November 2016 - Haskell-Cafe

Is it possible to get the selector functions when defining a generic class?
by chris＠kahn.pro 24 Nov '16

24 Nov '16

Hey all! I'm trying to understand what's going on in GHC.Generics and defining a generic class... I understand that there's a `Selector` class and `selName` function that can get the *name* of a selector, but is there a way to access the selector function itself? The documentation conveniently avoids examples involving records and is otherwise quite barren. So if I have a data type like... data Person = Person { name :: String , age :: Int } deriving Generic instance MyTypeClass Person I want my generic implementation of MyTypeClass to be able to access each selector function in the record, f :: Person -> String, g :: Person -> Int, etc. Chris

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Overriding CPP preprocessor on OSX
by Cody Goodman 24 Nov '16

24 Nov '16

I'm having some problems[0] with the derive-storable package on OSX I don't have on Ubuntu. I would like to use g++ rather than clang to preprocess CPP code. Is there a way to do this with cabal? Thanks! 0: https://github.com/mkloczko/derive-storable/issues/1

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Re: [Haskell-cafe] Is it possible to get the selector functions when defining a generic class?
by David Feuer 24 Nov '16

24 Nov '16

I don't know much about this database stuff, but I'm pretty sure you want to *build* the selector, rather than *extracting* it. You should use the Generics metadata to get the string for parsing/printing, but aside from that, you just have the products. On Nov 23, 2016 10:19 PM, "Chris Kahn" <chris(a)kahn.pro> wrote: Aaaand you'll get mine twice since I forgot to reply-all the first time :) Sure, so in postgresql-simple there are two classes for automatically generating functions that encode/decode database rows, `FromRow` and `ToRow`. In the Hasql library--another postgres library--the encoders and decoders must be written by hand for each user-defined type. I want to write a class that will automatically generate these. I successfully wrote a `FromRow` class that can generate Hasql's `Row` type, since it's basically identical to what's in postgresql-simple's `FromRow`. But in Hasql the encoder type, Params, is contravariant and encoders are defined like: personEncoder :: Params Person personEncoder = contramap name (value text) <> contramap age (value int) The `value text` part can be determined based on the type information, but it's also expecting a matching selector function. I'm at a total loss for how I could generate something like this. On 11/23/2016 10:06 PM, David Feuer wrote: > Sorry if anyone gets this twice; the first copy somehow went to a > non-existent Google Groups version of haskell-cafe. > > GHC.Generics doesn't offer any built-in support for such things. It > *looks* like there *might* be some support in packages built around > generics-sop. When you're working directly with GHC.Generics, the > notion of a record barely even makes sense. A record is seen as simply > a possibly-nested product. For example, ('a','b','c') will look > *approximately* like 'a' :*: ('b' :*: 'c'). You're generally not > "supposed" to care how large a record you may be dealing with, let > alone what field names it has. May I ask what you're actually trying > to do? Your specific request sounds peculiarly un-generic. > > On Wed, Nov 23, 2016 at 9:52 PM, <chris(a)kahn.pro> wrote: >> Hey all! >> >> I'm trying to understand what's going on in GHC.Generics and defining a >> generic class... I understand that there's a `Selector` class and `selName` >> function that can get the name of a selector, but is there a way to access >> the selector function itself? The documentation conveniently avoids examples >> involving records and is otherwise quite barren. >> >> So if I have a data type like... >> >> data Person = Person >> { name :: String >> , age :: Int >> } deriving Generic >> >> instance MyTypeClass Person >> >> I want my generic implementation of MyTypeClass to be able to access each >> selector function in the record, f :: Person -> String, g :: Person -> Int, >> etc. >> >> Chris >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post.

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ANN: Haskell Website Working Group
by Gershom B 24 Nov '16

24 Nov '16

The following text is reproduced from Neil Mitchell’s blog (I’m just the messanger) where it lives at: https://neilmitchell.blogspot.com/2016/11/the-haskellorg-website-working-gr… == The Haskell.org Website Working Group (HWWG) == Haskell represents both a language and a user community - and moreover a fantastic community full of friends, fun, and deep technical debate. Unfortunately, in recent times the community has started to fracture, e.g. Cabal vs Stack, haskell.org vs haskell-lang.org. These divisions have risen above technical disagreements and at some points turned personal. The solution, shepherded by Simon Peyton Jones, and agreed to by both members of the haskell.org committee and the maintainers of haskell-lang.org, is to form the Haskell Website Working Group (HWWG). The charter of the group is at the bottom of this post. The goal of the Haskell Website Working Group is to make sure the Haskell website caters to the needs of Haskell programmers, particularly beginners. In doing so we hope to either combine or differentiate haskell.org and haskell-lang.org, and give people clear recommendations of what "downloading Haskell" means. Concretely, we hope that either haskell-lang.org redirects to haskell.org, or that haskell-lang.org ends up being used for something very different from today. == The Haskell Website Working Group (HWWG) == - Scope and goals - * The HWWG is responsible for the design and content of the user-facing haskell.org web site, including tutorials, videos, news, resource, downloads, etc. * The HWWG is not responsible for: * The infrastructure of haskell.org * Toolchains, Hackage, compilers, etc * The HWWG focuses on serving users of Haskell, not suppliers of technology or libraries. * An explicit goal is to re-unite the haskell.org and haskell-lang.org web sites. - Expected mode of operation - * HWWG is not responsible for actually doing everything! The web site is on github. Anyone can make a pull request. The general expectation is that uncontroversial changes will be accepted and committed without much debate. * If there is disagreement about a proposed change, it's up to the HWWG to engage in (open) debate, and to seek input as widely as time allows, but finally to make a decision. - Membership - Initial membership comprises of: * Neil Mitchell (chair) * Nicolas Wu * Andrew Cowie * Vincent Hanquez * Ryan Trinkle * Chris Done It is expected the committee will change over time, but the mechanism has not yet been thought about. - Rules of engagement - * Recognising that honestly-held judgements may differ, we will be scrupulously polite both in public and in private. * Recognising that Haskell has many users, and that different users have different needs and tastes, we want haskell.org to be inclusive rather than exclusive, providing a variety of alternative resources (toolchains, tutorials, books, etc) clearly signposted with their intended audiences. * Ultimately the haskell.org committee owns the haskell.org URL, but it delegates authority for the design and content of the web site to the HWWG. In extremis, if the haskell.org committee believes that the HWWG is mismanaging the web site, it can revoke that delegation.

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debugging vector index out of range
by briand＠aracnet.com 23 Nov '16

23 Nov '16

Hi, I'm getting a vector index out of range. I'm trying to figure out how to get something to give the line number of the offender. However, attempting to use 'ghc -prof' gives me a problem because several of the libraries I'm using don't have profiled versions. Is there some way for me to debug this ? That is some way to debug it without inserting debug traces everywhere checking the vector indices ? Thanks.

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Intended behavior of -no-rtsopts-suggestions
by Michael Snoyman 23 Nov '16

23 Nov '16

I'm trying to compile an executable which will totally ignore RTS options flags (have the runtime completely ignore them). It seemed like a combo of -rtsopts=none and -no-rtsopts-suggestions[1] would achieve this. However, I can't figure out what the presence of that flag is doing. Can someone give me a pointer to what my expected behavior should be here, and even better how to achieve my goal of having the runtime ignore +RTS on the command line? $ cat Main.hs import System.Environment main = getArgs >>= print $ ghc -rtsopts=none -no-rtsopts-suggestions Main.hs -fforce-recomp && ./Main +RTS -s [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Main: RTS options are disabled. Link with -rtsopts to enable them. $ ghc -rtsopts=none Main.hs -fforce-recomp && ./Main +RTS -s [1 of 1] Compiling Main ( Main.hs, Main.o ) Linking Main ... Main: RTS options are disabled. Link with -rtsopts to enable them. [1] https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/phases.html…

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Bangalore Haskell User Group Meetups
by Harendra Kumar 22 Nov '16

22 Nov '16

This is for Haskellers in Bangalore, I apologize for wide distribution. We are having Haskell meetups in Bangalore on a regular basis now. If you are in Bangalore please join us by signing up at https://www.meetup.com/The-Bangalore-Haskell-User-Group/ . You can also join the gitter chat room at https://gitter.im/blrhaskell/Lobby . We have had a few lively and productive sessions in the recent past and hope to continue to do so in future. Looking forward to your participation. -harendra

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Haskell with ncurses
by Atrudyjane 21 Nov '16

21 Nov '16

As a beginning Haskeller working my way through the Haskell book, I'm looking ahead at different options for a user interface. The idea is to build various small tools for personal use. After reading around, it looks like web apps are preferred. Tried to use the Atom editor, and it just seemed a little slow on my machine. The interfaces don't need to be complex or "pretty", just clean looking. I would almost prefer something simple like ncurses. What are your opinions/experiences with using ncurses with Haskell? Or is that the wrong path altogether? Regards, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.

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[cabal-install bootstrap failed] ghc-pkg: missing --global-package-db option, location of global package database unknown
by CRUMEYROLLE Pierre 21 Nov '16

21 Nov '16

hello i try to reinstall cabal in a customized directory but bootstrap launch fail because ghc-pkg list is failed thanks for help $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.10.3 $ ./bootstrap.sh --no-doc Using gcc for C compiler. If this is not what you want, set CC. Using /usr/libexec/gcc/x86_64-redhat-linux/4.8.5/collect2 instead. Checking installed packages for ghc-7.10.3... ghc-pkg: missing --global-package-db option, location of global package database unknown Error during cabal-install bootstrap: running 'ghc-pkg list' failed $ ghc-pkg list ghc-pkg: missing --global-package-db option, location of global package database unknown

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Investigating Evaluation, Typing and Deduction -- WIP
by Rik Howard 21 Nov '16

21 Nov '16

Comments welcome. http://www.dcs.bbk.ac.uk/~rik/gallery/investigating-evaluation-typing-and-d…

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