May 2017 - Haskell-Cafe

Re: [Haskell-cafe] Foldable for (,)
by Olaf Klinke 04 May '17

04 May '17

Look at the Stream functor in Data.Stream [1]. Below I will argue by examples that while both Foldable and Traversable instances can be defined, the Foldable instance is less likely to produce terminating functions. Streams can be given a Traversable instance easily: instance Traversable Stream where sequenceA (Cons fa fas) = liftA2 Cons fa (sequenceA fas) For some applicatives this will not terminate. For streams of Maybes, sequenceA will either yield Just an infinite stream or return Nothing. Streams of IO actions also seem fine. A possible Foldable instance would suffer the same termination issues due to recursion. But with a monoid, folds over a stream will terminate less often, e.g. instance Foldable Stream where foldMap f (Cons x xs) = mappend (f x) (foldMap f xs) Then, all (xs :: Stream Bool) will diverge for the constant True stream and terminate for all others. -- Olaf [1] http://hackage.haskell.org/package/Stream-0.4.7.2/docs/Data-Stream.html

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Haskell type declaration summary
by Andrew Butterfield 04 May '17

04 May '17

Dear Haskellers, any quick and easy way to extract all the type declarations from a Haskell source file? (for easy reference) Regards, Andrew Butterfield School of Computer Science & Statistics Trinity College Dublin 2, Ireland

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Re: [Haskell-cafe] Haskell type declaration summary
by Andrew Butterfield 04 May '17

04 May '17

Hi Kevin, yes - I can do that, but it is tedious to issue each command, and I may not remember what all the types are called (I know that :browse will tell me what things exist in the current module with a capital letter) Also I want to be able to do this for a file different to the one currently loaded in GHCi. Something along the lines of (assuming a unix shell): $> showtypes MyBigMonsterHaskellFile.hs data X = A | B type Name = String newtype Y = Z Name .... $> (maybe with a blank line for each one..) Has no-one rustled up something like this - or do I need to look into Haskell parsing...? Cheers, Andrew > On 4 May 2017, at 16:30, Kevin Quick <quick(a)sparq.org> wrote: > > > You can use ghci's browse: > > $ ghci > Prelude> import Data.Maybe > Prelude Data.Maybe> :browse Data.Maybe > ... > Prelude Data.Maybe> :l MyFile.hs > MyFile Data.Maybe> :browse MyFile > ... > > Andrew Butterfield writes: > >> Dear Haskellers, >> any quick and easy way to extract all the type declarations from a Haskell source file? >> (for easy reference) >> >> Regards, >> >> Andrew Butterfield >> School of Computer Science & Statistics >> Trinity College >> Dublin 2, Ireland >> >> _______________________________________________ >> 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. > Andrew Butterfield School of Computer Science & Statistics Trinity College Dublin 2, Ireland

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How should I view this file? It looks like literate Haskell but isn't that file type.
by Jeffrey Brown 04 May '17

04 May '17

I am looking at Text.Markdown. The Types.hs file[1] looks like it's got comments that are meant to be viewed as some form other than plain text -- for instance, , msAddHeadingId :: Bool -- ^ If @True@, an @id@ attribute is added to the heading tag with the value equal to -- the text with only valid CSS identifier characters. -- -- > ## Executive Summary -- -- > <h2 id="executive-summary">Executive Summary</h2> -- -- Default: @False@ -- -- Since 0.1.13 I thought it might be literate Haskell, but the file ends in .hs, not .lhs. How should I view it? Preferably in Emacs? [1] https://github.com/snoyberg/markdown/blob/master/Text/Markdown/Types.hs -- 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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customize ghci error msg
by Johannes Waldmann 04 May '17

04 May '17

Dear Cafe. I have a "stack repl" which loads 841 modules in ghci. I have to do :set prompt "> " because the default prompt shows all modules loaded, and I never see a value or an error message without an enourmous amount of scrolling. But, even with this prompt - when I get some error on ":r", ghci says "... Failed, modules loaded: ..." and again I have to scroll back five miles. How could I change this message? - Thanks, J.W. https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#g…

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Re: [Haskell-cafe] [Biohaskell] hash map/associative structure
by Ketil Malde 04 May '17

04 May '17

> I know it may be a long shot, but did you consider using columnar data store like Apache Arrow? Arrow might be an option, but is there a Haskell interface? (Googling gives the obvious hits regarding arrows, and Google doesn't seem to care about me adding +apache to the search, it gives me result where "+apache" is overstruck.) > Without knowing more about your application it is a bit difficult to produce more hints. > What is your application? The short story is that I extract a number of 64-bit values from my data, and want to maintain frequency counts for each unique value. So there'll be on the order of 10^9 (plus/minus an order of magnitude) unique values, with counts ranging from one to a few million (and large values being rare). The long explanation is that I'm doing k-mer counts for molecular sequences, breaking DNA sequence data into overlapping words of fixed size (the parameter k), and counting their occurrences. I encode them as Word64, using two bits per nucleotide (the alphabet is A, C, G, and T). This is of course a fairly staple thing to do, and there is no lack of alternative programs that do it - but I'd like mine to work anyway, and it annoys me to have run into this particular bug. Whether it is my own fault, in the Judy FFI, the GHC runtime or libraries, the libjudy code, GHC compilation issues, or a hardware error. -k -- If I haven't seen further, it is by standing in the footprints of giants

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Foldable for (,)
by Jonathon Delgado 04 May '17

04 May '17

I sent the following post to the Beginners list a couple of weeks ago (which failed to furnish an actual concrete example that answered the question). Upon request I'm reposting it to Café: I've seen many threads, including the one going on now, about why we need to have: length (2,3) = 1 product (2,3) = 3 sum (2,3) = 3 or (True,False) = False but the justifications all go over my head. Is there a beginner-friendly explanation for why such seemingly unintuitive operations should be allowed by default?

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Typing Haskell in Haskell experience
by Christopher Done 03 May '17

03 May '17

I’ve been working through Typing Haskell in Haskell in detail ( https://gist.github.com/chrisdone/0075a16b32bfd4f62b7b) and I noticed that if I change e.g. SourcePrelude.hs from the Hackage package ( http://hackage.haskell.org/package/thih) to have a declaration like this: [("printZ", Just (Forall [] ([] :=> (tString))), [([], ap [econst showMfun, econst (("Z" :>: toScheme (TCon (Tycon "Nat" Star))))])])], In other words printChar = show (Z :: Nat) Where Z is neither defined nor an instance of Show, it type checks just fine. If I replace it with e.g. toScheme (TVar(Tyvar "k" Star)) i.e. “some thing of type k“, then it complains that the instance is ambiguous, which is true. So it seems that THIH will accept any type as an instance of any class without complaint as long as all the type variables have been filled in with the correct kind. This came as a surprise to me, as I’m using it in a project. There is apparently code in the type checker that pays attention to the class environment, and you provide instances to it, but it doesn’t seem to care what instances are provided or not. I believe I’ll have to do some extra work to check that the instances actually exist and are full resolved. I can’t tell whether this is a bug or intended behavior. I tried emailing the author of THIH but his email bounced back. Frankly the actual interesting bit of the type checker — the bit where we resolve class instances — is actually the hardest bit for me to read and the bit I’d like more exposition on. I’ve written a tokenizer, parser, renamer and thanks to THIH an inferer, and a stepper in a small dialect of Haskell I’m calling Duet. There’s an example here: http://chrisdone.com/toys/duet-gamma/ You can see the problem illustrated if you type e.g. the following in the top-left: data Nat = Z | S Natmain = show Z The type inferred is: main = ((show :: (Show Nat) => Nat -> String) (Z :: Nat) :: (Show Nat) => String) Which is at least enough information for me to do a lookup and insert an instance dictionary. But I sort of would have expected the type checker to say “no instance”. It does explicitly say that you have to do your own dependency analysis and renamer, but the instance part isn’t clear to me. Oh, and you also have to write your own kind inference for data type declarations. Who knew implementing a Haskell was so involved? So beware future travelers… THIH is awesome, but these are the things I’ve found missing. Ciao!

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ANNOUNCE: graphviz 2999.19.0.0
by Ivan Lazar Miljenovic 03 May '17

03 May '17

I've just released a new version of my Haskell bindings to the Graphviz graph visualisation toolkit. Note that the major version bump is solely because I had to remove what I thought was a useful feature (trying to allow you to do things like `"a" --> ["b", "c", "d"]` in Monadic graphs by automagically converting the "a" into ["a"]) turned out not to work in practice (as people will typically use this with literal values, but when combined with OverloadedStrings results in GHC getting confused as to what the type is). So unless you actually managed to use this functionality, then the API remains the same as 2999.18.* and you should just be able to bump your upper versions in your .cabal files (which you all do, right)? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Cabal and IPv6
by Serguey Zefirov 02 May '17

02 May '17

I have found that Cabal does not work on IPv6-only networks (which my machine happen to be part of). The problem is basically this: "ping hackage.haskell.org" does not resolve host while "ping6 hackage.haskell.org" does. Thus cabal cannot do anything useful on that machine. Do anyone have a solution for that? I was unable to find any on the internet. ;)

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