Re: threaded red-black tree
this might help: http://www.cs.kent.ac.uk/people/staff/smk/redblack/Untyped.hs andrew Lex Stein said: Hi, No one responded to my question several weeks ago about a purely functional implementation of a threaded, red-black tree. My message was sent about the same time as that flurry of silly emails about how to respond to homework questions. Was my message not responded to because it was classified as a homework question? I assure you this is officework, not homework. I ended up porting Okasaki's red-black tree implementation; hacking it apart with a bunch of mutation for the threading of the list through the tree. However, I'm still missing a deletion function and I haven't been able to find a prototype (Okasaki's red-black tree module lacks delete). My study of the RB-tree deletion routine in CLR hasn't yet yielded an adaptation for a functional environment. Any advice would be much appreciated. Thanks, Lex -- Lex Stein http://www.eecs.harvard.edu/~stein/ [EMAIL PROTECTED]cell: 617-233-0246 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe -- http://www.acooke.org/andrew ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Read file problems
HI, I have a recurrent probleme with function readFile and Monad : it is that i try to get a line, transform it and stock it in a list and what i want is that function return me the list. But hugs say me that in readFile my_file = \s - map cons_line (lines s) readFile is a IO String type but is used here as [[Char]] type... and i don't know what to do... Ghc say me that is the lambda abstraction that fails with map cons_line (lines s) , he couldn't match IO against [] ! Help me please, i understand well (i hope) how Monad work but here i don't see a solution. Sorry for my poor english but i'm already a student. Thanks for all. -- C, socks and sun ;-) and haskell bugs :-( ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Instance checking and phantom types
Nick Name wrote: Hi all, I have an example wich I don't understand: First of all, let's rename the constructors and types a bit to make things clearer add the instance in question, and remove the type signatures: module Main where class C t data T = MkT instance C T instance C () data C t = T1 t = MkT1 f1 = MkT1 data C t = T2 t = MkT2 t f2 = MkT2 () Then we can easily ask GHC: [EMAIL PROTECTED]:~ ghci -v0 Main.hs *Main :i T1 MkT1 f1 T2 MkT2 f2 -- T1 is a type constructor, defined at Main.hs:8 data (C t) = T1 t = MkT1 -- MkT1 is a data constructor, defined at Main.hs:8 MkT1 :: forall t. T1 t -- f1 is a variable, defined at Main.hs:10 f1 :: forall t. T1 t -- T2 is a type constructor, defined at Main.hs:12 data (C t) = T2 t = MkT2 t -- MkT2 is a data constructor, defined at Main.hs:12 MkT2 :: forall t. (C t) = t - T2 t -- f2 is a variable, defined at Main.hs:14 f2 :: T2 () The first function, f1, is accepted both by hugs and ghc, unlike the second wich is rejected. Why does this happen? Shouldn't f1 be rejected with no instance C () The reason is buried in http://haskell.org/onlinereport/decls.html#sect4.2.1 In a nutshell: The context in datatype declarations has only an effect for the *data* constructors of that type which use the type variables mentioned in the context. Contexts have no effect for the *type* constructor. IIRC the reason for this design decision was that contexts in type signatures should always be explicit. Cheers, S. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Instance checking and phantom types
Nick Name wrote: Got it ;) Thanks for prompt reply. What does should always be explicit mean? Is there a notion of explicit context that I should know? What I meant was the fact that you always have to write down *all* contexts involved in a type signature. Nothing is inherited under the hood by contexts in datatype declarations. Cheers, S. ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
