- Haskell-Cafe - Haskell.org

Braces vs. Whitespace
by Ashley Yakeley 22 Aug '01

22 Aug '01

At 2001-08-21 20:31, Dean Herington wrote (on the Haskell list): >Now realizing that the "where" keyword is optional when there are no >accompanying declarations, I think it would be preferable to omit the >"where" keyword to indicate that the absence of declarations is >intentional. Personally I prefer the explicit braces/semicolons syntax rather than worrying about whitespace: module DeepSeq where { class DeepSeq a where { deepSeq :: a -> b -> b; deepSeq = seq; -- default, for simple cases }; ... instance DeepSeq Ordering; instance DeepSeq Integer; instance DeepSeq Int; instance DeepSeq Float; instance DeepSeq Double; } ...or if you prefer... module DeepSeq where { class DeepSeq a where { deepSeq :: a -> b -> b; deepSeq = seq; -- default, for simple cases }; ... instance DeepSeq Ordering; instance DeepSeq Integer; instance DeepSeq Int; instance DeepSeq Float; instance DeepSeq Double; } etc. Basically you put a braced block after every 'do', 'in' and 'where', and semicolons after every declaration. When I was first learning Haskell, I got massively confused by the whitespace rules, especially with mixed tabs/spaces etc. I reckon the braces style is more readable to anyone used to C/C++/Java etc. -- Ashley Yakeley, Seattle WA

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Re: Subtyping
by Fritz K Ruehr 22 Aug '01

22 Aug '01

[sorry about that, let's try again...] Ross Paterson wrote (in regards variations on factorial): > Another version: > > fac = foldr (*) 1 . unfoldr (\n -> guard (n > 0) >> return (n, n-1)) Thanks for forwarding this; may I quote it on the web site? (with full attribution, of course) If so, you are welcome to give it a name and a pithy little "slogan" (also for the one below). > Also, the combinatory programmer would regard defining y by recursion > as cheating, and would probably prefer one of these versions: > > newtype SelfApp a = SA (SelfApp a -> a) > > selfApply :: SelfApp a -> a > selfApply (SA f) = f (SA f) > > yCurry :: (a -> a) -> a > yCurry f = selfApply (SA (\x -> f (selfApply x))) > > yTuring :: (a -> a) -> a > yTuring = selfApply (SA (\x f -> f (selfApply x f))) Yes, this is definitely the right way to go here. I comment on the site about the fact that we should really *define* Y (and I felt a bit guilty about it :) ), but I didn't try hard enough to get around the typing issue. You have done so quite elegantly. Of course, there is also a trade-off at some point between the impact of the various examples and their sheer number (I added an environment-model based interpreter yesterday that started to push this boundary). Still, more monads (arrows, even?) are needed. -- Fritz Ruehr PS: for those who missed it, the site is at: <http://www.willamette.edu/~fruehr/haskell/evolution.html>

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Re: Subtyping
by Fritz K Ruehr 22 Aug '01

22 Aug '01

Ross Paterson wrote (in regards variations on factorial):

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Re: GHC Mandrake problem
by Pixel 22 Aug '01

22 Aug '01

"phaywood(a)mail.iprimus.com.au" <phaywood(a)pop.iprimus.com.au> writes: > Hello, I tried to install version ghc 5.00.2 & prof from mandrake (vers 8.0 > with current updates). It installed fine, but i get this message when i try to > run ghci: > > > Loading package std ... ghc-5.00.2: can't find .o or .so/.DLL for: m (lib.so: > cannot open shared object file: No such file or directory) booh :'-( it tries to find m.o then tries lib.so > > Can anyone tell me what's missing, or how to fix this?? i don't know either. It used to work. I'm currently trying a rebuild...

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Re: Subtyping
by Fritz K Ruehr 21 Aug '01

21 Aug '01

Frank Atanassow wrote: > This is a situation where the explicit fixpoint datatype comes in handy: > > data Fix f = In (f (Fix f)) > > ... > > BTW, this is not a beginner, or even intermediate, Haskell programmer's > technique. For those who are interested in what exactly is or isn't a beginner or intermediate Haskell programmer's technique, I humbly offer the following <http://www.willamette.edu/~fruehr/haskell/evolution.html> which includes examples of the afore-mentioned approach, along with many others, from accumulating parameters to comonads. Fair warning: tongue held firmly in cheek ... -- Fritz Ruehr fruehr(a)willamette.edu

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Modular arithmetic
by Dylan Thurston 21 Aug '01

21 Aug '01

They said it couldn't be done! They said Haskell didn't have dependent types! Here's a way to mimic dependent types using existential types, illustrated by an implementation of modular arithmetic. To try it out, load modulus.hs and try something like Modulus> inModulus (mkModulus (1234567890123::Integer)) (^98765432198765) 2 1160454313058 to compute 2 to the 98765432198765'th power modulo 1234567890123. The key is the definitions at the top of TypeVal.hs: > class TypeVal a t | t -> a where > -- typeToVal should ignore its argument. > typeToVal :: t -> a > > data Wrapper a = forall t . (TypeVal a t) => Wrapper t > > class ValToType a where > valToType :: a -> Wrapper a `valToType' takes a value `x' and returns a (wrapped version of a) fake value in a new type; from the new type, `x' can be recovered by applying typeToVal. This code works under ghc. It uses existentially quantified data constructors, scoped type variables, and explicit universal quantification. Enjoy, Dylan Thurston

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Singular Type Constructor
by Ashley Yakeley 21 Aug '01

21 Aug '01

Consider: -- data T a = T -- Is there anything in the Prelude or a standard library that looks like this? Should there be? What should it be called? 'Singleton'? 'Reified'? 'T'? I looked but couldn't find anything. Such a simple type constructor is in fact very useful when you need to pass types around to disambiguate class instances. For instance: -- class HasTypeName t where getTypeName :: T t -> String instance HasTypeName Int where getTypeName T = "Int" instance HasTypeName () where getTypeName T = "()" intName = getTypeName (T :: T Int) -- -- Ashley Yakeley, Seattle WA

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deriving
by Mark Carroll 21 Aug '01

21 Aug '01

Does anyone know of any Haskell texts that introduce 'deriving' fairly comprehensively? All I've been able to find out from what I have is that your type can derive Eq, Ord, Show etc. and then they'll magically work as if you'd written the instance declarations yourself. I've no idea how you can make your own type classes that types can 'derive' from, etc. IYSWIM, or really what's going on at all. -- Mark

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Re: Subtyping
by Mansour Al-Mutairi 20 Aug '01

20 Aug '01

Hi, I am a newbie but I think that what you are proposing can be simulated in haskell. It is like subclassing in OOP. You can create a new type that is an extension of the older one. Unlike OOP overloading, you will need to create a new function that can work on the new type. >data SomeObject = SomeInt Int | SomeString String | _ will become: data SomeObject = SomeInt Int | SomeString String >...later... > >data SomeObject |= SomeReal Double will become: data SomeObject2 = SO SomeObject | SomeReal Double >same :: SomeObject -> SomeObject -> Bool >same (SomeInt a) (SomeInt b) = a == b >same (SomeString a) (SomeString b) = a == b >same _ _ = False > >...later... > >same (SomeReal a) (SomeReal b) = a == b will become: same :: SomeObject -> SomeObject -> Bool same (SomeInt a) (SomeInt b) = a == b same (SomeString a) (SomeString b) = a == b ...later... same2 :: SomeObject2 -> SomeObject2 -> Bool same2 (SomeReal a) (SomeReal b) = a == b same2 x y = same x y tell me if I am wrong. thanks. Mansour Al-Mutairi :) _________________________________________________________________ Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp

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Re: trying to run programs
by Patrik Jansson 20 Aug '01

20 Aug '01

[I move this thread to haskell-cafe.] On Mon, 20 Aug 2001, Johansson Mikael wrote: > I'm starting up using a GHC-compiler and have successfully compiled a simple > "Hello World" program. Thanks for the responses. > Now I'm trying to use some simple programs from haskell.org/hugs but there are > problems. > > AnsiDemo.hs:10: Variable not in scope: `writeStr' > > It seems some object are missing. I have tried to find them > in ghc/lib/imports and others but haven't succeded. > Is there some libraries I need to run these hugs-files on > my ghc-compiler? AnsiDemo.hs imports some hugs-specific files (AnsiInteract.hs, ...) which are also available in the hugs distribution but (somewhat confusing for your use) not in the demos directory. [Maybe the hugs example suite could be transformed into a stand-alone Haskell example suite? Any takers?] If you have hugs installed, then the necessary files are in ../lib/hugs (seen from the demos directory). I think the files for AnsiDemo are propoer Haskell 98 and not using any hugs-extensions so it should work. (Using ghci-5.0 it "compiles" OK, but on my Solaris xterm the printing is delayed - that might work better under Window$.) For use with ghc the required files could either be copied/moved into the demos directory, or you could start ghc with -i../lib/hugs added to the command line (so that it searches theat directory as well). I attach a tarball with the directory tree including lib and demos from my installation of hugs. Good luck, Patrik Jansson

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