Re: [Haskell-cafe] reasons why Template Haskell does not propose something similar to Python exec() or eval()
IIRC you can use haskell-src-exts to parse a string into TH AST structures.
There might even be a quasi-quoter for that; I don't have a real computer
at hand right more, so you'll need to do some research of your own.
On Aug 24, 2013 11:37 AM, TP paratribulati...@free.fr wrote:
Hi everybody,
I continue to learn and test Template Haskell (one more time thanks to John
Lato for his post at:
http://www.mail-archive.com/haskell-cafe@haskell.org/msg106696.html
that made me understand a lot of things).
I have a question about the way Template Haskell is working. Why Template
Haskell does not propose something similar to Python (or bash) exec() or
eval(), i.e. does not offer the possibility to take a (quoted) string in
input, to make abstract syntax in output (to be executed later in a splice
$()).
For example, in Python, to make an affectation 'a=a' programatically, I
can simply do (at runtime; even if I am here only concerned with what
Template Haskell could do, i.e. at compile time):
def f(s): return '%s = \'%s\'' % (s,s)
exec(f(a))
a
'a'
With Template Haskell, I am compelled to make a function returning the
abstract syntax corresponding to variable declaration:
ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s)
(see complete example in Post Scriptum).
This works fine, but it is less direct. I am sure that the Template Haskell
approach has many advantages, but I am unable to list them. I think it is
important to have the reasons in mind. Could you help me?
Thanks in advance,
TP
PS: the complete Haskell example:
---
module MakeVard where
import Language.Haskell.TH
makeVard :: Monad m = String - m [Dec]
-- Equivalent to %s = \%s\
makeVard s = return [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s)
[]
]
---
tested by
---
{-# LANGUAGE TemplateHaskell #-}
import MakeVard
$(makeVard a)
main = do
print a
---
resulting in
$ runghc -ddump-splices test.hs
test_makeVar.hs:1:1: Splicing declarations
makeVard a
==
test_makeVar.hs:4:3-14
a = a
a
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Re: [Haskell-cafe] Errors in non-monadic code
Except that people generally don't seem to agree what constitutes 'exceptional', even when disregarding the python world... On Aug 19, 2013 9:24 PM, Brandon Allbery allber...@gmail.com wrote: On Mon, Aug 19, 2013 at 2:59 PM, jabolo...@google.com wrote: I'd say that if you were in the context of the IO monad, maybe you'd prefer to use exceptions instead of 'Either' or 'Maybe'. Even in IO, exceptions should be reserved for truly exceptional conditions (of the program cannot safely continue variety), not merely for error checking when this can be described as a normal flow of evaluation. Exceptions are not simply alternative flow of control, even in procedural languages; they are *disruptions* of flow of control. -- brandon s allbery kf8nh sine nomine associates allber...@gmail.com ballb...@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] inv f g = f . g . f
Note that at least for the dropWhile example, there is a specialized function, dropWhileEnd, which is most likely more efficient than reversing the list twice. On Aug 17, 2013 3:35 PM, Tom Ellis tom-lists-haskell-cafe-2...@jaguarpaw.co.uk wrote: On Sat, Aug 17, 2013 at 11:11:07AM +0200, Christopher Done wrote: Anyone ever needed this? Me and John Wiegley were discussing a decent name for it, John suggested inv as in involution. E.g. inv reverse (take 10) inv reverse (dropWhile isDigit) trim = inv reverse (dropWhile isSpace) . dropWhile isSpace This sounds like a job for a lens, or similar. Tom ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Alternative name for return
It is a pure value in the context of the outer monad (the one you wrap it in). I'd say pure is still appropriate. On Aug 6, 2013 10:14 AM, Tom Ellis tom-lists-haskell-cafe-2...@jaguarpaw.co.uk wrote: On Tue, Aug 06, 2013 at 10:03:04AM +0200, J. Stutterheim wrote: `putStrLn Hi` is not a pure value... Why not? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Open Source project suitable for 2-3 persons this fall?
There's one of my hobby projects that could use some manpower to bootstrap it into a production-quality tool. It's an HTML templating system; right now it reads input templates in one of two languages (one very declarative, very similar to Python's jinja2, the other being a locally-pure functional language with a curly-bracket syntax), and it can output PHP and JavaScript as well as execute templates directly in Haskell code. There's also a rather experimental JSON-based intermediate format, designed for high-security scenarios (JSON can be parsed quickly, but because it cannot contain arbitrary code, a compromised compiler can be mitigated by whitelisting what the runtime can do; this way, only the runtime has to be audited, not the compiler). There is an incomplete implementation of a PHP library that can interpret this intermediate JSON format, but it hasn't been used in any production-quality project yet. The core part is the compiler, consisting of a Parsec-based parser, an AST, an optimizer, and a few backends. The unique selling points: * Pre-compile your templates - now you can have the cake (expressive clean template language) and eat it too (great performance, because it's pre-compiled to your web app's native language). * Use the same template for PHP, Haskell and JavaScript (and maybe others, too). * Bye bye XSS: HTML-encoding is the default. * Extensible: Hook up your own native functions - it's as simple as adding a function to your template input data, then calling it inside the template. It could use some work in a lot of areas; projects could include, for example: - Getting the JavaScript backend to support 100% of the language features (right now, only the direct-execution and PHP backends pass all tests) - Implement some glue to allow easy embedding in Haskell web applications (yesod / happstack / snap) - Add more features to the input language (some of which would also require some changes to the backends), e.g.: - range literals - list comprehensions - indexed for-each, or some way of getting the current iteration's index; very useful for things like zebra-stripe data grids and such - Add other backends (Python and Ruby would be the most obvious candidates, but I'm really open to anything) - Add more library functions - A better optimizer - Some tooling and integration support, e.g. some kind of mechanism to switch between executing on-the-fly (for development) and using pre-compiled templates (for production), something to implement graceful degrading on the client side (use client-side templates when possible, fall back to postbacks and server-side templates as needed), syntax highlighters for popular editors, etc. Anyway, here's the source: https://bitbucket.org/tdammers/hpaco The PHP lib/script that can produce and run a subset of the JSON intermediate format is here: https://bitbucket.org/tdammers/phpaco (Any contributions, feedback, etc., very welcome) On Thu, Jul 04, 2013 at 05:26:59PM +0200, Anders Bech Mellson wrote: Is there any project that needs working this fall which could be used as a university project? I am in the university (M.Sc. in software development), so I am mainly looking for project ideas (preferably concrete ones). We are 2-3 students that have ~10 hours pr week for 3 months to work on a project. Is there a listing somewhere with project ideas for contributing to the Haskell community? Thanks in advance, Anders ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] tangential request...
On Sun, Jun 23, 2013 at 09:18:18PM -0700, Mark Lentczner wrote: Thanks all, I’ve got what I needed. Brief results; Big variety in window and text sizes, but very few font and color choices. More than half the terminals seem to be basically default settings. Well, there's only so many monospace fonts that are beautiful, reasonably unicode-complete, easy on the eyes, programming-friendly AND free (-ish). Also, the default for the most popular terminal emulators on Linux (e.g. xterm, rxvt) is actually a bitmap font. Finally, 15% seem to be using horrid bitmap console fonts. _How can you stand to look at them?!?!_ (Don't worry, you'll have Plush soon enough...) Controversial topic there, tread carefully ;) . Lots of folks are borderline militant when it comes to their terminal fonts, whether they're bitmapped or not. Also, white-on-black vs. black-on-white seems to be an emotionally-charged question. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] tangential request...
On Mon, Jun 24, 2013 at 04:09:22PM +0100, Tom Ellis wrote: On Mon, Jun 24, 2013 at 08:02:17AM -0700, Mark Lentczner wrote: And yet, just four fonts make up over 75% of the sample - and two of those are essentially identical! Inconsolata and Consolas? My bet: - Bitstream Vera Sans Mono - DejaVu Sans Mono - Inconsolata - Whatever the default terminal font is on OS X ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] loop error message
On Sat, Jun 22, 2013 at 06:09:58AM +0300, Mihai Maruseac wrote: On Sat, Jun 22, 2013 at 12:41 AM, Omari Norman om...@smileystation.com wrote: I compiled some code with GHC 7.6.3 that produces a simple error at runtime myProgramName: loop At which point the program exits with code 1. Is there documentation for this error anywhere? Does it mean I have some infinite loop in my code somewhere? If so, does GHC catch all infinite loops? I have never gotten this error before. Thanks. Hi, Indeed, you have an infinite loop in your code. However, not all infinite loops are catcheable by the mechanism employed there, it would be very hard to do so. Some are even impossible to catch at all, a.k.a. the Halting Problem. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] data constructor names
On Sat, Jun 22, 2013 at 12:15:07PM +0100, Tom Ellis wrote: On Sat, Jun 22, 2013 at 04:26:14AM -0500, Brian Lewis wrote: Say you write data Callback = Error ... | ... [...] Then, later, you write data Error = ... [...] They're both good names, but there's a conflict. What do you mean by a conflict? That's fine as far as the compiler is concerned because constructors live in a different namespace from types. I think he meant something like: data Callback = Error Error | Success data Error = Error String ...in which case there is a clash for the identifier Error at the value level. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Nomyx 0.2 beta, the game where you can change the rules
On Wed, Jun 19, 2013 at 01:18:54PM +0200, Corentin Dupont wrote: Thanks Brent! I'm glad you like it. You will win a lot of money if new players come :) Indeed Brent proposed a rule that allows sponsorship: if you invite a player in, you win 50 ECU... Joined a game, though I don't really have time to look into it right now. One thing though: It appears that the game sends multipart/alternative e-mail messages, where the text/plain part is completely empty (0 bytes). Since I've configured my mail client to prefer text/plain over text/html, this is kind of inconvenient. Otherwise, good job. I'm curious where this is going. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Automating Hackage accounts
On Thu, Jun 13, 2013 at 09:44:03AM -0400, Andrew Pennebaker wrote: Could we add an HTML form for creating new Hackage accounts? Right now, our community is small enough that emailing r...@soi.city.ac.uk and waiting for a manual response isn't too bad of a problem, but as we grow, it would be nice for these sorts of things to be handled by a server, like with RubyGems and NPM. IMHO, a more pressing issue is SSL uploads and package signing. As it stands, anyone with a Hackage account can upload a new version of any given package, and some wire-sniffing is enough to reveal a legit user's password. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Automating Hackage accounts
On Thu, Jun 13, 2013 at 05:07:38PM +0300, Mihai Maruseac wrote: On Thu, Jun 13, 2013 at 5:02 PM, Tobias Dammers tdamm...@gmail.com wrote: On Thu, Jun 13, 2013 at 09:44:03AM -0400, Andrew Pennebaker wrote: Could we add an HTML form for creating new Hackage accounts? Right now, our community is small enough that emailing r...@soi.city.ac.uk and waiting for a manual response isn't too bad of a problem, but as we grow, it would be nice for these sorts of things to be handled by a server, like with RubyGems and NPM. IMHO, a more pressing issue is SSL uploads and package signing. As it stands, anyone with a Hackage account can upload a new version of any given package, and some wire-sniffing is enough to reveal a legit user's password. I'd try to solve the latest two things first before going into creating a specific form. On the other hand, maybe we can rig something up with Yesod or similar to solve all three points at the same time. I'm busy now with my masters disertation but I can attempt something in a month if it seems ok and no one else does it before that date. IIRC, there have been previous attempts, or at least a discussion. I can't remember what the result was, though. Either way, it'll take more than just a Yesod web application built over a weekend; signed packages would require package authors to, well, sign, so cabal would need features for that; you'd also have to extend it to *check* those signatures, and give the user options to refuse or allow unsigned packages. SSL should be relatively simple though, mostly a matter of updating cabal's configuration and installing a suitable certificate on the hackage server. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] (no subject)
On Mon, Jun 10, 2013 at 05:41:05PM +0530, Zed Becker wrote: Haskell, is arguably the best example of a design-by-committee language. You do realize that design-by-committee is generally understood to refer to the antipattern where a committee discusses a design to death and delivers an inconsistent, mediocre spec, as opposed to a situation where a leader figure takes the loose ends, runs with them, and turns them into a coherent, inspiring whole? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] whats with cabal and libgmp.so.3
On Tue, May 14, 2013 at 01:16:44PM +0530, Rustom Mody wrote: Today cabal suddenly started giving me errors that libgmp.so.3 is not found Moving away my old .cabal makes it work again Any explanations? [I am on debian testing. I think the causing factor was that debian switched major versions recently. There were a lot of updates... Nothing that looked like it was related to ghc] That could very well be the cause. My guess is that jessie has a different libgmp than wheezy. The stuff in your .cabal was linked against the version from wheezy, but now it's gone because of the dist upgrade (of which cabal is unaware); recompiling everything and linking against the new version fixes it. If you want to be sure, see if apt updated libgmp as part of the dist upgrade. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Robot - Simulate keyboard and mouse events under X11
On Thu, May 09, 2013 at 01:36:52PM +1200, Chris Wong wrote: On Tue, May 7, 2013 at 5:18 AM, Jeanne-Kamikaze jeannekamik...@gmail.com wrote: Looks like an interesting library. Will it be able to read pixels from a window at some point? Not sure -- I have no idea how screen capturing works in X11. Calling gnome-screenshot should probably cover most use cases. I just took 2 minutes looking at the source code for scrot (which is about as simplistic a screen grabber as you'll find), and it looks fairly simple: it uses giblib, which provides a function called gib_imlib_create_image_from_drawable, which does exactly that. Not sure if Haskell bindings for giblib exist already though. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Hackage checking maintainership of packages
On Mon, May 06, 2013 at 08:14:59PM +0800, Niklas Hambüchen wrote: On 06/05/13 20:06, Tillmann Rendel wrote: Is a human clicked the check box a good metric for a human commits themselves to this package? If the check box has the text Do you want this thing to be called 'maintained' on Hackage next to it, yes. The problem is that people tend to (truthfully) check such a box, then stop maintaining the package for whatever reasons, and never bother unchecking the box. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Backward compatibility
On Fri, May 03, 2013 at 01:23:55PM +0300, Guy wrote: That's what I thought of when I saw the original complaint - GHC is too backwards compatible! There's far too much boilerplate because of the Functor = Applicative = Monad mess. Float/Double being Enums is ridiculous. None of this gets fixed because of the fear that new GHC won't compile old code. PS The proposal to fix Functor = Applicative = Monad has patches attached for GHC and base, but the backwards compatibility bogeyman always seems to trump something that will break a lot of code. This kind of breaks everything changes would require something similar to what Python is doing with the 2 - 3 transition, and considering how painfully slowly it is progressing there, I understand perfectly well why people don't want to go there. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Backward compatibility
On Fri, May 03, 2013 at 03:35:08PM +0100, Ozgur Akgun wrote: Hi, On 3 May 2013 11:43, Tobias Dammers tdamm...@gmail.com wrote: PS The proposal to fix Functor = Applicative = Monad has patches attached for GHC and base, but the backwards compatibility bogeyman always seems to trump something that will break a lot of code. This kind of breaks everything changes would require something similar to what Python is doing with the 2 - 3 transition, and considering how painfully slowly it is progressing there, I understand perfectly well why people don't want to go there. There is one very big advantage in the Haskell-world though. Most of the struggle will be at the compile time. The biggest headache caused by the Python 2 - 3 transition is how you get a runtime error 2 weeks after you think you've fixed everything! (Yeah, I know code coverage analysis is an option when you don't have static type checking, but ...) Well; this is an additional problem that comes with the pragmatic choice for a language that doesn't believe in type checking and compile-time guarantees, but I think it is largely orthogonal. At least theoretically, you could have 100% test coverage, which would put the Python project in the same situation as the Haskell one (except that the tests themselves would also have to be converted...) The problem I'm talking about, though, is that such a change would split the ecosystem in two (say, Traditional Haskell and Functor-Correct Haskell), just like the 2/3 separation is all over the Python ecosystem and causes all sorts of headaches. Should I be using Python 2 or 3 is not a question that has a simple answer, and that *is* a problem. Cheers, Ozgur ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
