March 2001 - Haskell-Cafe

Re: constants and functions without arguments
by Tom Pledger 30 Mar '01

30 Mar '01

Andreas Leitner writes: : | The classic examlple for this is class POINT, which has an interface | like that: | -- | POINT | -- group 1 | x: DOUBLE | y: DOUBLE | -- group 2 | rho: DOUBLE | abs: DOUBLE | -- | Now, you can choose to either implement group 1 as attributes, or | group 2 and calculate the other one. With the uniform access | principle, which says that accessing attributes and functions with no | arguments happen in the same syntactic way, you can change the | implementaion without notice. No user of this class needs to know. | | I am to new to FP to say whether this is equaly important in | FP-languages, since the semantics are different (constants vs. | attributes and immutable values vs. objects), but for now I am just | currious wether it is possible theoretically. That uniformity of access can certainly be achieved in Haskell. Here's the Point example. I've done it with two constructors so as to get fewer floating point imprecisions, but some other possible representations (e.g. one with only the Polar constructor) would look identical outside the module. module Point(Point, cartesian, polar, x, y, r, theta) where -- data type with hidden constructors data Point = Cartesian Double Double -- corresponds to group 1 | Polar Double Double -- corresponds to group 2 -- proxy constructor functions cartesian x y = Cartesian x y polar r theta = Polar r theta -- attribute extraction functions x (Cartesian x y) = x x (Polar r theta) = r * cos theta y (Cartesian x y) = y y (Polar r theta) = r * sin theta r (Cartesian x y) = sqrt (x*x + y*y) r (Polar r theta) = r theta (Cartesian x y) = atan2 y x theta (Polar r theta) = theta

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Instances of Read and Show
by Ingo Sander 30 Mar '01

30 Mar '01

I have a question concerning the Read and Show classes. I have read the Haskell 98 report (section 6.3.3 The Read and Show Classes), but I still do not figure out how to make instances. I have a simple data type: data TVal a = Abst | Prst a I managed to define the function show, so that I can use it with the data type TVal instance (Show a) => Show (TVal a) where show Abst = "_" show (Prst x) = show x Signals> [Prst 1, Abst, Prst 2] [1,_,2] :: [TVal Integer] which is what I want to have. But how do I make a Read instance? I still do not understand the meaning of 'readsPrec' and 'readList' (and not of 'showPrec' and 'showList') either. Can somebody explain to me or give me a good reference so that I can make TVal an instance of the classes Read and Show. Thanks in advance for your help! Ingo Sander

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RE: "Lambda Dance", Haskell polemic, etc. on O'Reilly site
by Simon Marlow 30 Mar '01

30 Mar '01

> There are, perhaps good reasons for all these failures, but please > recognize them for what they are: failures to meet the needs of the > working programmer who just wants to get something done > (whether he/she > lives in YU, FR, JP, or US) > > I would love to be using Haskell for day to day programming work but I > can't at this time. Unless the community recognizes what is > required and > gets it done, I never will. There is one good reason that Haskell fails in these areas, and that's shortage of effort. We in the GHC team are actively trying to encourage our users to contribute: with an open CVS source tree, open development mailing lists, etc. We've had some good results (thanks to our developers!), but we need more. GHC is a large and complex system, and with trying to be all things to all people - a research vehicle / experimentation platform / production compiler, our developer resources are stretched a bit thin. I'm sure this goes for the other Haskell systems too. I see libraries as the biggest missing piece in the Haskell language. Let's face it, Joe Programmer isn't going to pass Haskell by just because it doesn't support a particular flavour of overlapping instance declarations. But he might if it doesn't have a networking library, or unicode support, or an XML parser. Please, join libraries(a)haskell.org. </rant> Cheers, Simon

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"Lambda Dance", Haskell polemic, etc. on O'Reilly site
by Fritz K Ruehr 30 Mar '01

30 Mar '01

Forgive me if the following links were announced here and I just missed them. I found them through the O'Reilley Meerkat "Open Wire Service", in the "Lambda" channel of the "Lang" category (see: <http://www.oreillynet.com/meerkat/index.php?&c=4623&t=ALL> ). The first link is the "Lambda Dance" page, presumably inspired by the infamous "Hamster Dance" (if you don't know the latter, you are better off for it, so don't ask): <http://lambdadance.spacebar.org/> "Beta-reduce the night away", as the site says! The second link is a little polemic entitled "Why People Aren't Using Haskell", which I presume would be of interest to readers of this list (the author is Dejan Jelovic, whom I don't recognize as a regular contributor here): <http://www.jelovic.com/articles/why_people_arent_using_haskell.htm> There are several other interesting looking links on the Meerkat site ("Moving FP from Academia", "Object Functional Patterns", etc.), but I will let you investigate on your own. Enjoy ... -- Fritz Ruehr, still humming to the lambda dance

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Printed Haskell 98 Report
by Mark Carroll 29 Mar '01

29 Mar '01

I don't suppose it's possible to get a reasonable printed copy of the Haskell 98 Report and Haskell 98 Library Report at a sensible price, is it? Just wondering, before I go pricing printing and binding. Thanks. -- Mark

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(no subject)
by Atenea Azcue 27 Mar '01

27 Mar '01

Hello My name is Atenea . I just want to ask you that how can i Creat a program that just say "hello word" and another that can add a "matrix" if you can help me I would really going to apreciate please. Thank you for your listening --------------------------------- Do You Yahoo!? Yahoo! Mail Personal Address - Get email at your own domain with Yahoo! Mail.

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(no subject)
by Atenea Azcue 27 Mar '01

27 Mar '01

Hello My name is Atenea : I�m what to ask you who can I make a program in haskell like une program that say hello word --------------------------------- Do You Yahoo!? Yahoo! Mail Personal Address - Get email at your own domain with Yahoo! Mail.

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Re: the constant functor
by Tom Pledger 27 Mar '01

27 Mar '01

Tobias Haeberlein writes: | | > What is wrong with: | > | > class Functor ConstInt where | > map f (Const n) = Const n | | Ups - absolutely nothing. | I actually tried it with | | data ConstInt a = Int This version doesn't refer to the type Int. Instead, it introduces a new nullary data constructor called Int, which you could use like this: instance Functor ConstInt where fmap f Int = Int ...but I don't think that's what you were after. If you use a type alias (type ConstInt a = Int) instead of declaring a data type, it will refer to the type Int. However, then you'd need to use ConstInt partially applied in the instance declaration, which Haskell doesn't allow. I think Gofer has a more liberal type system which does allow it. | and didnt manage to make this an instance of | class Functor. Sorry for giving the wrong | example.

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Re: the constant functor
by Koen Claessen 27 Mar '01

27 Mar '01

(moved reply to haskell-cafe) Hi Tobias, | data ConstInt a = Const Int | | Any ideas how to make 'ConstInt' an instance of class | Functor? What is wrong with: class Functor ConstInt where map f (Const n) = Const n ? /Koen. -- Koen Claessen http://www.cs.chalmers.se/~koen phone:+46-31-772 5424 mailto:koen@cs.chalmers.se ----------------------------------------------------- Chalmers University of Technology, Gothenburg, Sweden

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Re: Linearity Requirement for Patterns
by Michael Hanus 22 Mar '01

22 Mar '01

Fergus Henderson wrote: > My understanding, from talking with Michael Hanus a few years ago, is > that Curry is not referentially transparent, in the sense that you can't > replace equals with equals, because it permits nondeterministic functions. > In particular, `let x = f y in g x x' is not the same as `g (f y) (f y)' > in Curry. Of course, as you already mentioned, one has te be very precise when talking about referential transparency. In particular, one has to say what "equals" mean. Otherwise, one could also argue that Haskell is not referential transparent: if a Haskell program has a defining equation "f 1 = 1", the expressions "(f 1)" and "1" might not be identical (if there is another equation like "f x = 0" before). In Curry, each defining equation contributes to the semantics of a function independently of the other equations for the same function, and the overall goal is to compute all values of an expression (plus possible substitutions for free variables) which can be derived from this equational reading of all rules. In this sense, Curry is refentially transparent (although I have no formal definition of this notion but only soundness, completeness and optimality results). Your "counter example" comes from a misunderstanding of the "let" in Curry. Since the kernel language is based lazy evaluation w.r.t. a set of constructor-based rewrite rules, the "let" is considered as syntactic sugar for abbreviating some function argument. For instance, the meaning of the definition h = let x = f y in g x x is defined as an abbreviation for the rules h = aux (f y) aux x = g x x W.r.t. this semantics, there is no problem even with non-deterministic functions. If you have a precise definition of "referentially transparent" at hand so that you can make your statement more precise, I would be very interested to read it. As you have seen, the slogan "replacing equals by equals" is still to fuzzy since just looking for equality symbols in a language is not sufficient. Michael

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