- Haskell-Cafe - Haskell.org

Novice Haskell query
by Mark Carroll 31 Jul '01

31 Jul '01

Let's say I have: data Tree a = Branch a (Tree a) (Tree a) | Leaf instance Show a => Show (Tree a) where show (Branch datum left right) = show left ++ " <- " ++ show datum ++ " -> " ++ show right show Leaf = "*" y = Leaf x = (Branch 3 Leaf Leaf) Now, "show x" will work, and "show y" won't. How can I write things so that I can show both x and y? I don't seem to be able to write, say, instance Show a => Show (Tree a) where show (Branch datum left right) = show left ++ " <- " ++ show datum ++ " -> " ++ show right instance Show (Tree a) where show Leaf = "*" or, perhaps, instance Show (Tree a) where Show a => show (Branch datum left right) = show left ++ " <- " ++ show datum ++ " -> " ++ show right show Leaf = "*" ...or whatever other things come to mind. -- Mark

2 1

RE: Singular Type Constructor
by Ashley Yakeley 31 Jul '01

31 Jul '01

At 2001-07-31 01:55, Simon Marlow wrote: >The type T is undoubtedly useful, but I think the example you give isn't >a good one. It works perfectly well without the T data type: > > class HasTypeName t where > getTypeName :: t -> String Perhaps. I think your use of 'undefined' is a bit ugly. The type 't -> String' suggests you need a value of type 't' to get a name for 't', which is not what I want to imply. 'undefined :: t' is intended to represent the type 't', which is not something intuitively "undefined". -- Ashley Yakeley, Seattle WA

1 0

RE: Singular Type Constructor
by Simon Marlow 31 Jul '01

31 Jul '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) The type T is undoubtedly useful, but I think the example you give isn't a good one. It works perfectly well without the T data type: class HasTypeName t where getTypeName :: t -> String instance HasTypeName Int where getTypeName _ = "Int" instance HasTypeName () where getTypeName _ = "()" intName = getTypeName (undefined :: Int) (this is just like the Typeable class in the Dynamic library, BTW) Cheers, Simon

1 0

Interactive shell
by Mark Carroll 30 Jul '01

30 Jul '01

People may recall that I was looking for an interactive Haskell shell at which I could type things and experiment, as you'd expect to get with any Lisp environment. At people's suggestion I installed GHCi and got it working, but I seem quite limited in what I can type at it. For instance, it balks at my various attempts to define a new algebraic type along the lines of "data Foo = Bar | Baz", for use in future intercourse. Is there some magical incantation I'm missing, or does it really not let me do that, and if not then does anything else? Thanks. -- Mark

1 0

RE: computer language shootout
by Dejan Jelovic 30 Jul '01

30 Jul '01

Josef Svenningsson wrote: > Having said that I must also say that I think that ghc's I/O is fast > *enough*. It's just that it doesn't compare very well with other > languages. I've never had problems caused by slow I/O in Haskell. This is the favorite argument of proponents of slow languages ;). But as long as people wait for their computers (to boot up, to run a program, to compile a program, to do a computation, to...) speed is going to be a virtue. Dejan http://www.jelovic.com

1 0

RE: computer language shootout
by Simon Marlow 30 Jul '01

30 Jul '01

> So am I. Have any haskell-performance gurus looked at any of > the code? Many of > the slowest entries are written in a very elegant, high-level > style that is > probably also relatively slow. I've looked at a few. The most common factor in the worst performers seems to be the performance of String-related operations. I tried converting a few to use PackedStrings but that didn't help much. I suspect our PackedString implementation could do with some tuning, and we could gain considerable benefit from having an hGetLinePS function (like hGetLine but gets a PackedString). Cheers, Simon

3 2

RE: computer language shootout
by Simon Marlow 30 Jul '01

30 Jul '01

> On Mon, 30 Jul 2001, Simon Marlow wrote: > > > I've looked at a few. The most common factor in the worst > performers > > seems to be the performance of String-related operations. I tried > > converting a few to use PackedStrings but that didn't help much. I > > suspect our PackedString implementation could do with some > tuning, and > > we could gain considerable benefit from having an > hGetLinePS function > > (like hGetLine but gets a PackedString). > > > Well, I think the common factor is I/O. The lazy I/O seems to > be a real > bottleneck here. Trying to improve that would gain even more, I think. > I'm saying this because I did quite a bit of fiddling with some of the > examples (including trying PackedStrings). The conclusion I drew from > doing this was that the I/O performance was the problem. Agreed - I/O is indeed a bottleneck, but I believe it's the strings-as-lists-of-characters aspect of I/O rather than the lazy aspect that's the killer. Enclosed is a version of the spell-checker program that is about a factor of 3 faster than the one that Doug currently has; it uses PackedStrings and a home-grown hash table. The only down side is that it reads the entire input into a big PackedString before producing anything, and profiling suggests that it spends about half its time splitting the big string into lines. I think if I hack up an hGetLinePS I might be able to get another factor of 2 out of it. Cheers, Simon

1 0

ST Questions
by Ashley Yakeley 28 Jul '01

28 Jul '01

Two questions on module ST: <http://www.haskell.org/ghc/docs/latest/set/sec-st.html> 1. What's the 's' parameter for? 2. Do STRefs get garbage-collected once they're forgotten? For instance, if I have an action that calls newSTRef, uses the ref, and then forgets it, and I run the action repeatedly, am I at risk of running out of memory? -- Ashley Yakeley, Seattle WA

2 1

getCh
by Michael Ackerman 27 Jul '01

27 Jul '01

Hello, In Bird's book, p 332, he writes of a "Haskell primitive function, getCh, which is similar to getChar except that the character is not echoed". I can find no reference to getCh in the Report or the docs for the libraries. How can this be defined? Thanks. - Michael Ackerman

1 0

Re: Simple compiler question
by Wolfgang Lux 26 Jul '01

26 Jul '01

Mark Carroll wrote > Do any of the decent Haskell compilers allow you to just type function > definitions at an interpreter prompt and use them in subsequent > interactions, as you'd expect from a Lisp environment? I don't know whether you consider hbi (the interactive version of hbc) a decent compiler, but it supports entering definitions as well: > let { f x = x + x }; f :: (Prelude.Num a) => a -> a > f 3; 6 Wolfgang -- Wolfgang Lux Phone: +49-251-83-38263 Institut fuer Wirtschaftinformatik FAX: +49-251-83-38259 Universitaet Muenster Email: wlux(a)uni-muenster.de

1 0