- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] exceptions vs. Either
by David Menendez 04 Aug '04

04 Aug '04

David Menendez writes: > MR K P SCHUPKE writes: > > > I would suggest using the type system as I said earlier so: > > > > toNonEmptyList :: [a] -> Maybe (NonEmpty a) > > toNonEmptyList (a0:_) = Just (NonEmpty a) > > toNonEmptyList _ = Nothing > > > > Then redefine head: > > > > head :: NonEmpty a -> a > > head (NonEmpty (a0:_)) = a0 > > Oleg described a similar technique a few months ago. > > From <http://haskell.org/pipermail/haskell/2004-June/014271.html>: > > |> newtype NonEmpty a = NonEmpty [a] -- the NonEmpty constructor should > |> -- be hidden (not exported from its module) > |> > |> head' (NonEmpty a) = head a -- no error can occur! Can use unsafe > version > |> tail' (NonEmpty a) = tail a -- no error can occur! Can use unsafe > version > |> > |> -- trusted function: the only one that can use NonEmpty constructor. > |> fork_list_len f g x = if null x then f else g (NonEmpty x) > |> > |> revers x = revers' x [] > |> where > |> revers' x accum = fork_list_len accum (g accum) x > |> g accum x = revers' (tail' x) ((head' x):accum) > > We have these equivalences: > > toNonEmptyList == fork_list_len Nothing Just > fork_list_len d f == maybe d f . toNonEmptyList > > I think defining 'toNonEmptyList' in terms of 'fork_list_len' is > cleaner, but that's just my personal taste. (As it happens, I ended up > defining a very similar function 'cons' in my recursion module[1]). > > [1] > <http://www.eyrie.org/~zednenem/2004/hsce/Control.Recursion.html#v% > 3Acons> -- David Menendez <zednenem(a)psualum.com> <http://www.eyrie.org/~zednenem/>

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Re: [Haskell-cafe] exceptions vs. Either
by MR K P SCHUPKE 04 Aug '04

04 Aug '04

A lot of programming errors come from failure to correctly validate. I remember being taught to validate on input _and_ validate when you use a value. So I would say do both - use maybe for input functions, and exceptions for when you use a value. You can use the type system to make this more efficient. Imagine you want an integer between 10 and 20, you have a function like: newtype ValidInt = Valid Int validateInt :: Int -> Maybe ValidInt validateInt x | x>=10 && x<=20 = Just (Valid x) validateInt _ = Nothing Now when you validate on input - you produce an error if you get "Nothing" and in all the functions you use the value you use ValidInt instead of Int. This way you use the type system to differentiate between a valid int (does not need to be rechecked) and a normal int (which would need to be validated). Of course you must make sure you don't write more than one constructor function for ValidInt. This suggests a form of constrained constructor would be useful... Whilst this can be done in Haskell, it is not easy and involves reifying the value to a type, and using a type class to constrain the data statement. Really here we are dealing with dependant types, and this is much easier in a dependantly typed language like Epigram. Keean.

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Re: [Haskell-cafe] exceptions vs. Either
by MR K P SCHUPKE 03 Aug '04

03 Aug '04

>> Is there any easy way (TH?) to amend these to output the line number Or use GHC's reifyLocation 'function' in a mapException. Keean.

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Modifying Running Programs
by S. Alexander Jacobson 02 Aug '04

02 Aug '04

eBay modifies 30k lines of code per week while maintaining 99.92% uptime. Is there a way to force ghc (or some other Haskell implementation) to load new implementations of functions without restarting the program? Alternatively, is there a way of restarting a program with new code without having to reconstruct all its in memory state? -Alex- ______________________________________________________________ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com

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Re: [Haskell-cafe] Modifying Running Programs
by MR K P SCHUPKE 02 Aug '04

02 Aug '04

>Is there a way to force ghc (or some other Haskell >implementation) to load new implementations of >functions without restarting the program? Yes, if you use the dynamic loading interface fron ghci. Code can be loaded and unloaded at runtime (however I believe the actual memory is never recovered) Keean.

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Multiprocessor compiling.
by Matthew Roberts 02 Aug '04

02 Aug '04

It seems there is quite some interest in compiling code for multi-processor machines. As a research student, I am always looking for interesting projects to do. However, I am no haskell guru and have certainly never written a compiler. All the comments so far have been very interesting, but too many go over my head and I am not gettting a coherent view of what is going on. Let me try to tie this up. The basic computational model of functional languages seems inherantly parallilisable, but there are clearly many problems - and I don't really understand them. I just picked up the book on implementing the back end of a Haskell compiler and so I will eventually get there. For the meantime, and for all the others out there in my situation, could someone who knows tell us the fundamental reason it is hard to compile Haskell for multiple processor machines - in broad strokes. Cheers, Matt

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Hello and help request
by enrico.santoemma＠tin.it 01 Aug '04

01 Aug '04

Hello to everyone :) My name is Enrico, 40, and I write software for the industry at Beta 80 Group. After this short introduction, here is the help request: I'm managing the rewrite of an application, to port it to the 'classic' Java three-tiers architecture. Before I begin to feed the team, I want to see a fully functional prototype of the problem. I mean the app's functions, not the graphic. What I imagine is a data base of types and groups of functions. The idea is to study the problem, inject new functions while checking their coerence with the existing type infrastructure, and so on. In plain Java, I'd have wrote the Business Idea in informal language, extracted the types from it, assigned methods to types and then glued things together in some way. In Haskell I (should) have the opportunity to stay on the problem, decomposing it into sub-problems until I come to the soil. What I ask to this list is if and where Haskell is different from the object oriented way to decompose the problem. I'm very fresh on Haskell. Exactly... chapter 13 of SOE :) Are type classes different from Java classes? I think so, but I guess this means that one should ask different things to type classes, respect to what is being asked to Java classes. It is this difference that's unclear to me. Any help would be very appreciated Enrico

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Re: [Haskell-cafe] Multiprocessor compiling.
by MR K P SCHUPKE 01 Aug '04

01 Aug '04

>want to fork 3 threads to compute "a*b + c*d" of course this depends on what a b c and d are ... if they are 1000x1000 matricies then you may well want the '*' operations in separate threads... Keean.

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Re: [Haskell-cafe] Re: humor
by Jon Cast 31 Jul '04

31 Jul '04

Sean Perry <sean.perry(a)intransa.com> wrote: > Gabriel Ebner wrote: > > enrico.santoemma(a)tin.it writes: > > > > > >>Well... more seriously, does someone have got links to applications with > >>visible source? > > > > > > The only one I know is darcs: http://abridgegame.org/darcs > > > > which is actually quite readable, even for a newbie. He does like the > '$' operator a bit much for my tastes though. Obviously you haven't realized the great beauty of $. You cannot like $ to much. foldr should be defined as follows: foldr f z [] = id $ z foldr f z (x:xn) = (f $ x) $ foldr f z xn etc. :) Jon Cast

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humor
by enrico.santoemma＠tin.it 31 Jul '04

31 Jul '04

When I met Java some time ago, I said to myself: "Java is a great language to write IDEs for the Java language". Now that I'm looking for Haskell source code (perhpas that there is another way to learn a language than to spy on others?), I'm tempted to say: "Haskell is a great language to write libraries for the Haskell language" Well... more seriously, does someone have got links to applications with visible source? Enrico Santoemma

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