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Re: Tree handling
by Lars Lundgren 26 Feb '01

26 Feb '01

On Mon, 26 Feb 2001, Martin Gustafsson wrote: > Hello > > I'm a haskell newbie that tries to create a tree with arbitary numbers of childs. > I create the data structure but i can't do anything on it can someone please help > me with a small function that sums the values of the leafs, so i don´t loose my hair > so fast. > > The datastructure looks like this and a binary tree built with it would look like this: > > > data GeneralTree = Nil | Node (Integer,[GeneralTree]) > As you said you were a newbie I will ask a few questions about your datastructure. Do you know that there is no need to tuple the elements in the Node if you do not want to. You can write: data GeneralTree = Nil | Node Integer [GeneralTree] What is the intended difference between (Node 5 []) and (Node 5 [Nil]) ? > > tree = > (20, > [ > (-20,[(30,[Nil]),(20,[Nil])]), > (40,[(65,[Nil]),(-40,[Nil])]) > ] > ) This is not of type GeneralTree! (And its layout is messed up) Hint: write the type of every expression you write, and debugging will be much easier. tree :: GeneralTree ERROR tree.hs:8 - Type error in explicitly typed binding *** Term : tree *** Type : (a,[(b,[(c,[GeneralTree])])]) *** Does not match : GeneralTree This is an expression with type GeneralTree: tree :: GeneralTree tree = Node 20 [Node (-20) [Node 30 [Nil], Node 20 [Nil]], Node 40 [Node 65 [Nil], Node (-40) [Nil]]] Now it should be very easy to write a function to sum the nodes in a tree sumTree :: GeneralTree -> Integer sumTree Nil = 0 sumTree (Node n ts) = ... write this yourself hint - sum and map are very useful functions (defined in the prelude) as are recursion. Good luck! /Lars L

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RE: stack overflow
by Scott Turner 26 Feb '01

26 Feb '01

At 01:26 2001-02-26 -0800, Simon Peyton-Jones wrote: >And so on. So we build up a giant chain of thunks. >Finally we evaluate the giant chain, and that builds up >a giant stack. > ... >If GHC were to inline foldl more vigorously, this would [not] happen. I'd hate to have my programs rely on implementation-dependent optimizations. BTW, I've wondered why the Prelude provides foldl, which commonly leads to this trap, and does not provide the strict variant foldl', which is useful enough that it's defined internal to the Hugs prelude. Simple prejudice against strictness? -- Scott Turner p.turner(a)computer.org http://www.billygoat.org/pkturner

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25 Feb '01

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Re: unliftM
by Lars Lundgren 23 Feb '01

23 Feb '01

On 23 Feb 2001, Julian Assange wrote: > > Is there a standard construct for something of this ilk: > > unliftM :: Monad m a -> a > I do not know if it is a standard, but every monad usually has a "runMonad" function. For ST you have runST, for IO you have unsafePerformIO and for your own monad you need to define it. Note, that if you use unsafePerformIO you, the action must not have (important) sideeffects. The burden of proof is upon YOU. /Lars L

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Re: Inferring from context declarations
by D. Tweed 22 Feb '01

22 Feb '01

George Russell wrote: > > (3) Simon Peyton Jones' comments about dictionary passing are a red herring, > since they assume a particular form of compiler. Various (MLj, MLton) > ML compilers already inline out all polymorphism. Some C++ compilers/linkers > do it in a rather crude way as well, for templates. If you can do it, > you can forget about dictionary passing. [Standard disclaimer: I write prototype code that's never `finished' to ever-changing specs in a university environment; other people probably view things differently.] I'm not sure I'd agree about this. Note that there's two levels, inlining polymorphic functions at the call site and `instantiating polymorphic functions at each usage type' without doing the inlining. C++ compilers have to at least do the second because of the prevailing philosophy of what templates are (i.e., that they're safer function-macros). Some of the time this is what's wanted, but sometimes it imposes annoying compilation issues (the source code of the polymorphic function has to be available everytime you want to use the function on a new class, even if its not time critical, which isn't the case for Haskell). I also often write/generate very large polymorphic functions that in an ideal world (where compilers are can do _serious, serious_ magic) I'd prefer to work using something similar to a dictionary passing implementation. I'd argue that keeping flexibility about polymorphic function implementation (which assumes some default but can be overridden by the programmer) in Haskell compilers is a Good Thing. Given that, unless computing hardware really revolutionises, the `speed/memory' profile of todays desktop PC is going to recurr in wearable computers/PDAs/etc I believe that in 20 years time we'll still be figuring out the same trade-offs, and so need to keep flexibility. ___cheers,_dave________________________________________________________ www.cs.bris.ac.uk/~tweed/pi.htm|tweed's law: however many computers email: tweed(a)cs.bris.ac.uk | you have, half your time is spent work tel: (0117) 954-5250 | waiting for compilations to finish.

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Re: Primitive types and Prelude shenanigans
by Ketil Malde 22 Feb '01

22 Feb '01

qrczak(a)knm.org.pl (Marcin 'Qrczak' Kowalczyk) writes: > 'negate' defined, as it's obviously meaningless for them. We can put > 'fromInteger' in some class and 'negate' in its subclass, and make > only the former instance for natural numbers. Nitpick: not necessarily its subclass, either. We can probably imagine types where negate makes sense, but fromInteger does not, as well as vice versa. -kzm -- If I haven't seen further, it is by standing in the footprints of giants

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making a Set
by Tom Pledger 21 Feb '01

21 Feb '01

(moved to haskell-cafe) G Murali writes: | hi there, | | I'm tryng to get my concepts right here.. can you please help in | defining a funtion like | | makeSet :: (a->Bool)->Set a | | I understand that we need a new type Set like | data Set a = Set (a->Bool) what puzzles me is how to apply the funtion | to all elements belonging to type a. What other operations do you need to implement for "Set a"? Is there anything that can't be expressed in terms of those set membership functions you already have?

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RE: Primitive types and Prelude shenanigans
by Simon Peyton-Jones 21 Feb '01

21 Feb '01

I don't mind doing this, but can someone first give a brief justification about why it's a good idea, independent of the discussion that has taken place on this list? I'd like to add such an explanation to the code. Simon | -----Original Message----- | From: qrczak(a)knm.org.pl [mailto:qrczak@knm.org.pl] | Sent: 16 February 2001 17:42 | To: haskell-cafe(a)haskell.org | Subject: Re: Primitive types and Prelude shenanigans | | | Thu, 15 Feb 2001 20:56:20 -0800, William Lee Irwin III | <wli(a)holomorphy.com> pisze: | | > literal "5" gets mapped to (fromPositiveInteger 5) | > literal "-9" gets mapped to (fromNonZeroInteger -9) | | Note that when a discussed generic Prelude replacement | framework is done, and ghc's rules are changed to expand -9 to | negate (fromInteger 9) instead of fromInteger (-9), then you don't | need uglification of the fromInteger function to be able to define | types with only nonnegative numeric values. Just define your negate | in an appropriate class, different from the fromInteger's class. | | -- | __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ | \__/ | ^^ SYGNATURA ZASTEPCZA | QRCZAK | | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe(a)haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe |

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RE: newbie: running a state transformer in context of a state rea der
by Konst Sushenko 21 Feb '01

21 Feb '01

Marcin, thanks for your help. to implement the lift functionality i added these well known definitions: class (Monad m, Monad (t m)) => TransMonad t m where lift :: m a -> t m a instance (Monad m, Monad (State s m)) => TransMonad (State s) m where lift m = ST (\s -> m >>= (\a -> return (a,s))) but my lookahead function lookahead p = do { s <- fetch ; lift (evalState p s) } is typed as lookahead :: State MyState Maybe a -> State MyState Maybe (a,MyState) but i need lookahead :: State MyState Maybe a -> State MyState Maybe a apparently, the (>>=) and return used in the definition of lift above are for the monad (State s m), and not monad m... everything works if i do not use the TransMonad class, but define lift manually as: lift :: Parser a -> Parser a lift m = ST (\s -> unST m s >>= (\(a,_) -> return (a,s))) but this looks like a special case of the lift above, except the right hand side of 'bind' is executed in the right context. i am still missing something konst -----Original Message----- From: Marcin 'Qrczak' Kowalczyk [mailto:qrczak@knm.org.pl] Sent: Tuesday, February 20, 2001 10:17 AM To: haskell-cafe(a)haskell.org Subject: Re: newbie: running a state transformer in context of a state reader Mon, 19 Feb 2001 18:07:17 -0800, Konst Sushenko <konsu(a)microsoft.com> pisze: > now i am curious if it is possible to run the given parser (state > transformer) in a context of a state reader somehow, so as the state > gets preserved automatically. something that would let me omit the > calls to fetch and set methods. It should be possible to do something like this: lookahead:: Parser a -> Parser a lookahead p = do { s <- fetch ; lift (evalState p s) } where evalState :: Monad m => State s m a -> s -> m a lift :: Monad m => m a -> State s m a are functions which should be available or implementable in a monad transformer framework. I don't have the Hutton/Meijer's paper at hand so I don't know if they provided them and under which names. Such functions are provided e.g. in the framework provided with ghc (by Andy Gill, inspired by Mark P Jones' paper "Functional Programming with Overloading and Higher-Order Polymorphism"). This definition of lookahead uses a separate state transformer thread instead of making changes in place and undoing them later. I don't think that it could make sense to convert a state transformer to a state reader by replacing its internals, because p does want to transform the state locally; a value of type Parser a represents a state transformation. The changes must be isolated from the main parser, but they must happen in some context. -- __("< Marcin Kowalczyk * qrczak(a)knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTEPCZA QRCZAK _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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RE: Just for your fun and horror
by Manuel M. T. Chakravarty 20 Feb '01

20 Feb '01

Jon Cast <jcast(a)ou.edu> wrote, > Manuel M. T. Chakravarty writes: > > So, I guess, I have to extend my example to > > > > bar x = x + 42 > > > > I don't know if this counts, but gcc allows: > > int bar(int x)__attribute__(const) > { > return(x + 42); > } > > which is the exact C analogue of the Haskell syntax. Sorry, but I would say that it doesn't count as it is a compiler specific extension :-) Nevertheless, a good point. > The majority of `C > functions', I believe, (and especially in well-written code) are intended to > be true functions, not IO monads. They modify the state for > efficiency/ignorance reasons, not because of a conscious decision. Yes and no. I agree that they are often intended to be true functions. However, it is not only efficiency and ignorance which forces side effects on the C programmer. Restrictions of the language like the lack of call-by-reference arguments and (true) multi-valued returns force the use of pointers upon the programmer. Anyway, I don't want to do C bashing here - although, on this list, I might get away with it ;-) Cheers, Manuel

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