- Haskell-Cafe - Haskell.org

FP design
by Levi Stephen 08 Nov '07

08 Nov '07

Hi, I'm was wondering how most people work during when designing a functional program. Do you create data structures/types first? Do you work from some type signatures? For example, take a blog. Is the first step likely to be something like: data BlogEntry = BlogEntry { title::String,content::String,comments::[Comment] } type Blog = [BlogEntry] or more likely thinking about what functions will be required: addEntry :: BlogEntry -> Blog -> Blog displayBlog :: Blog -> HTML displayEntry :: BlogEntry -> HTML I'm trying to get my brain out of thinking OO thinking more functionally. Thanks, Levi lstephen.wordpress.com

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Type error with simple list function
by Fernando Rodriguez 08 Nov '07

08 Nov '07

Hi, This simple function definition that should rotate a list (put the first item in the last place) fails with a rather cryptic error with ghc: f :: [a] ->[a] ->[a] f (w : ws) = ws : w Couldn't match expected type `[a] -> [a]' against inferred type `[[a]]' In the expression: ws : w In the definition of `f': f (w : ws) = ws : w What's Haskell trying to tell me? I'm a newby so please forgive my ignorance. Thanks! Fernando

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type/class question: toString
by Graham Fawcett 08 Nov '07

08 Nov '07

Hi folks, Is there a way to declare a 'toString' function, such that toString x | <x is a String> = x toString x | <x's type is an instance of Show> = show x Perhaps, in the type system, there's a way to declare a ToString class, and somehow "inherit" all instances of Show as ToString instances? Thanks, Graham

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Should I step into a minefield? / Writing a trading studio in Haskell
by Joel Reymont 08 Nov '07

08 Nov '07

I need to pick among the usual list of suspects for a commercial product that I'm writing. The suspects are OCaml, Haskell and Lisp and the product is a trading studio. My idea is to write something like TradeStation [1] or NinjaTrader, only for the Mac. It would be quite nifty to use SPJ's financial combinator approach and, for example, embed Yi (Haskell editor). One of the key features of the product would be the ability to model your trading logic using a trading DSL. I'm thinking that this DSL could well be Haskell but I'm concerned about stepping into a minefield. I will need to embed GHC into the app, for example, and I understand that the GHC API does not offer unloading of code at the moment. I would prefer not to bundle GHC separately so I don't think the hs- plugins approach would work for me. Maybe I'm mistaken. Most of all, I'm concerned that my users will need to face the error reports from GHC and could get tripped by laziness, i.e. write something that would make the app run out of memory. Off the top of my head I can't figure out a way to limit what my users can do without analyzing the Haskell AST within the GHC API and complaining if necessary. Can someone with experience in offering a Haskell DSL to their users please comment? Notice that I'm not even mentioning being concerned with the unpredictable effects of laziness. There's probably a reason why Jane St Capital is using OCaml instead of Haskell. I'm not going to play in that league but my knee-jerk reaction is to use OCaml or Lisp and avoid laziness altogether. I just can't see how laziness can help in processing real-time price data. Thanks, Joel [1] http://www.tradestation.com/default_2.shtm [2] http://www.ninjatrader.com/webnew/index.htm -- http://wagerlabs.com

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Flymake Haskell
by Daisuke IKEGAMI 08 Nov '07

08 Nov '07

Hello emacsen users, Here is a setting to check your Haskell code /on-the-fly/ with 'flymake-mode'. (require 'flymake) ;; flymake for Haskell (defun flymake-Haskell-init () (flymake-simple-make-init-impl 'flymake-create-temp-with-folder-structure nil nil buffer-file-name 'flymake-get-Haskell-cmdline)) (defun flymake-get-Haskell-cmdline (source base-dir) (list "ghc" (list "--make" (concat "-i" base-dir) source))) (push '(".+\\.hs$" flymake-Haskell-init) flymake-allowed-file-name-masks) (push '(".+\\.lhs$" flymake-Haskell-init) flymake-allowed-file-name-masks) (push '("^\$.*\$:\$[0-9]+\$:\$[0-9]+\$: \$.*\$$" 1 2 3 4) flymake-err-line-patterns) The 'flymake-mode' has been developed at http://flymake.sourceforge.net/ Emacs22 has this mode already. dons recommends to me at IRC to write this code into the wiki, however, I don't know where we should put the snippet. You can watch a short demo at the following blog: http://madscientist.jp/~ikegami/diary/20071108.html#p01 I have some complaints though it works: - it's slow (hmm) - error and warning seems not distinguished (we may have to modify 'flymake-err-line-patterns') Any comment is appreciated. Best regards, Ike P.S. I like vi-clone, such as vim, too. ;-)

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words function
by Ryan Bloor 08 Nov '07

08 Nov '07

hi I am trying to create a function that uses the words function... I am doing the same thing to each element in a list so I am using mapping techniques. Code... --Define the main first function rStrings2Results :: ([String] -> String) -> [[String]] -> [String] rStrings2Results f(head:tail) = (f head : rStrings2Results f tail) I just want take a list and on the first member ("hello my name is ryan") to say [("hello", "my", "name", "is", "ryan"),..............] using the words function. _________________________________________________________________ 100’s of Music vouchers to be won with MSN Music https://www.musicmashup.co.uk

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generating Maybe
by Tim Newsham 08 Nov '07

08 Nov '07

Data.Maybe has functions for processing Maybe's but nothing useful for creating maybe. I think the following would be a very useful addition, a guarded function: guarded :: (a -> Bool) -> (a -> b) -> a -> Maybe b guarded p f x | p x = Just (f x) | otherwise = Nothing such a function in the std libs would make functions like "unfoldr" more attractive -- uses of foldr nearly always encapsulate this notion. Tim Newsham http://www.thenewsh.com/~newsham/

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type annotations elsewhere
by Jason Dusek 08 Nov '07

08 Nov '07

Haskell offers a few different annotations, for example strictness, to optimize programs. It would sure be nice if there were a way to pull the annotations out and put them somewhere else, enabling them, or not, at compile time. There could be several different annotation sets, even -- you could try different ones for different things. However, there are some real practical difficulties here -- how do you write an 'expresssion selector' to pair with the annotation, in order to find its target? It may involve walking the AST -- a fancy and potentially brittle way to go about optimization (a brittle thing to start with!). -- _jsn

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CPP, Leopard and Haskell Objective-C bindings
by Joel Reymont 08 Nov '07

08 Nov '07

It seems that the current approach taken by HOC is to strip preprocessing directives. This may not have been a problem before Leopard but Cocoa header files are now full of macros in most unusual places, e.g. @interface NSObject (NSDeprecatedMethods) + (void)poseAsClass:(Class)aClass DEPRECATED_IN_MAC_OS_X_VERSION_10_5_AND_LATER; @end I'm trying to figure out the easiest way to run CPP over the header files before processing them in HOC. Is there a CPP written in Haskell and packaged as a library? Does someone have a quick Haskell solution for running CPP over a header file (with include paths?), redirecting into a temp file and reading that back into Haskell? Should I preprocess all Cocoa header files using make and then read these in? Thanks, Joel -- http://wagerlabs.com

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ANN: infinity 0.3
by Austin Seipp 08 Nov '07

08 Nov '07

After quite a bit of work and other such stuff, I pretty much totally rewrote my previous IRC bot and instead updated it from v0.1 to v0.3. You may wonder where 0.2 went; so do I. This is basically an entire rewrite (I threw the first one out.) Now all code can be dynamically hot-swapped via the techniques described in Don's Yi paper. This can happen across multiple servers and as servers are taken down and the like. All code swaps near flawlessly, and even after the initial mechanism was working, the bot ran for quite a while with a good amount of success. I didn't plan to release it in this state, however, with it's dependencies on hs-plugins (which is ghc 6.6 only as of right now,) and my recent installation of GHC 6.8.1, I can't work on it because apparently although both versions appear to be installed, it seems as if ghc-6.6.1 is picking up configuration and package info from 6.8.1 (and therefore ghc-6.6.1 tells me it can't find packages like irc and plugins, although 'ghc-pkg-6.6.1 list' obviously shows they exist.) I also can't work on this on my OpenBSD box which only has 6.6.1, due to some arcane error with hs-plugins (can't find the __errno symbol, I believe.) So because of that, development is temporarily stalled and if you want to test it, you'll need 6.6.1 installed and working properly. I don't think this would have been *too* far from the actual, official release though. At this point I only planned minor tweaks and its release was pretty inevitable anyway. You can get a tarball from hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/infinity-0.3 The darcs repository is here: http://code.haskell.org/infinity/src/ Personally, it was real fun to work on this thing. I wrote almost all the base code for plugins, and for the Net and Monitor to use before the bot ever made its first connection, and a lot of it seemed to just piece together really easily. The code is undeniably cleaner and more extensible than the previous 0.1 code, but I still feel by most standards it's somewhat cluttered, so if anybody here would like to give any comments just from reading the source, that'd be pretty awesome. Until I can continue to work on it (whether hs-plugins is available for 6.8.1 or I get another box with 6.6.1 installed,) this will be considered the stable version by my definition, and all patches are of course welcome. darcs send austin at youareinferior dot net. Criticism is advised. -- - Austin

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