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Proposal: Add filterM or filterA to Data.Sequence
by David Feuer 30 Dec '14

30 Dec '14

This can be given exactly the same implementation as the one for lists: filterM :: (Applicative f) => (a -> f Bool) -> Seq a -> f (Seq a) filterM p = foldr go (pure empty) where go x r = f <$> p x <*> r where f flg ys = if flg then x <| ys else ys Bikeshed all you like over the name.

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Proposal: Move some functions from Control.Monad to Data.List
by David Feuer 29 Dec '14

29 Dec '14

Specifically: filterM, mapAndUnzipM (unless we can sensibly and efficiently generalize it from lists to arbitrary Functors or at least Traversables, which looks like it might be possible), zipWithM, zipWithM_, replicateM, replicateM_ These functions all have to do with monads, yes, but they're about using monads with lists. Some other types are perfectly capable of doing similar things with monads. David the Feuer

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Does anyone want intersperse for Data.Sequence?
by David Feuer 29 Dec '14

29 Dec '14

containers master now uses a new mechanism to implement <*> that can also be used directly to implement an efficient intersperse function corresponding to the one in Data.List. The real question is whether anyone wants one. The potential for clashing names is obviously a point against. The other is that the same asymptotic bounds (but almost certainly worse constant factors) can be obtained using intersperse x xs = drop 1 $ forwards $ Backwards (fromList [const x, id]) <*> Backwards xs

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Strange errors trying to install terminfo
by Sean McAfee 29 Dec '14

29 Dec '14

Hello-- I hope my question is appropriate here; I couldn't find an obviously better forum for what is apparently a cabal issue in a fair amount of searching. I've been trying to install the terminfo package on my Ubuntu system like so: > ~$ cabal install terminfo > Resolving dependencies... > Configuring terminfo-0.4.0.0... At this point, the command hangs, and I notice my mouse cursor has transformed into a crosshair image for some reason. I can't click on anything normally, but after several clicks around the screen, the command resumes: > /home/mcafee/node-v0.10.31/configure: line 10: syntax error near unexpected token `(' > /home/mcafee/node-v0.10.31/configure: line 10: `CC = os.environ.get('CC', 'cc')' > Failed to install terminfo-0.4.0.0 > cabal: Error: some packages failed to install: > terminfo-0.4.0.0 failed during the configure step. The exception was: > ExitFailure 2 I executed this command from within an Emacs shell buffer. I tried again from an xterm window and got the same result. I tried yet again from a textual virtual terminal, and the only difference was that this line was printed several times after the "Configuring" line: > import.im6: unable to open X server `' @ error/import.c/ImportImageCommand/368. Any help resolving this issue would be most welcome.

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Merge either into transformers
by Yitzchak Gale 28 Dec '14

28 Dec '14

Roman Cheplyaka wrote in February 2014: >> I proposed to merge either into transformers more than a year ago[1], >> and everyone seemed to agree. Could you please do it? >> >> [1]: http://www.haskell.org/pipermail/libraries/2012-December/019027.html Ross Paterson responded: > Not everyone agreed when we discussed this last August. My proposal then > was to introduce a new constructor ExceptT on the same pattern as ReaderT, > etc, and to deprecate ErrorT, and that's what I intend to include in the > next major release, after GHC 7.8 comes out. First of all, GHC 7.8 is well past. Now we are well into the 7.10 zone. I re-read the original thread and did not find anyone who disagreed with this, so I'm not sure what you mean by that. I also didn't find any mention of ExceptT there. The discussion was about merging the either library into transformers as is, with the bits that are problematic due to non-base dependencies written out by hand where possible or else omitted. That said, I think everyone will support you, without bikeshedding, if you decide to change any of the names and/or deprecate ErrorT. Thanks, Yitz

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Proposal: add intersperse and intercalate to Data.Sequence
by David Feuer 27 Dec '14

27 Dec '14

It seems there's enough interest to make it official. The usual two-week period begins now. Intersperse can be given very pleasant performance characteristics for sequences: the result can constructed incrementally as it is consumed. Intercalate cannot perform so pleasantly, but it seems odd to add one but not the other.

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Resurrecting yarr
by Dominic Steinitz 27 Dec '14

27 Dec '14

I am trying to resurrect the yarr package (https://hackage.haskell.org/package/yarr) Thanks to all the very helpful folk in the Haskell community it now compiles under 7.8.3 and the tests run apart from > ~/Dropbox/Private/yarr/tests $ ./canny 1 50 100 tree.png > luminosity 8.41 ms, 5.33 tics per index > canny: Oops! Entered absent arg $dVector1{v a6vi} [lid] Vector{tc rrL} > (VecList{tc rrR} (S{tc ruZ} Z{tc ruI})) > (VecList{tc rrR} > (S{tc ruZ} > (S{tc ruZ} (S{tc ruZ} (S{tc ruZ} (S{tc ruZ} Z{tc ruI}))))) > (Word{(w) tc 32R} > -> Word{(w) tc 32R} -> IO{tc 32I} Word{(w) tc 32R})) > canny: thread blocked indefinitely in an MVar operation Now according to the notes left by the original developer, this used to work under ghc 7.6.1. What is this message telling me? Let me know if I should provide more information. Many thanks Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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GHC 7.10.x migration guide
by Herbert Valerio Riedel 27 Dec '14

27 Dec '14

Hello! I've started collecting the most common compile-errors (and their respective fixes) I have encountered while compiling Hackage packages with GHC 7.10 in https://ghc.haskell.org/trac/ghc/wiki/Migration/7.10 Feel free to contribute to this migration guide. HTH, hvr

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Fwd: Latest Template Haskell Breaks Package
by Dominic Steinitz 25 Dec '14

25 Dec '14

I realise I should have sent this to the libraries list. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com Begin forwarded message: > From: Dominic Steinitz <dominic(a)steinitz.org> > Subject: Latest Template Haskell Breaks Package > Date: 23 December 2014 13:14:26 GMT > To: Haskell-Cafe <haskell-cafe(a)haskell.org> > > Hello Fellow Haskellers, > > I have become a maintainer for yarr (https://hackage.haskell.org/package/yarr) This no longer compiles with ghc-7.8.3 because it specifies base == 4.6. Relaxing this to base >=4.6 && <4.8 tells me I need a newer version of Template Haskell > >> rejecting: template-haskell-2.7.0.0, 2.6.0.0, 2.5.0.0, 2.4.0.1, 2.4.0.0, >> 2.3.0.1, 2.3.0.0, 2.2.0.0 (conflict: yarr => template-haskell>=2.8 && <2.9) > > If I now relax the constraint for Template Haskell I get a compiler error as there has been a breaking change from Template Haskell 2.9 to 2.10. > >> Data/Yarr/Utils/FixedVector/VecTuple.hs:45:16: >> Couldn't match expected type ‘TypeQ -> Q Dec’ >> with actual type ‘Q Dec’ >> The function ‘tySynInstD’ is applied to three arguments, >> but its type ‘Name -> TySynEqnQ -> DecQ’ has only two > > And indeed looking at the changes in http://git.haskell.org/packages/template-haskell.git/commitdiff/ccd7891c536… I can see that the function in question has changed. > >> -tySynInstD :: Name -> [TypeQ] -> TypeQ -> DecQ >> -tySynInstD tc tys rhs = >> +tySynInstD :: Name -> [TySynEqnQ] -> DecQ >> +tySynInstD tc eqns = > > Did I miss some announcement of this breaking change and the advice on what to do about it? > > If I did can someone please point me at the relevant document. If not then I feel sad and would be very grateful if someone could help me as I know very little about Template Haskell. > > Many thanks > > Dominic Steinitz > dominic(a)steinitz.org > http://idontgetoutmuch.wordpress.com >

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Proposal: (a -> Bool) -> a -> f a
by Nikita Volkov 24 Dec '14

24 Dec '14

I propose to add the following utility function into Control.Applicative: cond :: (Alternative f) => (a -> Bool) -> a -> f a cond p a = if p a then pure a else empty Following is a typical use case: \x -> fromMaybe 0 $ cond (> 0) $ x - 10 which is the same as \x -> let y = x - 10 in if y > 0 then y else 0 Why the first one is better: 1. The control flow is evident and goes from right to left step by step. It’s not scattered around like in the second example. 2. No need to interrupt to imagine a name for a temporary variable. 3. Less noise. Now, since it’s generalised to Alternative one can imagine tons of other useful cases for this. Alternative titles: - conditional - partial Best regards, Nikita

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