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Rose Tree
by Ryan Bloor 04 Nov '07

04 Nov '07

Hello, I need help... I am having trouble with rose trees. 1 2 3 4 5 6 7 8 9 10 11 That is the rose tree that I seek. Data Tree a = Empty | Leaf a | Node a [(Tree a)] example :: Tree (String, String) example = Node ("a","b") -- root node [ ........................................] -- end of tree What I want to do is create two functions that return either the children or parents of a given input, here a String. String -> Tree -> [String] I think _________________________________________________________________ The next generation of MSN Hotmail has arrived - Windows Live Hotmail http://www.newhotmail.co.uk

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Could someone explain winhugs does not accept this argument?
by karle 04 Nov '07

04 Nov '07

type Address = Int data Port = C | D deriving(Eq,Show) data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show) data Pkgtype = RTD | U deriving(Eq,Show) type Pkg = (Pkgtype,Address,Payload) type Table = Signal (Address,Port) system inA inB = (outC,outD) where route = update_pr inA inB s1 = lookup_pr route inA s2 = lookup_pr route inB (s3,s4) = unzipSY s1 (s5,s6) = unzipSY s2 s7 = add_list_pr s3 s5 s8 = add_list_pr s4 s6 outC = fifoSY s7 outD = fifoSY s8 lookup_pr::Signal Table->AbstExt Pkg->AbstExt Pkg lookup_pr t (Prst(x,y,z)) |(delive_adr_A t == y && x == U) = (Prst(x,y,z)) |otherwise = look_if_RTD_pkG_A t (Prst(x,y,z)) the error message is Type error in application *** Expression : delive_adr_A t *** Term : t *** Type : Signal Table *** Does not match : Signal (Int,Port) -- View this message in context: http://www.nabble.com/Could-someone-explain-winhugs-does-not-accept-this-ar… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Why does GHC limit stack size?
by Abhay Parvate 04 Nov '07

04 Nov '07

Hello all, Why is there a limitation on the stack size in GHC? Like heap where we can limit the size by -M RTS option but the default is unlimited, why not let the program use as big a stack as required? If not by default, then by a separate option? Some of the functions that we write in recursive fashion will usually cause a stack overflow, but will work fine if there is more stack (suppose we are not worried about efficiency). And these functions generally look nicer and compact than their tail recursive versions. Is this is a technical hurdle, or just a checkpoint for runaway programs? Can we have an RTS flag allowing unlimited stack size? Thanks, Abhay

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Re: Why can't Haskell be faster?
by Isaac Gouy 04 Nov '07

04 Nov '07

Ketil Malde wrote: > [LOC vs gz as a program complexity metric] Do either of those make sense as a "program /complexity/ metric"? Seems to me that's reading a lot more into those measurements than we should. It's slightly interesting that, while we're happily opining about LOCs and gz, no one has even tried to show that switching from LOCs to gz made a big difference in those "program bulk" rankings, or even provided a specific example that they feel shows how gz is misrepresentative - all opinion, no data. (Incidentally LOC measures source code "shape" as much as anything else - programs in statement heavy languages tend to be longer and thinner, and expression heavy languages tend to be shorter and wider.) __________________________________________________ Do You Yahoo!? Tired of spam? Yahoo! Mail has the best spam protection around http://mail.yahoo.com

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Compile-time evaluation
by Nicholas Messenger 04 Nov '07

04 Nov '07

{-# OPTIONS_GHC -fglasgow-exts -fno-monomorphism-restriction #-} -- Many people ask if GHC will evaluate toplevel constants at compile -- time, you know, since Haskell is pure it'd be great if those -- computations could be done once and not use up cycles during -- runtime. Not an entirely bad idea, I think. -- -- So I set about allowing just that: for arbitrary expressions to be -- evaluated, and the expanded expression spliced into client code. -- -- If you had some data in a file just out of convenience, you could say: -- > yourData = $(compileTimeIO $ parseFile $ readFile "data.txt") -- -- Or if you had an expensive computation that you want done at compile: -- > result = $(compileTimeEval $ expensiveComputation) -- -- I would appreciate comments. I wrote this completely blind with just -- the TH and Generics haddocks, so if I'm doing something tremendously -- stupid that can be improved, let me know. :) Especially if you can -- think of a less awkward way to go from Generics' data to TH -- expressions than using 'showConstr'... -- -- I wrote this with 6.6.1, in case there's any incompatibilities. Copy/ -- paste this post into CompileTime.hs, load into ghci, :set -fth, and -- futz around with the splices. -- -- -- Nicholas Messenger (nmessenger at gmail.com, omnId on #haskell) module CompileTime(compileTimeEval, compileTimeIO) where import Data.Generics import Language.Haskell.TH import Control.Monad import Data.Tree import Data.Ratio -- Expands a datum into an expression tree to be spliced into -- client code. compileTimeEval :: Data a => a -> ExpQ compileTimeEval = return . toExp -- Runs the IO action and splices in the evaluated result datum. compileTimeIO :: Data a => IO a -> ExpQ compileTimeIO = liftM toExp . runIO -- Does the work. :) toTree gets us a tree of constructors, so -- we mostly just have to fold the tree with AppE, except for -- TH's bizarre TupE. toExp :: Data d => d -> Exp toExp = applyAll . toTree where applyAll (Node k args) | isTuple k = TupE (map applyAll args) | otherwise = foldl AppE k (map applyAll args) isTuple (ConE n) = all (==',') (nameBase n) isTuple _ = False -- Synonym to shorten the definition of exp below type Ex a = a -> Exp -- Turns some datum into a tree of TH expressions representing -- that datum. The Exp at each node represents the constructor, -- the subtrees are its arguments. toTree :: Data d => d -> Tree Exp toTree x = Node (exp x) (gmapQ toTree x) where -- The various ways to turn a (Data d => d) into an -- Exp representing its constructor. any = ConE . mkName . deparen . showConstr . toConstr char = LitE . CharL int = sigged $ LitE . IntegerL . toInteger rat = sigged $ LitE . RationalL . toRational sigged f x = SigE (f x) (ConT . mkName . show $ typeOf x) -- The above functions combined together for different types. -- This is what gives the constructor Exp at each Node. There -- are definitely more types to cover that 'any' gets wrong... exp = any `extQ` (int::Ex Int) `extQ` (int::Ex Integer) `extQ` char `extQ` (rat::Ex Float) `extQ` (rat::Ex Double) `extQ` (rat::Ex Rational) -- Generics' showConstr puts parens around infix -- constructors. TH's ConE doesn't like 'em. deparen s = (if last s == ')' then init else id) . (if head s == '(' then tail else id) $ s

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Layout to non-layout code
by Maurício 03 Nov '07

03 Nov '07

Hi, I understand that many people like using layout in their code, and 99% of all Haskell examples use some kind of layout rule. However, sometimes, I would like not to use layout, so I can find errors easier (and maybe convert it to layout for presentation after all problems are solved). So, I wonder: would it be possible to implement a feature in, say, ghc, that would take code from input and output the same code with layout replaced by delimiting characters? For instance, it could take (untested code warning here): main = do s <- readLn putStrLn s and convert it to: main = do {s<-readLn; putStrLn s}; I imagine that would be easy, since the compiler has to do that as a first step anyway. If it's possible, how can I sugest that feature? Thanks, Maurício

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Announce: xmonad 0.4.1
by Don Stewart 03 Nov '07

03 Nov '07

The xmonad strike force is pleased to announce the 0.4.1 release of xmonad! (a tiling window manager) This is a maintainence release with simpler build dependencies. The X11-extras library is no longer required, and instead xmonad uses the improved X11 binding: X11-1.3.0 In other respects this release is functionally identical to xmonad 0.4. As usual, you can find the latest and greatest xmonad codes on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/xmonad http://hackage.haskell.org/cgi-bin/hackage-scripts/package/X11 Brought to you by the xmonad team: Spencer Janssen Don Stewart Jason Creighton Andrea Rossato David Roundy

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Please help from a newby
by karle 03 Nov '07

03 Nov '07

The same declaration.- type Address = Int data Port = C | D deriving(Eq,Show) data Payload = UP[Char] | RTDP(Address,Port) deriving(Eq,Show) data Pkgtype = RTD | U deriving(Eq,Show) type Pkg = (Pkgtype,Address,Payload) type Table = [(Address,Port)] update_table1::Table -> Pkg -> Table update_table1 [] (t,d,y) = [(t,d,y)] Error is Type error in explicitly typed binding *** Term : update_table1 *** Type : Table -> Pkg -> [(Pkgtype,Int,Payload)] *** Does not match : Table -> Pkg -> Table Please kindly suggest, thanks in advance. -- View this message in context: http://www.nabble.com/Please-help-from-a-newby-tf4740192.html#a13555338 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: Semantics of uniqueness types for IO
by Edsko de Vries 03 Nov '07

03 Nov '07

Hey, Just to continue the academic nitpicking.. :-) > Linear logic/typing does not quite capture uniqueness types since a term > with a unique type can always be copied to become non-unique, but a linear > type cannot become unrestricted. Actually, that isn't quite accurate. In linear logic, a term with a non-linear type can always be regarded as having a linear type, i.e. U -o !U is a theorem (my favourite reading of this theorem is "if you have an unlimited supply of bank notes, then you also have a single one"). The implication in the opposite direction is a falsity (from the fact that we have a single bank note, we cannot decude that we have an unlimited supply). By contrast, in uniqueness typing we certainly don't want to allow treat a term with a non-unique type as having a unique type, but we *may* want to allow to treat a term with a unique type as having a non-unique type (as you mention). However, you have to be very careful when you do that: if you allow to treat functions with a unique type as having a non-unique type, you loose all uniqueness guarantees unless you indicate in the type of the term whether or not the function has any unique elements in its closure. If you don't, you could for example construct a term that duplicates unique elements using dup x = (\f -> (f _|_, f _|_)) (\y -> x) (which would get type "forall a, unique a to pair of unique a's" if you allow to copy unique functions and then apply them). Edsko

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Re: Why can't Haskell be faster?
by Jeff.Harper＠handheld.com 03 Nov '07

03 Nov '07

Peter Hercek wrote: > * it is easy to mark stuff strict (even in function signatures > etc), so it is possible to save on unnecessary CAF creations Also, the Clean compiler has a strictness analyzer. The compiler will analyze code and find many (but not all) cases where a function argument can be made strict without changing the behavior of the program.

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