- Haskell-Cafe - Haskell.org

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.

2 2

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…

1 0

Bangalore Haskell User Group Meetups
by Harendra Kumar 23 Nov '16

23 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

1 0

Haskell with ncurses
by Atrudyjane 22 Nov '16

22 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.

4 4

[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

1 0

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…

1 1

Am I misunderstanding something about `StableName` and `hashStableName`?
by raichoo 21 Nov '16

21 Nov '16

Hi, I've been playing around with `System.Mem.StableName` and `hashStableName` and stumbled across a behavior that seems to be rather weird. First of all the documentation of `hashStableName` says the following: The Int returned is not necessarily unique; several StableNames may map to the same Int (in practice however, the chances of this are small, so the result of hashStableName makes a good hash key). OK, sounds fine, let's put this to the test. So I wrote a little program. module Main where import Control.Monad import System.Mem.StableName import System.Environment main :: IO () main = do args <- getArgs res <- forM [0..10000] $ \i -> do x <- makeStableName i let h' = hashStableName x unless (null args) $ print h' return h' putStrLn "---------------------------" print $ minimum res print $ maximum res Running this program without an argument only prints aus the minimum and maximum hash values that where produced. So surprises here: $ ./Test --------------------------- 1 10000 Here is how things get a bit weird when I make the progam print out the hash values while they are being produced I get a completely different result. Not only that but I get a whole lot of hash collisions, since the hashes produced only seem to be within the range of 1 and 260. $ ./Test print [...] 128 127 126 125 124 123 122 121 120 119 118 117 116 115 114 --------------------------- 1 260 Am I missing something fundamental here, because this behavior seems to be rather confusing. Kind regards, raichoo

3 2

Esqueleto maintainership
by Christopher Allen 21 Nov '16

21 Nov '16

Cross-posting my intent to take over maintainership of Esqueleto. Can read more here: https://github.com/haskell-infra/hackage-trustees/issues/84 This has gone on for awhile, but it's beginning to eat into maintenance of my work projects and it seems like there are others eager to contribute and help share the load where there wasn't 6 or 12 months ago. Cheers, Chris Allen

3 3

difficulty writing GEq instance
by Anders Papitto 21 Nov '16

21 Nov '16

Hello! I'm having a lot of trouble writing a Data.GADT.Compare.GEq instance - can anyone help me fill in the blank? For context - I'm generating Tag types automatically for using with DSum, and I need a GEq instance. That's part of an attempt to add efficient rendering of sum types to Reflex/Reflex-Dom (https://github.com/anderspapitto/reflex-sumtype- render/blob/master/src/ReflexHelpers.hs). I put this question on stackoverflow as well a day ago ( http://stackoverflow.com/questions/40698207/how-can-i- write-this-geq-instance). Here's the code (it's a full, standalone file - you can copy it into Foo.hs and run ghc to see the full error I'm facing). The error I get is that when I try to recursively call geq on the unwrapped x and y, I can't because ghc considers them to have different types - Quux a and Quux b. However, the whole point of why I'm trying to call geq is to see if a and b are the same, so I'm pretty confused. Note that I'm making use of the generics-sop library, which is where NP and NS and I come from. {-# LANGUAGE GADTs #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE RankNTypes #-} module Foo where import Data.GADT.Compare import Generics.SOP import qualified GHC.Generics as GHC data Quux i xs where Quux :: Quux (NP I xs) xs newtype GTag t i = GTag { unTag :: NS (Quux i) (Code t) } instance GEq (GTag t) where -- I don't know how to do this geq (GTag (S x)) (GTag (S y)) = let _ = x `geq` y in undefined

2 2

How to profile haskell web applications?
by Moritz Angermann 19 Nov '16

19 Nov '16

Hi, I’m looking to (perf, heap) profile a haskell web application. With ekg and the ekg wai middleware I can get some data on the rts, gc, and latency. What’s the approach towards performance (-p) and heap (-h) profiling for web applications? Cheers, moritz

1 0