February 2015 - Haskell-Cafe

Anonymous FFI calls
by Francesco Mazzoli 12 Feb '15

12 Feb '15

Hi, I am in a situation where it would be very useful to call C functions without an explicit FFI import. For example, I'd like to be able to do (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 instead of declaring the foreign import explicitely at the top level. Is there a way to do this or to achieve similar results in some other way? If not, I imagine it would be easy to implement such a facility in GHC, given that the code implementing calling to C functions must already be present to implement "proper" FFI imports. I think such an addition would be useful in many cases. Thanks, Francesco

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GHC Hackaton (GHC internals) slides
by Maurizio Vitale 12 Feb '15

12 Feb '15

Anybody knows if the slides (or higher quality video) for the GHC Hackaton are available? I'm not sure which year, this video has been posted in 2012: https://www.youtube.com/watch?v=lw7kbUvAmK4&list=PLBkRCigjPwyeCSD_DFxpd246Y… Thanks! Maurizio

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Re: [Haskell-cafe] Anonymous FFI calls
by Michael Sloan 12 Feb '15

12 Feb '15

It works, cool! So at least this ought to address Francesco's use-case. Is there anything wrong with my code? It definitely looks like a GHC bug. Yes, I've successfully generated instance declarations from the top-level. I may as well describe the usecase: pretty printing a value of any type in GHCI. While this feature list isn't the main point here, it'd be very cool to do things like: * Look up the value of mutable references such as IORefs * Print out functions as just their type * Be able to evaluate particular subtrees of the pretty print output (like present [1]) * Select a subportion of the output and get a variable out of it Usage would look like "$(pretty 'x)". This would reify the type of x and transitively generate `Pretty` instances for things, by using my th-reify-many package [2]. It then generates an expression which invokes `pretty` on `x`, using these new instances. This instance generation can do clever things like check if a type has a "Data" instance, and use it if available. In this way, we can construct a pretty printer for ghci which will work for any type. Certainly, this is still possible without instance generation, but it'd be quite clever to not need to re-generate the pretty printing code for every invocation. -Michael [1] https://github.com/chrisdone/present [2] https://github.com/mgsloan/th-reify-many On Wed, Feb 11, 2015 at 6:51 PM, Geoffrey Mainland <mainland(a)cs.drexel.edu> wrote: > Hi Michael, > > Try this out: > > https://gist.github.com/mainland/f18cf3827c4b0b64b835 > > Have you had success generating instance declarations from top-level > splices? > > Geoff > > On 02/11/2015 07:41 PM, Michael Sloan wrote: >> Hi! >> >> I don't mean to be accusatory with the whole documentation thing - I >> totally understand that most folks have a lot going on! In this >> particular case, I got very excited when I initially saw addTopDecls, >> because it says it allows you to "Add additional top-level >> declarations". After implementing something to use it, I got rather >> disappointed as it couldn't add instances. >> >> So, when I say "not implemented", it's just that the docs say it >> allows you to add additional top-level decls, when you can really only >> add functions, variable bindings, and foreign import decls. >> >> I didn't realize that not re-exporting these functions is due to their >> experimental status. That makes sense! I'm glad it was a conscious >> decision. I just tried writing a demo of addTopDecls, but >> unfortunately using it to add a function and referencing this function >> causes a GHC internal error: >> https://gist.github.com/mgsloan/53d7fa50338c696e5c80 . I haven't >> tried it with a foreign import yet. >> >> -Michael >> >> On Wed, Feb 11, 2015 at 3:51 PM, Geoffrey Mainland >> <mainland(a)cs.drexel.edu> wrote: >>> Typed Template Haskell happened at the end of my tenure at MSR, and >>> there was a mad rush to get it in to the compiler in time for 7.8.3 at >>> the same time I was starting a new job. I'm afraid the documentation is >>> indeed wanting, but the alternative was no typed Template Haskell. >>> >>> One side-effect was that the functions you note as present only in >>> Language.Haskell.TH.Syntax were not thoroughly vetted, so we didn't >>> re-export them from Language.Haskell.TH. >>> >>> I am willing to help with documentation if people want to use this >>> functionality. Has anyone attempted to use addTopDecls? Michael, when >>> you say "it's not implemented," what do you mean? What, exactly, is not >>> implemented? >>> >>> Cheers, >>> Geoff >>> >>> On 2/11/15 6:32 PM, Michael Sloan wrote: >>>> I'd love for the implementation to be as powerful as the documentation >>>> suggests it is :D >>>> >>>> However, yes, in the meantime, fixing the documentation would be great! >>>> >>>> Also, I meant to say that addTopDecls is only exported by >>>> "Language.Haskell.TH.Syntax". While this is a digression, there are a >>>> few other handy functions that are oddly left out of >>>> "Language.Haskell.TH": addDependentFile, addModFinalizer, and possibly >>>> more. >>>> >>>> -Michael >>>> >>>> On Wed, Feb 11, 2015 at 3:25 PM, Simon Peyton Jones >>>> <simonpj(a)microsoft.com> wrote: >>>>> I would LOVE someone to improve the documentation for addTopDecls. Manuel Chakravarty and Geoff Mainland were responsible for the implementation. >>>>> >>>>> Simon >>>>> >>>>> | -----Original Message----- >>>>> | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Michael >>>>> | Sloan >>>>> | Sent: 11 February 2015 23:19 >>>>> | To: Francesco Mazzoli >>>>> | Cc: ghc-devs(a)haskell.org; haskell >>>>> | Subject: Re: [Haskell-cafe] Anonymous FFI calls >>>>> | >>>>> | It seems like addTopDecls[1] will able to help here. Unfortunately, >>>>> | the function is not well documented and not very discoverable because >>>>> | it's only exported by Language.Haskell.TH. >>>>> | >>>>> | The documentation doesn't mention that it can only be used to create >>>>> | new top level functions and FFI imports[2]. I think that adding FFI >>>>> | imports was the main motivation for implementing it. In the past >>>>> | I've wanted to generate instances via this function, but unfortunately >>>>> | it's not implemented.. >>>>> | >>>>> | Hope that helps! >>>>> | -Michael >>>>> | >>>>> | [1] http://hackage.haskell.org/package/template-haskell- >>>>> | 2.9.0.0/docs/Language-Haskell-TH-Syntax.html#v:addTopDecls >>>>> | >>>>> | [2] >>>>> | https://github.com/ghc/ghc/blob/1d982ba10f590828b78eba992e73315dee33f78a/ >>>>> | compiler/typecheck/TcSplice.hs#L818 >>>>> | >>>>> | On Wed, Feb 11, 2015 at 2:26 AM, Francesco Mazzoli <f(a)mazzo.li> wrote: >>>>> | > Hi, >>>>> | > >>>>> | > I am in a situation where it would be very useful to call C functions >>>>> | > without an explicit FFI import. For example, I'd like to be able to do >>>>> | > >>>>> | > (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 >>>>> | > >>>>> | > instead of declaring the foreign import explicitely at the top level. >>>>> | > >>>>> | > Is there a way to do this or to achieve similar results in some other >>>>> | > way? >>>>> | > >>>>> | > If not, I imagine it would be easy to implement such a facility in GHC, >>>>> | > given that the code implementing calling to C functions must already be >>>>> | > present to implement "proper" FFI imports. I think such an addition >>>>> | > would be useful in many cases. >>>>> | > >>>>> | > Thanks, >>>>> | > Francesco >>>>> | > _______________________________________________ >>>>> | > Haskell-Cafe mailing list >>>>> | > Haskell-Cafe(a)haskell.org >>>>> | > http://www.haskell.org/mailman/listinfo/haskell-cafe >>>>> | _______________________________________________ >>>>> | ghc-devs mailing list >>>>> | ghc-devs(a)haskell.org >>>>> | http://www.haskell.org/mailman/listinfo/ghc-devs >

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Re: [Haskell-cafe] Anonymous FFI calls
by Michael Sloan 12 Feb '15

12 Feb '15

Hi! I don't mean to be accusatory with the whole documentation thing - I totally understand that most folks have a lot going on! In this particular case, I got very excited when I initially saw addTopDecls, because it says it allows you to "Add additional top-level declarations". After implementing something to use it, I got rather disappointed as it couldn't add instances. So, when I say "not implemented", it's just that the docs say it allows you to add additional top-level decls, when you can really only add functions, variable bindings, and foreign import decls. I didn't realize that not re-exporting these functions is due to their experimental status. That makes sense! I'm glad it was a conscious decision. I just tried writing a demo of addTopDecls, but unfortunately using it to add a function and referencing this function causes a GHC internal error: https://gist.github.com/mgsloan/53d7fa50338c696e5c80 . I haven't tried it with a foreign import yet. -Michael On Wed, Feb 11, 2015 at 3:51 PM, Geoffrey Mainland <mainland(a)cs.drexel.edu> wrote: > Typed Template Haskell happened at the end of my tenure at MSR, and > there was a mad rush to get it in to the compiler in time for 7.8.3 at > the same time I was starting a new job. I'm afraid the documentation is > indeed wanting, but the alternative was no typed Template Haskell. > > One side-effect was that the functions you note as present only in > Language.Haskell.TH.Syntax were not thoroughly vetted, so we didn't > re-export them from Language.Haskell.TH. > > I am willing to help with documentation if people want to use this > functionality. Has anyone attempted to use addTopDecls? Michael, when > you say "it's not implemented," what do you mean? What, exactly, is not > implemented? > > Cheers, > Geoff > > On 2/11/15 6:32 PM, Michael Sloan wrote: >> I'd love for the implementation to be as powerful as the documentation >> suggests it is :D >> >> However, yes, in the meantime, fixing the documentation would be great! >> >> Also, I meant to say that addTopDecls is only exported by >> "Language.Haskell.TH.Syntax". While this is a digression, there are a >> few other handy functions that are oddly left out of >> "Language.Haskell.TH": addDependentFile, addModFinalizer, and possibly >> more. >> >> -Michael >> >> On Wed, Feb 11, 2015 at 3:25 PM, Simon Peyton Jones >> <simonpj(a)microsoft.com> wrote: >>> I would LOVE someone to improve the documentation for addTopDecls. Manuel Chakravarty and Geoff Mainland were responsible for the implementation. >>> >>> Simon >>> >>> | -----Original Message----- >>> | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Michael >>> | Sloan >>> | Sent: 11 February 2015 23:19 >>> | To: Francesco Mazzoli >>> | Cc: ghc-devs(a)haskell.org; haskell >>> | Subject: Re: [Haskell-cafe] Anonymous FFI calls >>> | >>> | It seems like addTopDecls[1] will able to help here. Unfortunately, >>> | the function is not well documented and not very discoverable because >>> | it's only exported by Language.Haskell.TH. >>> | >>> | The documentation doesn't mention that it can only be used to create >>> | new top level functions and FFI imports[2]. I think that adding FFI >>> | imports was the main motivation for implementing it. In the past >>> | I've wanted to generate instances via this function, but unfortunately >>> | it's not implemented.. >>> | >>> | Hope that helps! >>> | -Michael >>> | >>> | [1] http://hackage.haskell.org/package/template-haskell- >>> | 2.9.0.0/docs/Language-Haskell-TH-Syntax.html#v:addTopDecls >>> | >>> | [2] >>> | https://github.com/ghc/ghc/blob/1d982ba10f590828b78eba992e73315dee33f78a/ >>> | compiler/typecheck/TcSplice.hs#L818 >>> | >>> | On Wed, Feb 11, 2015 at 2:26 AM, Francesco Mazzoli <f(a)mazzo.li> wrote: >>> | > Hi, >>> | > >>> | > I am in a situation where it would be very useful to call C functions >>> | > without an explicit FFI import. For example, I'd like to be able to do >>> | > >>> | > (foreign import ccall "cadd" :: CInt -> CInt -> CInt) 1 2 >>> | > >>> | > instead of declaring the foreign import explicitely at the top level. >>> | > >>> | > Is there a way to do this or to achieve similar results in some other >>> | > way? >>> | > >>> | > If not, I imagine it would be easy to implement such a facility in GHC, >>> | > given that the code implementing calling to C functions must already be >>> | > present to implement "proper" FFI imports. I think such an addition >>> | > would be useful in many cases. >>> | > >>> | > Thanks, >>> | > Francesco >>> | > _______________________________________________ >>> | > Haskell-Cafe mailing list >>> | > Haskell-Cafe(a)haskell.org >>> | > http://www.haskell.org/mailman/listinfo/haskell-cafe >>> | _______________________________________________ >>> | ghc-devs mailing list >>> | ghc-devs(a)haskell.org >>> | http://www.haskell.org/mailman/listinfo/ghc-devs >

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Applying for Haskell opportunities at Facebook
by Zongzhe Yuan 11 Feb '15

11 Feb '15

Hi I am a student in the University of Nottingham in year 2 and I am quite interested in Haskell. May I apply for the summer internship in this summer in London? I have learnt Haskell during the module Introduction to Functional Programming last Year and have the highest mark among the student from Ningbo campus. I could solve countdown problem and sudoku right now. I know Monad and know the basic idea about that. This semester I take the module Advanced Functional Programming taught by professor Graham Hutton. By the end of this semester, i will be good at monad using and learn more about the efficiency about the programming. Apart from Haskell, I also good at algorithm and Java programming. I deal the entire image processing procedure algorithm with a candy detect application. https://github.com/ZongzheYuan/ImageProcessing.git Here is my code and i am improving it continuously. I really looking forward to working at FaceBook and especially with Haskell. If i could have this opportunity, it will become my pleasure. Greeting Zongzhe Yuan 11/2/2015 This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Applying for Haskell opportunities at Facebook
by Zongzhe Yuan 11 Feb '15

11 Feb '15

Hi I am a student in the University of Nottingham in year 2 and I am quite interested in Haskell. May I apply for the summer internship in this summer in London? I have learnt Haskell during the module Introduction to Functional Programming last Year and have the highest mark among the student from Ningbo campus. I could solve countdown problem and sudoku right now. I know Monad and know the basic idea about that. This semester I take the module Advanced Functional Programming taught by professor Graham Hutton. By the end of this semester, i will be good at monad using and learn more about the efficiency about the programming. Apart from Haskell, I also good at algorithm and Java programming. I deal the entire image processing procedure algorithm with a candy detect application. https://github.com/ZongzheYuan/ImageProcessing.git Here is my code and i am improving it continuously. I really looking forward to working at FaceBook and especially with Haskell. If i could have this opportunity, it will become my pleasure. Greeting Zongzhe Yuan 11/2/2015 This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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Applying for Haskell opportunities at Facebook
by Zongzhe Yuan 11 Feb '15

11 Feb '15

Hi I am a student in the University of Nottingham in year 2 and I am quite interested in Haskell. May I apply for the summer internship in this summer in London? I have learnt Haskell during the module Introduction to Functional Programming last Year and have the highest mark among the student from Ningbo campus. I could solve countdown problem and sudoku right now. I know Monad and know the basic idea about that. This semester I take the module Advanced Functional Programming taught by professor Graham Hutton. By the end of this semester, i will be good at monad using and learn more about the efficiency about the programming. Apart from Haskell, I also good at algorithm and Java programming. I deal the entire image processing procedure algorithm with a candy detect application. https://github.com/ZongzheYuan/ImageProcessing.git Here is my code and i am improving it continuously. I really looking forward to working at FaceBook and especially with Haskell. If i could have this opportunity, it will become my pleasure. Greeting Zongzhe Yuan 11/2/2015 This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please send it back to me, and immediately delete it. Please do not use, copy or disclose the information contained in this message or in any attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. This message has been checked for viruses but the contents of an attachment may still contain software viruses which could damage your computer system, you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.

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svgcairo/ghc vis cabal install error..
by madhub 11 Feb '15

11 Feb '15

Hi !, I tried installing svgcairo ( for installing ghc vis ). I am getting following error. Could you please help me out install ghc vis. Thanks in advance Madhu — environment details. I am on mac yosomite with gcc provided by OS X. Following is the gcc —version 22:54:34:~$ gcc --version Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn) Target: x86_64-apple-darwin14.1.0 Thread model: posix export PKG_CONFIG_PATH=/usr/local/lib/pkgconfig:/opt/X11/lib/pkgconfig export LDFLAGS=-L/usr/local/lib export CFLAGS=-I/usr/local/include export LIBS=-L/usr/local/lib svgcairo error =============== Configuring svgcairo-0.13.0.1... Building svgcairo-0.13.0.1... Preprocessing library svgcairo-0.13.0.1... gtk2hsC2hs: Error in C header file. /usr/include/sys/qos.h:124: (column 34) [FATAL] >>> Syntax error! The symbol `__attribute__' does not fit here. cabal: Error: some packages failed to install: svgcairo-0.13.0.1 failed during the building phase. The exception was: ExitFailure 1

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Re: [Haskell-cafe] [Haskell] Rank-N types with (.) composition
by Tyson Whitehead 11 Feb '15

11 Feb '15

On 11/02/15 04:39 AM, Simon Peyton Jones wrote: > ($) has its own *typing rule*; it does not have a special type. It's very ad hoc, but ($) is used so much to decrease parens that (e1 $ e2) is almost special syntax! I do like the idea of being able to end a function application with a lambda, let, if, case, or do without having to use $ though. Would something like the following grammar rules be problematic for parsing? infixexp→ lexp qop infixexp | - infixexp | [fexp] lexp (*) | aexp (*) lexp → \ apat1 … apatn -> exp | let decls in exp | if exp [;] then exp [;] else exp | case exp of { alts } | do { stmts } (*) fexp → [fexp] aexp aexp → qvar | gcon | literal | ( exp ) | ( exp1 , … , expk ) | [ exp1 , … , expk ] | [ exp1 [, exp2] .. [exp3] ] | [ exp | qual1 , … , qualn ] | ( infixexp qop ) | ( qop⟨-⟩ infixexp ) | qcon { fbind1 , … , fbindn } | aexp⟨qcon⟩ { fbind1 , … , fbindn } https://www.haskell.org/onlinereport/haskell2010/haskellch3.html were I've added (*) to where I have added or removed lines (basically I've moved function application from lexp to infixexp in order to allow the last exp in a function application to be an lexp). > At the moment the *only* robust way to pass a polymorphic function to a polymorphic function (here, you are passing Wrap to (.)) is to wrap it in a newtype, much as Wrap does. Good to know. Thanks! -Tyson

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Regex-applicative and Data.Text
by Konstantine Rybnikov 11 Feb '15

11 Feb '15

I just tried some regex-applicative and it's amazing! Very nice library, thanks Roman! However, I can't figure out the best way to work with Data.Text.Text instead of String. The token would be Text, I guess, but then it breaks in composition, since type of `few anySym` would now return `[Text]`, not `Text`. Am I understanding this correctly that intention is to in issue #8? [0] Or is there a clever way to work with them today? Example code: {-# LANGUAGE OverloadedStrings #-} import qualified Data.Text as T import Data.Text (Text) import Text.Regex.Applicative main = do let input = "foo:\n--- blablabla\ttheend" let r1 = sym "foo:\n" *> sym "--- " *> few anySym <* sym "\t" <* few anySym :: RE Text Text putStrLn (show (input =~ r1)) Error is something like (this is an error for a bit different code, but should be very similar): Main.hs:14:40: Couldn't match type ‘[Text]’ with ‘Text’ Expected type: RE Text Text Actual type: RE Text [Text] In the second argument of ‘(*>)’, namely ‘few anySym’ In the first argument of ‘(<*)’, namely ‘few anySym *> sym "Actual stderr output differs from expected:" *> sym "--- " *> few anySym’ Thanks! [0]: https://github.com/feuerbach/regex-applicative/issues/8

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