December 2007 - Haskell-Cafe

list utilities -- shouldn't these be in the hierarchical libs somewhere?
by Thomas Hartman 18 Dec '07

18 Dec '07

I found http://haskell.cs.yale.edu/haskell-report/List.html had many useful "one off" type list functions such as "subsequences" and "permutations" which are nowhere to be found in hoogle, Data.List, or the haskell hierarchical libs Shouldn't these be included somewhere? thomas. --- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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New to Haskell
by Cristian Baboi 18 Dec '07

18 Dec '07

A few days ago, for various reasons, I've started to look at Haskell. At first I was quite impressed, after reading some FAQ, and some tutorials. Evrything was nice and easy ... until I've started writing some code on my own. What I should have been told about upfront: - the syntax for an expression - the syntax for a block - the adhoc syntax rules (how to distinguish among a tuple and a pharanthesized expression and how to find the start and end of a block for example ) - the fact that lambda expressions are not the same thing as "algebraic data" values - what guarantees are made by the LANGUAGE that an IO action (such as do putStrLn "Hello world" ) is not performed twice - the lambda expressions can be written (input) but cannot be printed (output) The biggest problem for me, so far, is the last one. Here is some strange example: module Hugs where aa::Int aa=7 cc:: (Int->Int)->(Int->Int->Int)->Int->(Int->Int) cc a op b = \x-> case x of { _ | x==aa -> x+1 ; _-> a x `op` b } f::Int->Int f(1)=1 f(2)=2 f(_)=3 g::Int->Int g(1)=13 g(2)=23 g(_)=33 h::[Int->Int] -> Int ->Int h [] x = x h [rr] x = let { u=Hugs.f ; v=Hugs.g } in case rr of { u -> Hugs.g(x)+aa ; v -> Hugs.f(x)+aa ; _ ->rr (x) + aa } h (rr:ll) x = h [rr] x + h (ll) x What I don't understand is why I'm forced to use guards like x==aa in cc, when aa is clearly bounded (is 7) and why in function h, the bounded u and v become free variables in the case expression. ________ Information from NOD32 ________ This message was checked by NOD32 Antivirus System for Linux Mail Servers. part000.txt - is OK http://www.eset.com

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Multiple statements with Where
by insertjokehere 18 Dec '07

18 Dec '07

Hi all, I am having problems adding multiple definitions with where for example in my code --A parser for recognising binary operations parseBinaryOp :: String -> String -> [(Expr, Expr, String)] parseBinaryOp op str | (elem op binops) && (notElem '(' (snd bm)) && (notElem ')' (snd bm)) && (elem nstr!!1 binops) = [(EInt 1, EInt 1, "HERE!")] | otherwise = [] where bm = bracketMatch str nstr = words (snd (bracketMatch str)) I get the error message parse error on input `=' Essentially this function is supposed to parse binary operations in a string, nstr is just a [String], binops is a list of Strings. The types appear to be fine, and GHCI dosnt say that this is the problem, the problem seems to lie with that definition of nstr after the definition of bm. I believe I have followed the definitions correctly, so I am at a loss for how to solve this problem. The list that is given for the first case is only a placeholder, once I get past this problem I should be able to make the function operate properly Any help would be much appreciated, please tell me if you need more info. Thankyou :-) -- View this message in context: http://www.nabble.com/Multiple-statements-with-Where-tp14397482p14397482.ht… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Re: [Haskell-cafe] Re: New to Haskell
by Miguel Mitrofanov 18 Dec '07

18 Dec '07

> > Yes, and Haskell can do it also. But C, I guess, can't print out a > > source code for a function (well, there can be some weird dialects of C > > I'm not aware about). Haskell can't do it either. > Well, LISP can, if I remember it right. Only in an interpreter, if I remember it right.

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Re: [Haskell-cafe] Re: New to Haskell
by Miguel Mitrofanov 18 Dec '07

18 Dec '07

> >> - the lambda expressions can be written (input) but cannot > >> be printed (output) > > This is a fundamental property of the language. A lambda > > expression is programme and at runtime the system doesn't > > know one lambda expression from another (all it can do with > > one is apply it to something). > Even C can apply a function variable to an argument (function pointers). Yes, and Haskell can do it also. But C, I guess, can't print out a source code for a function (well, there can be some weird dialects of C I'm not aware about). Haskell can't do it either.

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Re: [Haskell-cafe] New to Haskell
by Miguel Mitrofanov 18 Dec '07

18 Dec '07

> >> - the syntax for an expression > >> - the syntax for a block > > Don't see your point. > The point is the syntax is introduced as transformation of layout form to > non layout form. > As a user, I just want to be able to spot the basic components of a source > file without thinking about transformation rules. Well, most users are. > > Oh, that's pretty easy, parenthesized expression is not divided by a > > comma. > Thanks! What is the end of a block ? What introduce new blocks ? Not sure what you mean by "block" here. > Is this legal (`plus`) x y ? Never tried to write this myself, it looks stupid. > It's this a tuple ? ([a,b,c,d ]) ? No, since all commas are in the subexpression. > > then your putStrLn would be performed twice. IO actions are first-class > > values, that's a feature, not a bug. > What guarantees that by running the main, the string "Hello world" will be > printed exactly twice ? What kind of guarantees do you want? > I just want the sistem to be able to print one of these expressions ! > Its this too much to ask ? Yes, 'cause it means you want to embed almost all source code into the compiled program.

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Re: [Haskell-cafe] OOP'er with (hopefully) trivial questions.....
by Miguel Mitrofanov 18 Dec '07

18 Dec '07

> class Shape a where { > intersect :: Shape b => a -> b -> Bool > } > data Shape a = { intersect :: Shape b => a -> b -> Bool } > in fact, the syntax is rather similar, too! :) Um, well, and how are you going to implement it?

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Compiling GLFW / SOE on Windows
by Peter Verswyvelen 17 Dec '07

17 Dec '07

Hi, This might be a tip for people trying to compile the GLFW package bundled with SOE on Windows. I had to because I slightly modified the GLFW source code. The SOE readme.txt says: "For Win32 users, you'll need MinGW environment to compile GLFW. If "make win32-mgw" is unable to execute compile.bat propperly, use "compile.bat mgw". Please also make sure that your MinGW path should come before the GHC's gcc-lib path in the PATH environment." However, the readme.html file has the following note: Do not use the MSYS shell for compiling GLFW, because the supplied batch file "compile.bat" will only work under a Windows command prompt (or MS-DOS prompt). To get it working, I basically needed to modify my PATH environment variable so that it contains the MSYS bin folder (in my case C:\app\msys\1.0\bin), and then run "COMPILE.bat make mgw" file using the Windows command prompt (CMD.EXE) instead of the MSYS unix shell emulator... Cheers, Peter

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Questions about the Functor class and it's use in "Data types à la carte"
by Corey O'Connor 17 Dec '07

17 Dec '07

I'm working through the interesting paper "Data type à la carte" and am confused by the Functor instance for Val. I think this stems from some confusion of mine regarding the Functor class in general. The Functor instance I'm confused about is: instance Functor Val where fmap f (Val x ) = Val x where Val is defined as: data Val e = Val Int Is this the only valid Functor instance for the Val type? Even though I'd, naively, expect the Functor instance to look like: instance Functor Val where fmap f (Val x) = Val (f x) I suspect that would not work out due to the type of the Val constructor. Is this correct? The reason I find all this odd is because I'm not sure how the type class Functor relates to the category theory concept of a functor. How does declaring a type constructor to be an instance of the Functor class relate to a functor? Is the type constructor considered a functor? Any help would be, of course, greatly appreciated. -Corey O'Connor

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gtk2hs treeview problem (Windows)
by Jian Fan 17 Dec '07

17 Dec '07

I installed ghc 6.6.1 and 0.9.12.1. I'm trying to use the tree view widget. To enable search in treeview, I added following 2 lines: New.treeViewAppendColumn view col3 -- my addition New.treeViewSetEnableSearch view True New.treeViewSetSearchColumn view 0 in demo/treeList/TreeDemo.hs The program crashes if I press any key. The console shows: C:\Gtk2Hs\demos\treeList>TreeDemo.exe (TreeDemo.exe:3064): GLib-GObject-WARNING **: gtype.c:3339: type id `0' is inval id (TreeDemo.exe:3064): GLib-GObject-WARNING **: can't peek value table for type `< invalid>' which is not currently referenced I also tried ghc 6.8.1 with the same result. Any hints? Thanks. Jian

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