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Re: [Haskell-cafe] What's the problem with iota's type signature?
by michael rice 29 May '09

29 May '09

And I thought everyone was puzzling these type signatures out all by themselves. ;-) The Haskell learning curve is about the steepest I've ever seen for a computer language, but I think I'm catching on. Thanks for the info. Michael --- On Thu, 5/28/09, wren ng thornton <wren(a)freegeek.org> wrote: From: wren ng thornton <wren(a)freegeek.org> Subject: Re: [Haskell-cafe] What's the problem with iota's type signature? To: "Haskell-cafe" <haskell-cafe(a)haskell.org> Date: Thursday, May 28, 2009, 11:14 PM michael rice wrote: > Yeah, I went back and tried double again, though I'd swear I got the dang thing to compile (and run) w/o errors. > > I guess I meant Num. So Num is a class and Int and Integer are types? What are the other classes? Docs? > > Unification, for me, is pattern matching ala Prolog. What's the meaning in Haskell? Docs? > > I've been digging into this stuff for months and it's still tripping me up. Very frustrating. As others have mentioned, using GHCi can give you a lot of information. GHCi commands begin with a colon followed by the command (kinda like vi), whereas anything not beginning with a colon is treated as a Haskell expression to be evaluated. The :? command will list all the GHCi commands, which is a good place to start. The :type command takes a Haskell expression and prints the type inferred for it. You can also use the command ":set +t" which will make GHCi print the type after evaluating each expression; you may want to use this until you get the hang of things. The :info command takes a whitespace separated list of identifiers and will print their type and other information (e.g. what module it comes from, what type class a function belongs to, all the functions that are members of a type class, fixity of operators,...) As far as type classes and the like, there are a bunch. Hoogle[1] is an excellent API search engine for looking up this kind of stuff. Just enter the name of a function, type class, data type, or enter a type signature. The results give links to the online Haddock documentation (either GHC official docs, or Hackage docs). Unification is used when doing type inference. It's the same unification as in Prolog, only at the type level. Because of the way inference works, it happens that the |a| and |b| type variables must be unified for both of the functions you gave. A whole lot more can be said here, though I'm not sure how much you care :) The Wikipedia page on Hindley--Milner[2] gives a good starting point, though Haskell's type inference has more bells and whistles than the basic HM algorithm. [1] http://haskell.org/hoogle/ [2] http://en.wikipedia.org/wiki/Hindley-Milner -- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Re: [Haskell-cafe] Problem w/YAHT code example
by michael rice 29 May '09

29 May '09

Thanks! I just copied the code from the PDF file. Whatever that character was it wasn't the correct one. Michael --- On Thu, 5/28/09, Andrew Wagner <wagner.andrew(a)gmail.com> wrote: From: Andrew Wagner <wagner.andrew(a)gmail.com> Subject: Re: [Haskell-cafe] Problem w/YAHT code example To: "michael rice" <nowgate(a)yahoo.com> Cc: haskell-cafe(a)haskell.org Date: Thursday, May 28, 2009, 10:01 PM I'm not sure what you're using at the end of the identifier "search'", but it needs to be the single quote that's on the same key as the double quotes. I suspect that's where it's blowing up. On Thu, May 28, 2009 at 9:56 PM, michael rice <nowgate(a)yahoo.com> wrote: This code, from YAHT (Section 8.4.2, PDF pg. 119 of 192) seems to have a problem. Code below. Michael ================= [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l graph [1 of 1] Compiling Main ( graph.hs, interpreted ) graph.hs:6:24: lexical error at character '\8217' Failed, modules loaded: none. ============== Prelude> data Graph v e = Graph [(Int,v)] [(Int,Int,e)] search :: Graph v e -> Int -> Int -> Maybe [Int] search g@(Graph vl el) src dst | src == dst = Just [src] | otherwise = search’ el where search’ [] = Nothing search’ ((u,v,_):es) | src == u = case search g v dst of Just p -> Just (u:p) Nothing -> search’ es | otherwise = search’ es _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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We tried this functional, higher-order stuff with LISP and look what happened...
by Jason Dusek 29 May '09

29 May '09

What can we say to that? I'm well practiced in handling those who reject types outright (Python programmers), those who reject what is too different (C programmers), those who can not live without objects (Java programmers), those who insist we must move everything to message passing (Erlang programmers). It's not too often that I meet an embittered LISP programmer -- one who's well acquainted with a bold and well-supported community of functional programmers whose shooting star soon descended to dig a smoking hole in the ground. Who's to say Haskell (and the more typeful languages in general) do not find themselves in the same situation in just a few years' time? Is avoiding success at all costs really enough? -- Jason Dusek

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Re: [Haskell-cafe] What's the problem with iota's type signature?
by michael rice 29 May '09

29 May '09

Yeah, I went back and tried double again, though I'd swear I got the dang thing to compile (and run) w/o errors. I guess I meant Num. So Num is a class and Int and Integer are types? What are the other classes? Docs? Unification, for me, is pattern matching ala Prolog. What's the meaning in Haskell? Docs? I've been digging into this stuff for months and it's still tripping me up. Very frustrating. Thanks. Michael --- On Thu, 5/28/09, wren ng thornton <wren(a)freegeek.org> wrote: From: wren ng thornton <wren(a)freegeek.org> Subject: Re: [Haskell-cafe] What's the problem with iota's type signature? To: "Haskell-cafe" <haskell-cafe(a)haskell.org> Date: Thursday, May 28, 2009, 12:40 AM michael rice wrote: > Still exploring monads. I don't understand why the type signature for double is OK, It isn't. The |a| and |b| variables must unify: Prelude> :t \x -> Just (x+x) \x -> Just (x+x) :: (Num a) => a -> Maybe a Prelude> :t (\x -> Just (x+x)) :: Num a => a -> Maybe b <interactive>:1:13: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the polymorphic type `forall a b. (Num a) => a -> Maybe b' at <interactive>:1:0 `a' is a rigid type variable bound by the polymorphic type `forall a b. (Num a) => a -> Maybe b' at <interactive>:1:0 In the first argument of `Just', namely `(x + x)' In the expression: Just (x + x) > but not the one for iota. Again the |a| and |b| must unify. Also, Int isn't a type class (did you mean Num?) Also, the use of (..) requires the Enum class. Prelude> :t \n -> [1..n] \n -> [1..n] :: (Enum t, Num t) => t -> [t] Prelude> [3,4,5] >>= (\n -> [1..n]) [1,2,3,1,2,3,4,1,2,3,4,5] -- Live well, ~wren _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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Problem w/YAHT code example
by michael rice 29 May '09

29 May '09

This code, from YAHT (Section 8.4.2, PDF pg. 119 of 192) seems to have a problem. Code below. Michael ================= [michael@localhost ~]$ ghci GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Prelude> :l graph [1 of 1] Compiling Main ( graph.hs, interpreted ) graph.hs:6:24: lexical error at character '\8217' Failed, modules loaded: none. ============== Prelude> data Graph v e = Graph [(Int,v)] [(Int,Int,e)] search :: Graph v e -> Int -> Int -> Maybe [Int] search g@(Graph vl el) src dst | src == dst = Just [src] | otherwise = search’ el where search’ [] = Nothing search’ ((u,v,_):es) | src == u = case search g v dst of Just p -> Just (u:p) Nothing -> search’ es | otherwise = search’ es

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Who takes care of Haskell mailing lists?
by Maurício 29 May '09

29 May '09

Hi, I would like to create a mailing list for Portuguese speaking Haskell programmers. I tried checking haskell.org mailing lists page, but the only contact e-mail I see is 'mailman(a)haskell.org', and a message sent to that address is replied with an automatic message saying I'm not authorized to use it. Is there someone I could contact about that? Thanks, Maurício

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Parsec float
by Bartosz Wójcik 28 May '09

28 May '09

Hi Everybody (especially Parsec Creator), is there any reason why float parses only positive numbers? I find following defition: float = lexeme floating <?> "float" floating = do{ n <- decimal ; fractExponent n } If floating was defined like floating = do{ n <- integer ... or floating = do{ n <- int ... instead then it would parse negative ones as well. Best regards, Bartek

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ANNOUNCE: Hac φ: Haskell hackathon in Philadelphia, July 24-26
by Brent Yorgey 28 May '09

28 May '09

Greetings, I am very pleased to officially announce Hac phi, a Haskell hackathon/get-together to be held July 24-26 at the University of Pennsylvania in Philadelphia. The hackathon will officially kick off at 2:30 Friday afternoon, and go until 5pm on Sunday (with breaks for sleep, of course). Thanks to everyone who has already expressed interest in attending, it's shaping up to be a fun event. I want to stress that everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, please officially register [1], even if you already put your name on the wiki. Registration, travel, lodging and many other details can now be found on the Hac phi wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in Philadelphia! -The Hac φ team [1] http://haskell.org/haskellwiki/Hac_%CF%86/Register [2] http://haskell.org/haskellwiki/Hac_%CF%86 [3] http://haskell.org/haskellwiki/Hac_%CF%86/Talks

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Parsec float
by Malcolm Wallace 28 May '09

28 May '09

>> is there any reason why float parses only positive numbers? It is usual in parsing libraries to separate the recognition of a leading sign from recognition of the number itself: the sign-only parser can be reused in many contexts, e.g. in the Haskell'98 Numeric library, there is readSigned :: Real a => ReadS a -> ReadS a which takes a secondary parser as its argument. You can instantiate that argument to any of readDec :: Num a => ReadS a readHex :: Num a => ReadS a readFloat :: Num a => ReadS a etc. I do not know if the equivalent of "readSigned" exists in Parsec, but it in polyparse's Text.Parse module, it is called "parseSigned", rather unsurprisingly. Regards, Malcolm

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Stack overflow
by Krzysztof Skrzętnicki 28 May '09

28 May '09

Hello Cafe I'm currently writing an app with heavy use of message passing. To see which messages takes most of the bandwidth I wrote the following code: -- data Counter = CNT !Int !Int !Int !Int cntMsg (CNT a b c d) (MoveOther _ _) = (CNT a+1 b c d) cntMsg (CNT a b c d) (MoveSelf _) = (CNT a b+1 c d) cntMsg (CNT a b c d) (NilMsg) = (CNT a b c+1 d) cntMsg (CNT a b c d) (RoundEnd) = (CNT a b c d+1) emptyCnt = CNT 0 0 0 0 showCnt (CNT a b c d) = printf "CNT MoveOther=%d MoveSelf=%d NilMsg=%d RoundEnd=%d" a b c d -- The code for modifying the counter: (\ msg -> atomicModifyIORef ioref (\ cnt -> (cntMsg cnt msg,()))) Running it without increased stack blows it. With 200M stack I get after a second or so: CNT MoveOther=2125764 MoveSelf=0 NilMsg=0 RoundEnd=2916 The datatype itself is strict. So where is the thunk actually accumulating? Best regards Christopher Skrzętnicki

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