June 2014 - Haskell-Cafe

ANNOUNCE: haste-perch
by Alberto G. Corona 18 Jun '14

18 Jun '14

Hi, haste-perch defines builder elements (perchs) for Haste.DOM elements that are appendable, so that dynamic HTML can be created in the client in a natural way, like textual HTML, but programmatically and with the advantage of static type checking. It can be ported to other haskell-js compilers. http://hackage.haskell.org/package/haste-perch This program, when compiled with haste: main= do withElem "idelem" $ build $ do div $ do div $ do p "hello" p ! atr "style" "color:red" $ "world" return () Creates these element: <div id= "idelem"> <-- was already in the HTML <div> <div> <p> hello </p> <p style= "color:red"> world </p> </div> </div> </div> Since the creation is in the browser, that permit quite dynamic pages for data presentation, and interctive textual (a.k.a "serious") applications and, in general the development of client-side web frameworks using haskell with the haste compiler. See the README in the git repository: https://github.com/agocorona/haste-perch -- Alberto.

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What are the problems with instances for polymorphic types?
by Gábor Lehel 18 Jun '14

18 Jun '14

In other words instances for forall-types, such as: instance Foo (forall a. [a]) where ... It feels obvious to me that there *would* be problems with this, but I'm curious about what, exactly, they are. Could someone familiar with the matter either elaborate on them, or refer me to an existing explanation, a previous discussion, or something of the sort? I *don't* have any kind of use case in mind, I'm merely seeking a better understanding of the type-system issues involved. (I attempted Google, but didn't have much success.) Thanks in advance.

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Building static libraries with -fPIC
by Nikolay Amiantov 18 Jun '14

18 Jun '14

Hello all, I'm trying to build a shared (.so) library written in Haskell with "foreign export"s, which is statically linked with all needed dependencies. To do this, as far as I understood, I need to build all my dependencies (I use Cabal) with -fPIC flag, and also somehow get RTS static library built with -fPIC. Two questions regarding to this: 1) How can I add a build flag which will be added while building all my dependencies in Cabal. 2) How can I get needed RTS library. 3) How can I deal with "base"? Can I just build it with Cabal, so this question reduces to (1)? Thanks, Nikolay.

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Warnings with OverloadedLists
by Nikolay Amiantov 18 Jun '14

18 Jun '14

Hello, I've bumped into strange GHC behaviour with OverloadedLists and pattern matching. Consider this example: {-# LANGUAGE OverloadedLists #-} len :: [a] -> Int len [] = 0 len (_:t) = 1 + len t If I load this with "ghci -Wall", I get a warning: Test.hs:4:1: Warning: Pattern match(es) are non-exhaustive In an equation for ‘len’: Patterns not matched: [] If I disable OverloadedLists extension, warning goes away. Can someone clear this behaviour out for me? It looks like a bug, but I'm not sure. Thanks for help, Nikolay.

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Support of multi-constructor types by UNPACK pragma
by Nikita Volkov 18 Jun '14

18 Jun '14

Currently the pragma only supports single-constructor types. So in the following example it will simply be ignored: data A = A1 Char | A2 {-# UNPACK #-} !B data B = B1 Int | B2 Bool However it seems to be easily solvable by changing the type A to something like the following during unpacking: data A = A1 Char | A2_1 Int | -- from B1 A2_2 Bool -- from B2 Am I missing something? Why is it not implemented?

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ANN: haddock-library 1.0.0
by Mateusz Kowalczyk 18 Jun '14

18 Jun '14

Hi, haddock-library is a package aiming to provide bits of Haddock to the rest of the Haskell world without imposing the GHC dependency that we have on the ‘haddock’ package itself. Please note that this does not deprecate the ‘haddock’ package which aims to expose other bits of Haddock, including those talking with GHC. Specifically, 1.0.0 exposes the types used by the comment parser and the parser itself. This allows you to supply the Haddock parser with String and get a Haddock ADT back out to play with. This means that if in the past you were processing Haddock strings with your own solution (or copy + pasted Alex parser we had) then you should now switch to this package instead. The API is subject to change so upper version bound is advised. Tested on 7.6.3, 7.8.x and current HEAD. The parser exposed is what is used in Haddock itself which now internally depends on this package. There are plans to move more parts into this package, exposing as much useful functionality as possible without the GHC dependency. New releases are independent of the GHC release cycle, unlike Haddock itself. To file issues for this package, use the Haddock issue tracker that's now on GitHub. Thanks -- Mateusz K.

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return with mutable object
by Baojun Wang 18 Jun '14

18 Jun '14

Hi List, Per my understanding, return x would make a new copy of the object. What if the returned object is mutable? Will this make a new (mutable) object? My concern is if I created a very large mutable object, does return mutable make a full copy of the original mutable data, or just copy a reference (pointer?)? Thanks baojun

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"Spinless Tagless G-Machine" erratum?
by Tom Ellis 17 Jun '14

17 Jun '14

I am reading SPJ's seminal work "Implementing lazy functional languages on stock hardware: the Spinless Tagless G-machine" (1992) and I am confused by something which may be a minor notational error, or may be a key detail I am misunderstanding. The paper is available here http://citeseer.ist.psu.edu/viewdoc/summary?doi=10.1.1.53.3729 On page 26 we have aList = {} \n Cons {thing,nil} nil = {} \n Nil {} but I have not seen this use of the notation elsewhere in the paper. It strikes me that this should be aList = {} \n {} -> Cons {thing,nil} nil = {} \n {} -> Nil {} Is my intuition correct, or am I missing a key detail? Thanks, Tom

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Package Maintenance: hfuse
by Montez Fitzpatrick 17 Jun '14

17 Jun '14

I would like to take over maintenance of the following package: "hfuse." I have attempted to e-mail the original package author and maintainer with no response.

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Fwd: testing platform
by Roelof Wobben 17 Jun '14

17 Jun '14

Hello, IM learning Haskell by this book : Programming Haskell. Which test platform can I also learn the best or can I better first learn Haskell and later the test platforms ? Roelof

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