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";" in do
by Daryoush Mehrtash 30 Dec '10

30 Dec '10

Why do people put ";" in do {}, or "," in data fields, at the beginning of the line? -- Daryoush Weblog: http://onfp.blogspot.com/

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Incorrectly inferring type [t]
by william murphy 30 Dec '10

30 Dec '10

Hi All, I've spent a lot of time trying to write a version of concat, which concatenates lists of any "depth": So: concat'' [[[1,2],[3,4]],[[5]]] would return: [1,2,3,4,5] The code is: concat'' :: [a] -> [b] concat'' ((y:ys):xs) = (concat'' (y:ys)) ++ (concat'' xs) concat'' [] = [] concat'' (x:xs) = (x:xs) And the inevitable error is: test.hs:298:12: Couldn't match expected type `a' against inferred type `[t]' `a' is a rigid type variable bound by the type signature for `concat''' at test.hs:297:13 In the pattern: y : ys In the pattern: (y : ys) : xs In the definition of `concat''': concat'' ((y : ys) : xs) = (concat'' (y : ys)) ++ (concat'' xs) test.hs:300:24: Couldn't match expected type `b' against inferred type `[t]' `b' is a rigid type variable bound by the type signature for `concat''' at test.hs:297:20 In the first argument of `(:)', namely `x' In the expression: (x : xs) In the definition of `concat''': concat'' (x : xs) = (x : xs) Failed, modules loaded: none. Any help or advice would be appreciated. Will

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Lambda Calculus: Bound and Free formal definitions
by Mark Spezzano 30 Dec '10

30 Dec '10

Hi, Presently I am going through AJT Davie's text "An Introduction to Functional Programming Systems Using Haskell". On page 84, regarding formal definitions of FREE and BOUND variables he gives Defn 5.2 as The variable X is free in the expression E in the following cases a) <omitted> b) If E is a combination E1 E2 then X is free in E if and only if X is free in E1 or X is free in E2 c) <omitted> Then in Defn 5.3 he states The variable X is bound in the expression E in the following cases a) <omitted> b) If E is a combination E1 E2 then X is free in E if and only if X is free in E1 or X is free in E2. c) <omitted> Now, are these definitions correct? They seem to contradict each other....and they don't make much sense on their own either (try every combination of E1 and E2 for bound and free and you'll see what I mean). If it is correct then please give some examples of E1 and E2 showing exactly why. Personally I think that there's an error in the book. You can see the full text on Google Books (page 84) Thanks for reading! Mark Spezzano

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SampleVar semantics, documentation vs. source
by Eric Stansifer 30 Dec '10

30 Dec '10

Hi, doc: http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/Cont… source: http://www.haskell.org/ghc/docs/7.0-latest/html/libraries/base-4.3.0.0/src/… The documentation for Control.Concurrent.SampleVar implies that a SampleVar is either "filled" or "empty". The source code suggests that there are three possible states: "full" (when readers == 1), "empty" (when readers == 0), and "empty with blocked readers" (when readers < 0). In particular, the isEmptySampleVar function, isEmptySampleVar :: SampleVar a -> IO Bool isEmptySampleVar (SampleVar svar) = do (readers, _) <- readMVar svar return (readers == 0) returns false in the third state, when readers < 0. Can someone clarify the semantics of SampleVar? Thanks, Eric

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what is the status of haskell's mail libraries?
by Günther Schmidt 30 Dec '10

30 Dec '10

Hi all, I was just looking for mail libraries on hackage. You know libraries where I can construct an email, or retrieve on from the server. With retrieving an email I mean something with a bit more structure than a String, or, God help me, a ByteString. Where are we on this subject? I really cannot tell from the first glance. Günther

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How to make such code?
by Magicloud Magiclouds 30 Dec '10

30 Dec '10

Hi, In Haskell's way, normally, we write: if condition then expression_true else expression_false But I just heard that, in fancy way, we could do: do ifNeedToDo >> doIt bluh May I have any clue here? -- 竹密岂妨流水过山高哪阻野云飞

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Re: [Haskell-cafe] [Haskell-beginners] Tying the knot
by Antoine Latter 30 Dec '10

30 Dec '10

On Wed, Dec 29, 2010 at 7:22 PM, aditya siram <aditya.siram(a)gmail.com> wrote: > My brain turns into strange braid when I see this kind of thing. I > don't quite understand it and I've never used it in real world code > but I'll try and explain anyway. Caveat emptor. > Once when I was parsing a group of source files into an AST, the source files included 'copy' directives that allowed pieces of syntax to be a clone of some other piece of syntax - even across source files. So I did the whole process of parsing in the Reader monad, which was parametrized over the result of parsing all of the files. When I hit a copy directive, I simply called 'ask' and looked up the element I wanted to copy. It doesn't allow for separate processing of source files, but I didn't really need it. Antoine > First forget about 'labelLeaves' and think a function that only > returned the leaf count: > count :: Tree a -> Int > count tree = c > where > c = count' tree > > count' (Branch a b) = na+nb > where > na = count' a > nb = count' b > count' (Leaf _) = 1 > >> count $ Branch (Leaf "hello") (Leaf "world") > 2 > > Now look at 'n' and imagine it was a memory location. Mentally > substitute some hex address (like 0x0000) if it makes it easier. > Here's what the function looks like now: > > labelLeaves :: Tree a -> Tree Int > labelLeaves tree = tree' > where > (0x0000, tree') = label 0x0000 tree -- n is both result and argument! > > label 0x0000 (Branch a b) = (na+nb, Branch a' b') > where > (na,a') = label 0x0000 a > (nb,b') = label 0x0000 b > label 0x0000 (Leaf _) = (1, Leaf 0x0000) > > So if labelLeaves is given (Branch (Leaf "hello") (Leaf "world")) as > an argument, and we continue to think of 'n' as a memory location the > function returns something like: > (Branch (Leaf 0x0000) (Leaf 0x0000)) > > The part of the function where the leaves are counted up is exactly > like my 'count' example above, but when the function is done instead > of just returning it this line: > (n,tree') = label n tree > assigns the final count to 'n'. If 'n' is a memory location the final > leaf count would be sitting in 0x0000. Subbing the value at that > location into the result we get: > (Branch (Leaf 2) (Leaf 2)) > > > -deech > > On Wed, Dec 29, 2010 at 4:52 PM, Patrick LeBoutillier > <patrick.leboutillier(a)gmail.com> wrote: >> Heinrich, >> >>> A canonical example is the following solution to the problem of labeling all >>> the leaves in a tree with the total leaf count: >>> >>> data Tree a = Branch (Tree a) (Tree a) | Leaf a >>> >>> labelLeaves :: Tree a -> Tree Int >>> labelLeaves tree = tree' >>> where >>> (n, tree') = label n tree -- n is both result and argument! >>> >>> label n (Branch a b) = (na+nb, Branch a' b') >>> where >>> (na,a') = label n a >>> (nb,b') = label n b >>> label n (Leaf _) = (1, Leaf n) >>> >> >> This looks completely freaky to me... how does it work? Is it the >> laziness that allows the sum to be calculated first while preserving >> the structure (as thunks?), and then once the value of n is known it >> is propagated back down the tree and the actual tree values >> constructed? Anyways this is really amazing to my newbie eyes... >> >> Patrick >> -- >> ===================== >> Patrick LeBoutillier >> Rosemère, Québec, Canada >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://www.haskell.org/mailman/listinfo/beginners >

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What are these comments for {-# SCC "Mangler" #-}
by Aaron Gray 30 Dec '10

30 Dec '10

What are these comments for in Happy ? {-# SCC "Mangler" #-} Many thanks in advance, Aaron

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Reader monad
by michael rice 29 Dec '10

29 Dec '10

From: Control.Monad.Reader type Reader r = ReaderT r IdentityThe parameterizable reader monad. Computations are functions of a shared environment. The return function ignores the environment, while >>= passes the inherited environment to both subcomputations ============ Is there an unparameterizable reader monad? Michael

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Manatee-0.1.6 release!
by Andy Stewart 29 Dec '10

29 Dec '10

Hi all, Manatee-0.1.6 release, sorry for late, I was tired of the many trivial this month. http://www.youtube.com/watch?v=weS6zys3U8k hackage.haskell.org/package/manatee New version Manatee support customize and hot-swap, like elisp for Emacs, but much much fast and safe. Please look http://www.flickr.com/photos/48809572@N02/5304662424/lightbox/ to understand hot-swap framework. Example, copy IrcClient.hs (https://patch-tag.com/r/AndyStewart/manatee-ircclient/snapshot/current/cont… IrcClient.hs) to directory ~/.manatee/config/ then you can modify IrcClient.hs when manatee-ircclient is running, you just need press C-u to dynamic loading new version to *running* manatee-ircclient and won't lose state. Because GHC won't release memory after you dynamic-linking new version, so i recommend use this features as *develop mode* to accelerate develop. Once you adjust finish, you can press C-i to compile current configure file to build new manatee-ircclient. Next step, I will focus my time on IDE features.... Enjoy! -- Andy

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