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Re: Tutorial for literate Haskell
by b.i.mills＠massey.ac.nz 03 Mar '03

03 Mar '03

Hi, Since I sent this to the haskell list in the first place, I'd better let everyone know that it all worked out. > Hmm, there were no problems in simply doing so. Ok, I've cut your example down a bit (just from a minimalist tendency). The complete modified code is ... \documentclass{report} \usepackage{verbatim} % vital \begin{document} \newenvironment{code}{\footnotesize\verbatim}{\endverbatim\normalsize} HaskellModule.lhs: Maybe some text... We call our module Foo, because this name is very meaningful. \begin{code} module Foo where \end{code} \end{document} Works just fine and seems robust. My original problem ... an over-developed tendency to go it alone? I was not using package verbatim, which does help, doesn't it. > I hope this will help. Yup, thanks. Bruce.

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First-class types
by oleg＠pobox.com 03 Mar '03

03 Mar '03

The following is a more flexible alternative to overloading. We essentially define a function on types and invoke it, seemingly at run time. No Dynamics or unsafe computations are employed. We only need existential types, multi-parameter classes and functional dependencies. The code also shows how to manipulate values which cannot be manipulated. As an example, we define a _function_ (not a method!) 'add' such that: Main> add True False 1 Main> add () (5::Int) 5 Main> add () (5::Float) 5.0 Main> add (4::Int) True 5 Main> add (10::Int) (5::Float) 15.0 The example works both in GHC and Hugs. The signature of add is quite revealing: forall b a2 a1. (Num b, Coerce a2 b, Coerce a1 b, D a1 a2 b) => a1 -> a2 -> b That is, 'add' is capable of adding any two _things_, provided they both can be "generalized" and "coerced" into a number. The "type function" D computes the generalization of two types -- in a manner we specify. The function add is actually quite simple: > add x y = let general_type = typeof x y > x' = coerce x general_type > y' = coerce y general_type > in x' + y' The function 'typeof' is also interesting. It has a type forall b a2 a1. (D a1 a2 b) => a1 -> a2 -> b Note that it returns 'b' forall b! In a sense, the function performs a type computation at run time. The code follows. We should note that we could have achieved the same effect by defining an appropriate class with the method 'add'. In our solution however, the acts of generalizing, coercion, and addition are all separated. If we later decide to subtract things rather than add them, we do not need to alter the class and all the instances. We merely need to introduce the subtraction function. This makes the maintenance of the code easier. The distinct characteristic of the following code is an indirect manipulation of an untouchable, existential value. We were able to force a value to be of a specific type without any means of accessing the value directly. > class Type a where > name :: a -> String > instance Type Bool where > name a = "Bool" > instance Type Int where > name a = "Int" > --instance Type Char where > -- name a = "Char" > instance Type () where > name a = "()" > instance Type Float where > name a = "Float" > -- Type generalization function: type -> type -> type > class (Type b) => D a1 a2 b | a1 a2-> b > instance D Bool Bool Int > instance D Int Bool Int > instance D Bool Int Int > instance D Int Int Int > instance D () Int Int > instance D Int () Int > instance D () () Int > instance D Int Float Float > instance D () Float Float > instance D Float Int Float > instance D Float Float Float > -- The coercion function > class Coerce a b where > coerce :: a -> b -> b > instance Coerce () Int where > coerce _ _ = 0 > instance Coerce () Float where > coerce _ _ = 0 > instance Coerce Int Int where > coerce = const > instance Coerce Float Float where > coerce = const > instance Coerce Int Float where > coerce x _ = fromInteger $ toInteger x > instance Coerce Bool Int where > coerce True _ = 1 > coerce False _ = 0 > newtype M a1 a2 = M (forall b.(D a1 a2 b) => b) > data M1 a1 a2 = M1 a1 a2 (M a1 a2) > typeof v1 v2 = case (M1 v1 v2 (M undefined)) of M1 _ _ (M y) -> y > typeof1 v1 v2 = case (M1 v1 v2 (M undefined)) of M1 _ _ y -> y > add x y = let general_type = typeof x y > x' = coerce x general_type > y' = coerce y general_type > in x' + y'

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Re: Tutorial for literate Haskell
by b.i.mills＠massey.ac.nz 03 Mar '03

03 Mar '03

Yo, Steffen Mazanek wrote: > I do Literate Programming this way: > At first I define a Latex environment "code" as "verbatim" > e.g. so: \newenvironment{code}{\footnotesize\verbatim}{\endverbatim\normalsize} When I ran into the same question some time ago I tried that, but found that the \verbatim was interpreted to0 literally, so that the \end{code} does not terminate it. Could you give a complete short example that works for you? My own solution was to copy the definition of verbatim from the base files, and define code the same way in a separate style file. Regards, Bruce.

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looking for Database Interface in Haskell
by Johannes Waldmann 03 Mar '03

03 Mar '03

Dear all, we want to access a (MySQL) data base, running on a linux server, from a Haskell program. We planned to use http://www.volker-wysk.de/mysql-hs/ but it depends on earlier versions of hdirect (0.17?) and ghc(-4?). I built hdirect-0.19 (?) (from the ghc CVS) but the Foreign interfaces seem to have changed a bit since mysql-hs-0.10.1 was written. Any hints on what direction to take? Thanks, -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe(a)informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/207 --

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Re: How to force UNIX text files as output instead of DOS text files??
by Alexandre Weffort Thenorio 03 Mar '03

03 Mar '03

Thanks a lot m8. I got it. I was supplying it with --make but not with -o. Thanks again. I guess everything is fine now thanks to you guys.I wanna learn more about catching errors but that comes later when I finish this program. Best Regards Alex ----- Original Message ----- From: "Nils Decker" <nils(a)ndecker.de> To: "Alexandre Weffort Thenorio" <alethenorio(a)home.se> Sent: Monday, March 03, 2003 7:19 PM Subject: Re: How to force UNIX text files as output instead of DOS text files?? > "Alexandre Weffort Thenorio" <alethenorio(a)home.se> wrote: > > Hi Great I got it working. Thanks a lot guys. My only problem now is > > when compiling. I am getting: > > > > hextodat.0(.text+0x34):fake: undefined reference to '_stginit_IOExts' > > > > Anybody can tell me what could be wrong?? I can run --make but when > > trying to compile with -o flag I get this error. > > AFAIK, you have to supply -package lang to the linker too when using seperate > compilation. > > > e.g: > ghc -package lang -o foo.o foo.hs > ghc -package lang -o foo foo.o ... > > Regards > Nils > > -- > Unix is user friendly - it's just picky about it's friends. > > Nils Decker <nils(a)ndecker.de> >

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RE: fundeps for extended Monad definition
by oleg＠pobox.com 03 Mar '03

03 Mar '03

| > The reason, which is thoroughly explained in Simon Peyton-Jones' | > message, is that the given type signature is wrong: it should read | > f1 :: (exists b. (C Int b) => Int -> b) > Can you give an example of its use? Yes, I can. > class (Show a, Show b) => C a b | a -> b where > doit:: a -> b -> String > instance C Int Int where > doit a b = (show a) > instance C Bool Bool where > doit a b = if a then "everything" else "nothing" > newtype M a = M (forall b.(C a b) => b) > f :: Int -> M Int > f x = M undefined > g :: Bool -> M Bool > g x = M undefined > test1 a = case (f a) of > M b -> doit a b > test2 a = case (g a) of > M b -> doit a b I wonder if the Obfuscated Haskell contest has an entry for the most useless type (with no uses). However, if a type can be used for the contest, it is no longer the most useless. This makes one wonder if the rules of the contest implicitly contain the Russel paradox.

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RE: fundeps for extended Monad definition
by Simon Peyton-Jones 03 Mar '03

03 Mar '03

| > The reason, which is thoroughly explained in Simon Peyton-Jones' | > message, is that the given type signature is wrong: it should read | > f1 :: (exists b. (C Int b) => Int -> b) | | Right. Simon pointed out that this is a pretty useless function, but not | entirely so, since the result of it is not of type 'forall b. b', but | rather of 'forall b. C Int b => b'. Thus, if the C class has a function | which takes a 'b' as an argument, then this value does have use. I disagree. Can you give an example of its use? Simon

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Persistent data
by Sengan.Baring-Gould＠nsc.com 03 Mar '03

03 Mar '03

Is there some way to reduce the cost of garbage collection over large persistent datastructures without resorting to escaping to C to malloc memory outside the heap? The program I'm working is part database, which cannot discard information. The net result is that I see figures like 82.9% of the time taken by garbage collection. The heap profile looks like a charging capacitor: a linear increase (as expected) which is slowly dilated as time increases by the garbage collector thrashing memory. When I worked on the LOLITA natural language processor we solved the problem by moving a lot of the data out to C++, so that the heap only contains things soon to be freed. I know generational garbage collection is supposed to help, but it doesn't seem to. Is there a pure Haskell solution to this problem? Sengan

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How to force UNIX text files as output instead of DOS text files??
by Alexandre Weffort Thenorio 03 Mar '03

03 Mar '03

I am working on a small program which writes a text file but I need this file to be in UNIX format and as it is a Windows program I am compiling it, the output file tends to be in DSO format. Is there anyway that I can force it to create the file in UNIX format??? Best Regards NooK

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AW: GUI for Windows
by Markus.Schnell＠infineon.com 03 Mar '03

03 Mar '03

Thanks, Axel, Daan, I will have a try with ObjectIO. (At least the examples work ;-)) Markus

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