- Haskell-Cafe - Haskell.org

Restrict values in type
by Luke Clifton 16 Jan '14

16 Jan '14

Hi, I'm quite new to Haskell, and have been loving exploring it. I've always been a huge fan of languages that let me catch errors at compile time, finding dynamic languages like Python a nightmare to work in. I'm finding with Haskell I can take this compile time checking even further than most static languages and it has gotten me rather excited. So I was wondering if there is a Haskell way of solving my problem. I'm trying to represent an image made up of a list of strokes. Strokes are either lines, arcs or spots, and can be made using different pen shapes. data Image = Image [Stroke] data Stroke = Line Point Point PenShape | Arc Point Point Point PenShape | Spot Point PenShape data PenShape = Circle Float | Rectangle Float Float | ArbitraryPen -- Stuff (not relevant) And this is all great and works. But now I have a problem. I want to extend this such that Arc strokes are only allowed to have the Circle pen shape, and Lines are only allowed to have the Rectangle or Circle pen shapes. What is the best way of enforcing this in the type system. I could make more Strokes like LineCircle, LineRectangle, Arc, PointCircle, PointRectangle, PointArbitrary and get rid of the PenShape type altogether. But this doesn't really feel good to me (and seems like the amount of work I have to do is bigger than it needs to be, especially if I added more basic pen shapes). I thought about making the different PenShapes different types, using typeclasses and making Stroke an algebraic data type, but then my strokes would be of different types, and I wouldn't be able to have a list of strokes. I have been looking at DataKinds and GADTs, but I can't quite figure out if they actually help me here at all. I'm sure there is a way to do this, I'm just not googling properly. What I want to write is... data Image = Image [Stroke] data Stroke = Line Point Point (Circle or Rectangle) | Arc Point Point Point Circle | Spot Point PenShape data PenShape = Circle Float | Rectangle Float Float | ArbitraryPen -- Stuff (not relevant) Regards, Luke

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Postdocs / Research Programmer for Efficient First-Class Automatic Differentiation
by Barak A. Pearlmutter 15 Jan '14

15 Jan '14

Postdocs / Research Programmer for Compositional Learning via Generalized Automatic Differentiation The goal of this project is to make a qualitative improvement in our ability to write sophisticated numeric code, by giving numeric programmers access to _fast_, _robust_, _general_, _accurate_ differentiation operators. To be technical: we are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination in a production-quality fast system suitable for numeric computing in general, and compositional machine learning methods in particular. Our research prototype compilers generate object code competitive with the fastest current systems, which are based on FORTRAN. And the combined expressive power of first-class AD operators and function programming allows very succinct code for many machine learning algorithms, as well as for some algorithms in computer vision and signal processing. Specific sub-projects include: compiler and numeric programming environment construction; writing, simplifying, and generalising, machine learning and other numeric algorithms; and associated Type Theory/Lambda Calculus/PLT/Real Computation issues. TO THE PROGRAMMING LANGUAGE COMMUNITY, we seek to contribute a way to make numeric software faster, more robust, and easier to write. TO THE MACHINE LEARNING COMMUNITY, in addition to making it easier to write efficient numeric codes, we seek to contribute a system that embodies "compositionality", in that gradient optimisation can be automatically and efficiently performed on systems themselves consisting of many components, even when such components may internally take derivatives or perform optimisation. (Examples of this include, say, optimisation of the rules of a multi-player game to cause the players' actions to satisfy some desiderata, where the players themselves optimise their own strategies using simple models of their opponents which they optimise according to their opponents' observed behaviour.) To this end, we are seeking to fill three positions (postdoctoral or research programmer, or in exceptional cases graduate students) with interest and experience in at least one of: programming language theory, automatic differentiation, machine learning, numerics, mathematical logic. Informal announcement: http://www.bcl.hamilton.ie/~barak/ad-fp-positions.html Formal job postings on http://humanresources.nuim.ie/vacancies.shtml, in particular http://humanresources.nuim.ie/documents/JobSpecPostdoc2_Final.pdf and http://humanresources.nuim.ie/documents/JobSpecProgrammer_Final.pdf Inquiries to: -- Barak A. Pearlmutter <barak(a)cs.nuim.ie> Hamilton Institute & Dept Computer Science NUI Maynooth, Co. Kildare, Ireland

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GHC API: runStmt not taking into account reloaded module
by JP Moresmau 14 Jan '14

14 Jan '14

It's late here and I'm probably overlooking something stupid, so I'd like if somebody could put my nose on it... I'm using the GHC API to evaluate statements. I use runStmt to get a RunResult, lookupName to get the ID for the bound names, obtainTermFromId to get the term and showTerm to display it. So I can call a function from the loaded module with some parameters and get the result. Good! However, if I reload a module and I change the implementation of the function, runStmt still returns the old value! I know the reload worked because if I added new names, getNamesInScope returns the new names. What do I need to do to make sure the new function definitions are used? I've perused the source code of InteractiveEval and ghci but nothing stood out. I am calling setContext after load. Thanks a million! -- JP Moresmau http://jpmoresmau.blogspot.com/

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Future imports in GHC/Base
by João Cristóvão 14 Jan '14

14 Jan '14

Hello all, I was wondering if there is any particular reason for Haskell/GHC not having a kind of "import from Future", similar to Python import __future__, or, in a different take, stuff like modernizr for javascript. Was this done already? My quite simple use case: I could really use the Foldable instance for Either, and also ended up defining my own 'isLeft' and 'isRight'. Felt guilty of re-inventing the wheel after seeing it defined elsewhere, it was so obvious, of course. Then I see it defined in GHC Head - but unfortunatelly not all packages support it yet. And I can only guess that installing base 4.7 on top of GHC 7.6.3 would certainly result in Cabal Hell. I ended up copying the parts that I needed. Any one else doing this? My proposal is simple, and quite restricted: 1) Include only stuff with no dependencies on new compiler features 2) Focus mainly on additional typeclasses instances 3) Or new functions, that otherwise should not interfere with existing code. 4) Use CPP to translate the code into NOP when the new compiler finally enters the Haskell Platform. Thus, a very simple "FutureGHC" package (I'm open to alternative names) would, for isLeft/isRight, be like the code proposed below - taken from GHC Head libraries, of course. What do you think? Joao {-# LANGUAGE CPP #-} -- (...) module Data.Either.GHC707 ( module Data.Either #if __GLASGOW_HASKELL__ < 707 , isLeft , isRight #endif ) where #if __GLASGOW_HASKELL__ < 707 import Data.Either -- | Return `True` if the given value is a `Left`-value, `False` otherwise. -- -- /Since: 4.7.0.0/ isLeft :: Either a b -> Bool isLeft (Left _) = True isLeft (Right _) = False -- | Return `True` if the given value is a `Right`-value, `False` otherwise. -- -- /Since: 4.7.0.0/ isRight :: Either a b -> Bool isRight (Left _) = False isRight (Right _) = True #endif

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Re: [Haskell-cafe] Records vs Tuples
by oleg＠okmij.org 14 Jan '14

14 Jan '14

Cary Cherng wrote: > What exactly is the purpose in having both records and tuples? They > seem to behave rather similarly although records do have the extra > naming of the fields that tuples don't. In SML there is really no difference: In fact, in Standard ML, tuples are simply a special case of records; for example, the type int * string * bool is the same as the type { 1 : int, 2 : string, 3 : bool }. http://en.wikibooks.org/wiki/Standard_ML_Programming/Types In some programming languages (C, OCaml, Haskell), records, or structures, have to be declared and are not extensible. OCaml also has extensible records, which don't have to be declared and whose field names may be reused. Tuples may be thought of as `extensible' records that don't have to be declared (and HList takes this thought quite seriously). > data Rec1 = Rec1 { a :: Int, b :: Bool} > data Rec2 = Rec2 { a :: Int, b :: Bool} > foo :: Rec1 -> Bool > > Rec1 and Rec2 could be in totally different code libraries. I've read > that preventing Rec2 being used in foo is good for the type safety in > that Rec1 and Rec2 are likely intended to have semantically different > meanings and allowing interchangeability breaks this. The reason is not type safety but type inference, or the ease of it. Given > data Rec1 = Rec1 { rec1a :: Int, rec1b :: Bool} > data Rec2 = Rec2 { rec2a :: Int, rec2b :: Bool} and the term \x -> rec2a x + 1 the type checker quickly determines that x must be of a type Rec2, and the inferred type is Rec2 -> Int. With extensible records, using OCaml: # fun x -> x#rec2a + 1;; - : < rec2a : int; .. > -> int = <fun> we infer a polymorphic type. That's all nice, until we look at the OCaml type checker and see the complexity of inference. Open records bring up lots of interesting problems (e.g., variance), which interact in interesting and not yet worked out ways with other features (e.g., GADTs). Also, open records can often mask problems. For example, in Haskell \x -> rec1a x + rec2a x won't type check but the corresponding code in OCaml will -- although the programmer may have never intended a record with both fields rec1a and rec2a. OCaml community has had a lot of experience with extensible and non-extensible records (and their dual -- extensible and non-extensible variants). The conclusion seems to be that both are needed. Often a programmer does mean closed records/variants; and non-extensible forms match the programmer's intent closely and give better, and earlier, error messages.

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Records vs Tuples
by Cary Cherng 14 Jan '14

14 Jan '14

What exactly is the purpose in having both records and tuples? They seem to behave rather similarly although records do have the extra naming of the fields that tuples don't.

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OT - targeting many systems
by Marc Weber 13 Jan '14

13 Jan '14

I love Haskell, and it gets many things right. However eg its hard to target JS in an efficient way. Sometimes I just need a quick and dirty (somewhat typesafe) program which gets the job done - sometimes its js, sometimes a tool in console, sometimse web/client code (also smartphone applets). haxe.org comes close, but while the language is fun to use it still has some shortcomings - and sometimes I hit unexpected bugs. The real question I have is: - Haskell is cool - being lazy is cool - having OO is cool (because you can target Java/Flash/.. easily) - how to get best of all worlds? So I think about creating my own language is should look like more like a source code generator than language which contains different sub universes. The idea is to code some parts - such as a parser - in a Haskell like dialect, then reuse it in quick and dirty OO world (Java like). Of course additional targets could be just plain old C, because C++ is known to be very complex - and there are tons of legacy code which should be maintained (such as Vim) - but there is no true upstream, because coding C takes so much more time. I also just don't get why it should not be possible to write code once (such as parsing snippets or highlighting), then reuse it for Vim, Emacs, Yi, .. There is no way, you cannot merge such editors - but eventually there is a way to "code once" and compile down to elisp, C, Haskell ... Live is too short to duplicate much work .. I'm writing to this mailinglist because the knowledge in the Haskell community is incredible - and I want to know * who would join such an effort (maybe write drafts, get funded by kickstarter) * does such attempt already exist ? Languages like impredicative.com/ur, Haskell, lisp, Java, Haxe cleary also illustrate the power of "meta programming" or "source code generation at compile time", yet they all come with their own limitations, eg ur cannot create generate code which is useful outside of a web server and so on. So I feel I'm reinventing the wheel again and again - I'd like to stop this. So the goal would be trying to write "functors/code" which you can instantiate with different memeory management models, tell to target different backends etc. I love Haskell, its community and what it achieved, but somehow I feel that its only 80% of the true story getting everything done. Eg haxe already allows me to write code like such - and then compile it twice: once to create a .js file, once to create server side js/php/neko/whatsoever. class FormComponentGeoLocation { new (opts){ this.opts = opts; } function check(value){ // check with regular expression that value has lat lon numbers } #if SERVER function assignValuesFromInputs(post_data, values){ var value = post_data[opts.name]; check(value); values[opts.name] = post_data[opts.name]; } #end #if js additional client code eg use geo location apis to fill in lat/lon function assignValuesFromInputs(values); var value = post_data[opts.name]; check(value); values[opts.name] = value; } #end } So this is what I'd call modular code I can reuse which has all logic it needs. While Haskell is great I'm unsure wether it allows me to create modules by code (I know about template haskell, but that's only about adding code) - and while being lazy is a feature sometimes I'd like to be strict. (I know that people work on simplifying this) ideas/thougts/links/you'd like to join understanding whether such a goal would be reachable at all? Marc Weber

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any idea why binary isn't bulding on OSX
by Anatoly Yakovenko 13 Jan '14

13 Jan '14

src/Data/Binary/Get.hs:423:3: error: invalid preprocessing directive #-} ^ src/Data/Binary/Get.hs:511:53: warning: missing terminating ' character [-Winvalid-pp-token] -- host order, host endian form, for the machine you're on. On a 64 bit ^ 12 warnings and 1 error generated. Failed to install binary-0.7.1.0 cabal: Error: some packages failed to install: JuicyPixels-3.1.2 depends on binary-0.7.1.0 which failed to install. binary-0.7.1.0 failed during the building phase. The exception was: ExitFailure 1

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Structural typing of records in Haskell?
by Cary Cherng 13 Jan '14

13 Jan '14

Are there statically typed languages that treat records with structural typing, either imperative or functional? Why should records not be structurally typed in Haskell? From what I understand, in the below foo cannot take a Rec2 even though Rec1 and Rec2 are essentially the same. data Rec1 = Rec1 { a :: Int, b :: Bool} data Rec2 = Rec2 { a :: Int, b :: Bool} foo :: Rec1 -> Bool Rec1 and Rec2 could be in totally different code libraries. I've read that preventing Rec2 being used in foo is good for the type safety in that Rec1 and Rec2 are likely intended to have semantically different meanings and allowing interchangeability breaks this. But then why is map structurally typed. map takes an argument of type a -> b and suppose some other higher order function bar also takes an argument of type a -> b. Should map instead have the below type which prevents a function of type a -> b semantically intended for bar from being accidentally used in map. newtype Mapper a b = Mapper { fn :: a -> b } map :: Mapper a b -> [a] -> [b] map _ [] = [] map f (x:xs) = (fn f) x : map f xs If there is a mechanism that prevents something of type Rec2 from accidentally being used in foo, then why shouldn't there be something analogous that prevents something of type a -> b (meant for bar) from accidentally being used in map?

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upgrading from Xcode4 to Xcode5 on Mountain Lion
by Angel Alvarez (GMAIL) 13 Jan '14

13 Jan '14

Hi, I have pending the Xcode 4 -> 5 upgrade for a while in my mountain lion, have any of you any pointers to fixes needed in order to get Xcode 5 runnignwith the current haskell platform? from time to time I've seen people having trouble and don't know really if its recommended to upgrade on Mountain Lion... regards, Angel Alvarez (GMAIL) angeljalvarezmiguel(a)gmail.com

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