December 2015 - Haskell-Cafe

Re: [Haskell-cafe] Formatting strict Text
by fr33domlover 24 Dec '15

24 Dec '15

On Thu, 24 Dec 2015 00:13:34 +0200 fr33domlover <fr33domlover(a)riseup.net> wrote: > Hello, > > I have code which uses Text.Printf.printf with String. Now I'm moving the code > to use strict Text everywhere (I picked strict since all my strings are very > short, mostly less than 400 chars). What should I use instead of printf now? > > One solution I found is the text-format package, which seems to simply split a > Text on occurences of "{}" and insert values there. Are there other simple and > efficient options? All I need is to append Text values and sometimes Ints too, > so it's enough to have > > (1) efficient append and concat > (2) A function that is :: Int -> Text > > Why is strict Text not a Monoid instance? And Builder has no `toStrictText` > function? There is a function to convert Int to Builder and a Buildable > instance for Int, but for a simple Int->Text it seems I'm left with an ugly > `T.pack . show`. > > > Suggestions / advice / comments on this? > > Thanks in advance :) > > fr33 Since Text does have a Monoid instance after all, another question: The docs say 'append' is subject to fusion, but it isn't said about 'concat'. When I need to append several texts together, does it matter whether I just use <> between then or I 'concat' a list of them? Any one of them recommended / more efficient? I don't have any performance issues in my program, I just want to create good habits now and not get used to bad ones :P --fr33

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Re: [Haskell-cafe] Formatting strict Text
by Adam Bergmark 24 Dec '15

24 Dec '15

Data.Text definitely has a Monoid instance, it's defined in Data.Text. - Adam On Wed, Dec 23, 2015 at 11:13 PM, fr33domlover <fr33domlover(a)riseup.net> wrote: > Hello, > > I have code which uses Text.Printf.printf with String. Now I'm moving the > code > to use strict Text everywhere (I picked strict since all my strings are > very > short, mostly less than 400 chars). What should I use instead of printf > now? > > One solution I found is the text-format package, which seems to simply > split a > Text on occurences of "{}" and insert values there. Are there other simple > and > efficient options? All I need is to append Text values and sometimes Ints > too, > so it's enough to have > > (1) efficient append and concat > (2) A function that is :: Int -> Text > > Why is strict Text not a Monoid instance? And Builder has no `toStrictText` > function? There is a function to convert Int to Builder and a Buildable > instance for Int, but for a simple Int->Text it seems I'm left with an ugly > `T.pack . show`. > > > Suggestions / advice / comments on this? > > Thanks in advance :) > > fr33 > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >

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Formatting strict Text
by fr33domlover 23 Dec '15

23 Dec '15

Hello, I have code which uses Text.Printf.printf with String. Now I'm moving the code to use strict Text everywhere (I picked strict since all my strings are very short, mostly less than 400 chars). What should I use instead of printf now? One solution I found is the text-format package, which seems to simply split a Text on occurences of "{}" and insert values there. Are there other simple and efficient options? All I need is to append Text values and sometimes Ints too, so it's enough to have (1) efficient append and concat (2) A function that is :: Int -> Text Why is strict Text not a Monoid instance? And Builder has no `toStrictText` function? There is a function to convert Int to Builder and a Buildable instance for Int, but for a simple Int->Text it seems I'm left with an ugly `T.pack . show`. Suggestions / advice / comments on this? Thanks in advance :) fr33

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wiki account creation
by Artúr Poór 23 Dec '15

23 Dec '15

Hello, I would like to create a new wiki account. The automatic account creation has been disabled, and requests should be sent to domain(a)haskell.org. However, my email has been returned as undelivered. Could someone help with the account creation? Artúr

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NoMonomorphismRestriction forces odd situation when using contexts
by Noon Silk 23 Dec '15

23 Dec '15

Hello, Consider the following program: ------------------------------------------------------------------- -- bar.hs {-# LANGUAGE NoMonomorphismRestriction #-} data Status = Foo | Bar data Rec m a = Rec { get :: m a , status :: Status } defRec :: (Monad m) => Rec m a defRec = undefined myRec :: (Monad m) => Rec m a myRec = Rec x y where Rec x y = defRec ------------------------------------------------------------------- It doesn't compile (under GHC 7.10.2), the error is: bar.hs:16:7: No instance for (Monad t0) The type variable ‘t0’ is ambiguous When checking that ‘y’ has the inferred type y :: Status Probable cause: the inferred type is ambiguous In an equation for ‘myRec’: myRec = Rec x y where Rec x y = defRec Failed, modules loaded: none. If you remove the language extension, it does. Keeping the NoMonomorphismRestriction, it can be forced to typecheck by putting in a "dummy" type that satifies the constraint when obtaining the field `status` (that doesn't depend on `m`. ------------------------------------------------------------------- -- bar.hs {-# LANGUAGE NoMonomorphismRestriction #-} data Status = Foo | Bar data Rec m a = Rec { get :: m a , status :: Status } defRec :: (Monad m) => Rec m a defRec = undefined myRec :: (Monad m) => Rec m a myRec = Rec x y where s :: Rec Maybe a -> Status s = status x = get defRec -- y = s defRec ------------------------------------------------------------------- So, what's going on here? Is this a feature of this extension? It seems like the ability to destructure and then recombine is pretty fundamental, and it shouldn't break in the way that it does. (I.e. it's kinda crazy to have to put in a dummy type satisfying the constraint; is there some way to pass down the error -- Noon Silk, ن https://silky.github.io/ "Every morning when I wake up, I experience an exquisite joy — the joy of being this signature."

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Lazy evaluation illustrated for Haskell divers
by Takenobu Tani 22 Dec '15

22 Dec '15

Dear cafe, I'm studying and enjoying for Haskell. I'm drawing some illustrations about lazy evaluation in Haskell. Of course, I know a lot of pretty good documents. My document is one of the starter-kits to them. Here is: Lazy evaluation illustrated for Haskellers http://takenobu-hs.github.io/downloads/haskell_lazy_evaluation.pdf https://github.com/takenobu-hs/lazy_evaluation If there are misunderstandings, please teach me. I'll correct them. I apology if my responses is late. This is my night work (fun) =) I wish you a happy new year. Enjoy, lazy!, Takenobu

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Data.Map docs missing on hackage
by Andrew Butterfield 22 Dec '15

22 Dec '15

Is there a problem on hackage ?- when I try to look up Data.Map (via Hoogle) I get an error: ------------------- Not Found There is no documentation for containers-0.5.7.1. See https://hackage.haskell.org/package/containers/docs/Data-Map.html for the latest version. ---------------- Going to that link gives the exact same error Andrew Butterfield School of Computer Science & Statistics Trinity College Dublin 2, Ireland

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Re: [Haskell-cafe] Q: language-c upstream repo (fwd)
by Ivan Zakharyaschev 22 Dec '15

22 Dec '15

---------- Forwarded message ---------- Date: Tue, 22 Dec 2015 05:59:17 +0000 From: Benedikt Huber <benedikt.huber(a)gmail.com> To: Ivan Zakharyaschev <imz(a)altlinux.org>, haskell-cafe(a)haskell.org Subject: Re: Q: language-c upstream repo Hi Ivan, the hub.darcs.net repository is a mirror of http://code.haskell.org/language-c (the official upstream) that was created to simplify contributing. Conversion to darcs-2 was not straightforward IIRC, and the update of the repository did not work out.That's why code.haskell.org is still darcs-1. Best, Benedikt Ivan Zakharyaschev <imz(a)altlinux.org> schrieb am So., 20. Dez. 2015 um 16:32 Uhr: Hello! I've been interested in hacking language-c library a bit for my needs. (If it turns out useful and successful, I'll tell about it.) And I've cloned the repo listed at https://hackage.haskell.org/package/language-c-0.4.7 , namely: http://code.haskell.org/language-c Apart from my special hacks, I've also fixed some typos and so on. But then I discovered that if I want to publish them on hub.darcs.net , I need a darcs-2 repo, and the initial one which I used for cloning is a darcs-1 repo. I could do the conversion to darcs-2, but I read that it is not reproducible: run several times on the same set of patches it will give different results. As I've discovered also a language-c repo at hub.darcs.net which looks like an upstream repo: http://hub.darcs.net/visq/language-c , I'm in doubt whether I should continue making my patches against the darcs-1 repo which is said to be upstream at hackage or against http://hub.darcs.net/visq/language-c whichis not officially declared as the upstream but looks more fresh. ..if I want to be able to send some of my patches upstream. Best regards, Ivan

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[ANNOUNCE] sbvPlugin: Run SMT solvers over Haskell "theorems"
by Levent Erkok 22 Dec '15

22 Dec '15

Hello all, I'm happy to announce the first public release of sbvPlugin, a new GHC-core plugin based on the SBV library: http://hackage.haskell.org/package/sbvPlugin The plugin can be used to prove (or refute!) theorems in your Haskell code, using the SBV library as its backend. Here's a most simple example: {-# OPTIONS_GHC -fplugin=Data.SBV.Plugin #-} module Test where import Data.SBV.Plugin {-# ANN test theorem #-} test :: Integer -> Integer -> Bool test x y = x + y >= x - y When compiled (or loaded into ghci), you'll see that the plugin will attempt to prove the given theorem, and will produce a counter-example since it is not valid! (This assumes, you've installed an SBV-supported SMT solver. Z3 from Microsoft is a good choice: http://github.com/Z3Prover/z3) <http://github.com/Z3Prover/z3> The plugin is still in its early days and the examples are rather sparse. But see the following for one worked out instance: (Thanks to Anthony Cowley for the original idea.) http://hackage.haskell.org/package/sbvPlugin/docs/Data-SBV-Plugin-Examples-… The sbvPlugin takes advantage of the SBV library ( http://hackage.haskell.org/package/sbv) to directly reason about Haskell programs at GHC's core level. However, the plugin is *not* a replacement for SBV. It only exposes a few features of SBV directly at the Haskell level; which hopefully makes it useful and more friendly than SBV; but it is neither intended nor possible for it to replace the SBV library itself. In particular, the plugin is limited to certain types (mostly numeric), and a few composite ones (lists and tuples.) If the plugin does not "understand" a Haskell type/expression, it'll use a technique called "uninterpretation" to black-box it. In those cases, the counter-examples produced might be bogus, though proofs will still be valid. Note that any definitions that are outside of the module being compiled will also go uninterpreted, including all the Prelude functions. The plugin is still in its early days, and there are a number of rough edges. Any feedback is most welcome! Happy hacking, -Levent.

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Re: [Haskell-cafe] More disciplined alternative to rewrite rules?
by Johannes Waldmann 21 Dec '15

21 Dec '15

What graph(s) do you want to rewrite? How would this help with respect to A, B, and C? The program is a term (tree), the runtime data structure (heap) is a graph - you want rewriting at run-time then? But the purpose is to move work from run-time to compile-time. - J.W.

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