hackage export "Data.List" module? Another useful question could be - what all packages export symbol named 'x'? Hackage search or hoogle are not helpful in providing a precise answer to these. I am assuming that related functionality usually lands up in the same part of the module namespace. This fact can help us in discovering related or alternative packages or functions available in a given area. For example when I am looking for list related functions I can search for who all exports "Data.List". I guess creating such an index should not be difficult. All the necessary information is available in the packages' .cabal files, we just need to extract it and create a module name to package name index. -harendra

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