November 2007 - Haskell-Cafe

FP design
by Donn Cave 08 Nov '07

08 Nov '07

I have been working on a little Java project lately at work, by way of an introduction to the language, and naturally I often have cause to regret that it isn't Haskell instead. But in the specific matter I'm wrestling with, the Java library's OOP model is, to its credit, allowing me to do some things. I'm using their standard LDAP client library, but swapping in my own function to read X509 certificates for the SSL. Actually, swapping in my own SSL socket "implementation", which in my case just calls the standard library SSL socket implementation to do most of the work. Now it's not like I can't imagine it working better - it may be a little fragile, for one thing - but I have wondered what facilities a Haskell design could have drawn on to de-couple implementation components like that. Let's say you download an IMAP mail client library, and look to see if it can operate on a UNIX pipe; on an SSL socket; authenticate with GSSAPI Kerberos 5 -- when none of those things are supported out of the box. (As I have needed, and done, all three of those with the standard Python IMAP library module.) You may also want its I/O operations to integrate with some dispatching core, for a GUI. But of course you also want the basic interface to be simple in this area - the IMAP protocol itself is complicated enough! So I'm the one user in a thousand that will want to provide my own I/O functions, for example. In the old world, I guess I would be looking for some extended API where my I/O functions are parameters to the "open" or "init" function, and the IMAP functions take over from there. In a more pure functional oriented model, could it be an extended API that exposes the IMAP functionality as operations on data, and leaves it to me to deal with the I/O? Donn Cave, donn(a)drizzle.com

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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 07 Nov '07

07 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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