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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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SmallCheck Depth
by Omari Norman 11 Feb '15

11 Feb '15

The original SmallCheck paper [0] gives a type data Name = P | Q | R and states that all values of type Name have depth 0. To me this suggests that all values of type Bool would have depth 0 as well, seeing as Bool would simply be data Bool = True | False Sure enough, if I load up ghci with SmallCheck 0.2.1, I get > series 0 :: [Bool] [True, False] But in QuickCheck 1.1.1 I get > list 0 series :: [Bool] [] Very puzzling to me is that it seems the definition of series for Bool has not changed in the new version. Both versions have something like series = cons0 True \/ cons0 False So what explains the different behavior? Thanks. --Omari [0] http://www.cs.york.ac.uk/fp/smallcheck/smallcheck.pdf

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

11 Feb '15

On Wed, 11 Feb 2015, 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 already wondered why runST $ do foo bar worked in some versions of GHC and not in others. I guess it is better to tell the programmer the truth, i.e. remove the special treatment of ($). If you save a programmer from wondering about why "runST $ do ... " does not work with the special typing rule for ($), the programmer will later wonder why (.) does not work or why his privately defined function application operator (e.g. for reversed order of argument and function) will not work. I think that I stay with "runST (do ...)" in order to be compatible with several GHC versions.

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

11 Feb '15

On Tue, 10 Feb 2015, Tyson Whitehead wrote: > I came across something that seems a bit strange to me. Here is a > simplified version (the original was trying to move from a lens > ReifiedFold to a lens-action ReifiedMonadicFold) You are on Haskell(a)haskell.org here. Could you please move to haskell-cafe?

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stuck at the first part of chapter 1 of the CIS course
by Roelof Wobben 11 Feb '15

11 Feb '15

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Using Haskell types in a database
by Nikita Karetnikov 11 Feb '15

11 Feb '15

Suppose I need to represent a user in a database. I could make a table like this one (using Yesod's config/models syntax): User name Text email Text Maybe is_verified Bool default=false last_login UTCTime default=now() Instead, it would be better to do the following, so I couldn't swap name and email by accident or feed a wrong value to is_verified and last_login: User name UserName email UserEmail v Maybe last_login UserLastLogin where newtype UserName = UserName Text deriving (Show, Read) derivePersistField "UserName" newtype UserLastLogin = UserLastLogin UTCTime deriving (Show, Read) derivePersistField "UserLastLogin" data Verified data Unverified newtype UserEmail v = UserEmail Text deriving (Show, Read) derivePersistField "UserEmail" (So I could define functions like sendAnnouncement :: UserEmail Verified -> IO () sendVerification :: UserEmail Unverified -> IO ()) The Haskell part is not new and has been successfully used in the large (see http://code.haskell.org/~dons/talks/dons-google-2015-01-27.pdf, pp. 17--21), but I wonder whether it's reasonable in the database layer. The first table will be mapped to this in PostgreSQL: User name character varying not null email character varying is_verified boolean not null default false last_login timestamp with time zone not null default now() while the second will use character varying for everything. The only downsides of the latter approach I can think of: 1. You can no longer rely on the database to ensure (basic) type safety, so everything needs to be done in the Haskell layer. Otherwise, you will get runtime errors. 2. I haven't benchmarked this, but I assume that comparing, say, booleans should be faster than comparing varchars. So, is it worth it in practice? Do I need to worry about these issues? And are there any other problems?

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