March 2015 - Haskell-Cafe

Multiple compiles with cabal
by Ernesto Rodriguez 12 Mar '15

12 Mar '15

Dear Cafe, I have a program [1] which half of the code is meant to be compiled with GHCJS and the other half with GHC (Or any other Haskell compiler). Currently I do the compilation separately and simply include the compiled JS as a static resource for the regular project. Has anyone run into a similar scenario? Can Cabal handle this? Is there some way of having multiple compiles w/o requiring a shell script to initialize them? Thank you & Cheers N. [1] https://github.com/netogallo/Cryptographer (annoying bc Github now believes my project is like 99% JS since the GHCJS runtime is bundled as a static resource) -- Ernesto Rodriguez Masters Student Computer Science Utrecht University www.netowork.me github.com/netogallo

2 1

Mathematical functions with multiple arguments
by Sumit Sahrawat, Maths & Computing, IIT (BHU) 12 Mar '15

12 Mar '15

Hi everybody, I have a function of type plot :: ([Double] -> Double) -- A function to plot -> [(Double, Double)] -- Range for all arguments -> IO () I want to enforce the fact that ranges for all arguments should be provided. Is there a way to make the type system enforce it? -- Regards Sumit Sahrawat

3 3

Is there a name for this?
by Eric 11 Mar '15

11 Mar '15

Given a class Foo: > class Foo f where> bar :: f -> Bool> baz :: f -> Char -> Int does the record type Foo' > data Foo' = Foo' {> bar' :: Bool,> baz' :: Char -> Int> }> > instance Foo Foo' where> bar = bar'> baz = baz' where the fields of the record Foo' are just implementations of the functions of Foo have any special name? In some sense it's a canonical instance of Foo; we could trivially write a universal > toFoo' :: Foo f => f -> Foo' function. I ran across this pattern and wondered whether it was useful enough to have received a name. It vaguely resembles a typeclass dictionary or a vtable in an OOP language. --Eric

3 3

missing rseq?
by Kees Bleijenberg 11 Mar '15

11 Mar '15

In the book Parallel and Concurrent Programming in Haskell http://chimera.labs.oreilly.com/books/1230000000929/ch02.html#sec_par-eval-s udoku2 a list of sudokus is solved in parallel. In version 2 (sudoku2.hs) the program splits the list of sudokus in 2 seperate lists and solves these lists in parallel. In version3 (sudoku3.hs) parMap is used. What I don't understand is why in sudoku2 the program has to wait until the parallel computations are finished with rseqs while in sudoku3.hs there is no rseq (not in the main program nor in parMap)? Why can't program sudoku3.hs terminate before all parallel calculations are finished as in Example 2-1. rpar/rpar? Kees

2 2

Basic question about recursive data type...
by Nicholls, Mark 10 Mar '15

10 Mar '15

a long time since I did some Haskell and my brain has reverted to its OO dogma... > {-# LANGUAGE GADTs, TypeFamilies, TypeOperators, MultiParamTypeClasses, FlexibleInstances, UndecidableInstances, FlexibleContexts #-} consider > data Next input output where > End :: Next () () > Next :: (input -> output) -> Next input output this is a bit like curried function... I can construct > y :: Next Integer (Next String (Next () ())) > y = Next (\x -> > if (x == 1) then > Next (\y -> > if (y == "s") then > End > else > End) > else > Next (\y -> > if (y == "t") then > End > else > End)) which is a bit like "Integer->String->()" I can execute it... > exe :: Next input output -> input -> output > exe (Next f) i = f i > exe End () = () > p :: Next String (Next () ()) > p = exe y 1 ok.... but how do I constrain all the outputs to be of type Next? I don't want this to be valid > z :: Next Integer Integer > z = Next (\x -> x + 1) CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

1 1

How to abstract away set representation
by martin 10 Mar '15

10 Mar '15

Hello all, I was trying to construct a data type "Profile" which allows some set-like behavior. I started with functions to count, add and filter profiles. type Count = Int data Prof a = P { pCount :: Count, pFilter :: (a -> Bool) -> Prof a, pAdd :: (Prof a) -> (Prof a) } | NoProfile When I tried to actually implement a Profile, the first two functions were easy: exProfile :: [Int] -> Prof Int exProfile xs = P { pCount = length xs, pFilter = \p -> exProfile [x | x<-xs, p x], } But I didn't know how to implement pAdd. I was tempted to write something like pAdd = exProfile (xs ++ ys) But I wouldn't know where to take the ys from. I also hesitate to declare a toList function (which would solve the problem) in Prof. I would only do this *because* it is handy in exProfile, not because it is an intrinsic feature of Prof. What would be a good way to work with set-like behavior without being tied to one particular implementation?

3 5

[] == []
by Rustom Mody 10 Mar '15

10 Mar '15

My impression is that earlier [] == [] used to give a type error. Now it gives True. So 1. Which instance of == is being used? 2. Is there some option controlling this behavior?

5 10

raspberrypi and ghci
by Vasili I. Galchin 10 Mar '15

10 Mar '15

Hello, I have a B++ pi. I did a "apt-get install haskell-platform. No " ghci"!. So I did a google on this issue. Circa 2013 ghci seemed to be an issue with raspberrypi/arm. What is the current status of "ghci" on the pi/arm??. Vasili

2 3

Fwd: Freelance and full-time Haskell development jobs in tech startups
by Vasili I. Galchin 10 Mar '15

10 Mar '15

Hello, Was this posted on this mail list? I am afraid of phishing. Vasili ---------- Forwarded message ---------- From: Vassili van der Mersch <vassvdm(a)sevendays.io> Date: Fri, Mar 6, 2015 at 10:21 AM Subject: Freelance and full-time Haskell development jobs in tech startups To: "biohaskell(a)biohaskell.org" <biohaskell(a)biohaskell.org> Hi BioHaskell, I'm a co-founder at Sevendays - it's a website where tech startups can find quality programmers and designers (amongst other jobs): https://www.sevendays.co <https://www.sevendays.co/?utm_source=Email_2&utm_medium=email&utm_campaign=…> We're looking for great programmers and designers who like to work at startups, either on freelance or permanent basis, on site or remotely. We're growing fast and we're constantly looking for talented developers - in particular, Haskell dev skills are highly demanded at the moment. We never take a cut - you'll keep 100% of your earnings. I'd be grateful if you could have a look and tell me what you think. Have a great day! Vassili ------------------------------------------------------------------------------- Co-founder at Sevendays - https://www.sevendays.co Customer support: info(a)sevendays.co Follow us on Twitter: https://twitter.com/sevendaysapp

4 3

Leuven Haskell User Group meeting March 17 @ 19:00
by Tom Schrijvers 10 Mar '15

10 Mar '15

Dear all, Our next Leuven Haskell User Group meeting will take place on Tuesday, March 17 at 19h00. For details see here: https://groups.google.com/d/msg/leuven-haskell/q863tfr2Na4/Yp6AKMVzMcIJ Everyone is welcome! Tom -- prof. dr. ir. Tom Schrijvers Research Professor KU Leuven Department of Computer Science Celestijnenlaan 200A 3001 Leuven Belgium Phone: +32 16 327 830 http://people.cs.kuleuven.be/~tom.schrijvers/

1 0