February 2012 - Beginners

liftM2, how does it work?
by franco00＠gmx.com 24 Feb '12

24 Feb '12

I am a bit puzzled by liftM2. I know what it does, but let's take a look at the implementation. liftM2 :: (Monad m) => (a1 -> a2 -> r) -> m a1 -> m a2 -> m r liftM2 f m1 m2 = do { x1 <- m1; x2 <- m2; return (f x1 x2) } Which could be rewritten (I am a bit uncomfortable with the do notation): m1 >>= \x1 -> m2 >>= \x2 -> return (f x1 x2) so: liftM2 (+) [1,2] [3,4] = [4,5,5,6] The instance of the List monad tells me that: instance Monad [] where m >>= k = foldr ((++) . k) [] m m >> k = foldr ((++) . (\ _ -> k)) [] m return x = [x] fail _ = [] That foldr is just doing a concatMap, it seems to me. But I still don't get the | return (f x1 x2) | part. The first thing I thought was: return ( (+) [1,2] [3,4] ) but of course this is not the IO monad, the bind operator does not just fetch stuff from outer space. What is there really 'inside' x1 and x2 when the return statement is evaluated, though? I think I intuitively got it (x1 and x2 are bound to m1 and m2, every f x1 x2 expression is evaluated and then put together in a list), but it would be helpful to see what really happens to interiorise it. I started from the first step (>>=) m1 = foldr. ((++) . k) [] m1 -- partially applying m1 but cannot really proceed from there. Any help will be appreciated, if you think the question is not clear enough, do not hesitate to ask.

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Text.JSON.JSON instance for Data.Text.Text
by Vincent Ambo 24 Feb '12

24 Feb '12

Hej, I'm trying to create a JSON instance for Data.Text.Text. The CouchDB package uses Text.JSON, otherwise I would've considered using Aeson … This is what I've got so far: http://hpaste.org/64281 showJSON and readJSON work with this, but fromJSON and toJSON from Text.JSON.Generic don't. I'm not really sure how to 'tell' these functions how to use my instance. Any ideas? :] Vincent

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More Context for Failures (Data.Aeson)
by Darrin Thompson 24 Feb '12

24 Feb '12

I had a problem last night where I was parsing a lot of non-trivial and annoyingly inconsistent json. These json documents have a very bad habit of replacing lists with single objects when a particular document contains a singleton list, which is maddening, but I can work around it when I know to expect it. That's just one example of the kinds of failures due to inconsistent documents. My parsing code looks like this: (Data.Aeson, btw) instance FromJSON Whatever where parseJSON (Object o) = do a <- parseSomething o b <- parseSomethingElse o c <- ... andManyMore <- ... return $ Whatever a b c ... parseJSON _ = fail "dude" In my code I might have 10 sub parsers so any of them might fail. When the parser fails I see a failure message like "expected [a] but got Object" and no additional context. And of course if I was in a language with a stack I could just look at the stack. (And I'd be miserable in other ways. So I'm not exactly complaining...) These documents are long enough that I burned a lot of time comparing Aeson dumps looking for where I got an object instead of an array. Wouldn't it be neato if I could do this... instance FromJSON Whatever where parseJSON (Object o) = do a <- failureContext "branchA" $ parseSomething o b <- failureContext "branchB" parseSomethingElse o return $ Whatever a b parseJSON _ = fail "dude" Now when it dies, the error message will be "branchB: expected [a] but got Object". This seems like a useful utility. Does it already exist and I don't know it? Would it be easy to write? I'm a little bit out of my depth here. I've never tried to handle exceptions in Haskell before. Seems like it would be something like: failureContext :: (MonadIOSomething m) => String -> m a -> m a failureContext errorContextName = -- Catch exceptions and rewrite messages -- or wrap the exception in something and rethrow like Erlang or Java -- Darrin

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Haskell Platform on OS X Lion with Xcode 4.3
by Philippe Sismondi 21 Feb '12

21 Feb '12

Greetings. I have been had the Haskell Platform installed on my Mac for a long time. Last week I got a new faster Mac which is running OS X 10.7.3 and has Xcode 4.3 installed. I cannot install the Haskell Platform on it because the HP complains that it cannot find the developer tools. (BTW, Xcode 4.3 *does* appear to include the 10.5 SDK. Has anyone had luck installing the HP in this environment? (I know that this is not the ideal mailing list for this question. The haskell-platform mailing list seems to be broken at the moment, as is the haskell-platform bug reporting trac website.) - P -

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Adding to a package
by AbdulSattar Mohammed 21 Feb '12

21 Feb '12

I just submitted a patch to Wai, but I didn't like the process I went through while testing it on my machine. I cloned the repo, modified it and ran runghc Setup configure --prefix=$HOME --user runghc Setup install That registered as something like wai-2.1.1.1. ( I don't know what it exactly looked like, but it appended a ".1" to the previous version.) When I ran my test code, it used the latest version. Fine. But, is this the way everyone does that? I mean, adding a new version? I can't think of any other way, but I'm just against registering the package into the GHC registry. How does everyone do that? -- Warm Regards, AbdulSattar Mohammed

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A question about import
by bahadýr altan 19 Feb '12

19 Feb '12

Hi, when I try to use import like these: import System.Random import Control.Monad I get this error: "Syntax error in input (unexpected keyword "import")". Can you tell me why this happens and how I can fix it? Thanks..

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A simple remove function
by bahadýr altan 19 Feb '12

19 Feb '12

Hello. I'm trying to write a code which deletes the wanted element in a list. Like this : myremove "doaltan" 3 = "doltan". I wrote the code below but it doesn't even compile. Could you help me with fixing the code? Thanks! myremove x s = (take (s-1) x):(drop (s+1) x)

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A question about pattern matching
by bahadýr altan 18 Feb '12

18 Feb '12

Hello. I'm wondering what " (Eq a) =>" part in the code below does.. I couldn't figure it myself.. Thanks in advance :) isPalindrome :: (Eq a) => [a] -> Bool isPalindrome xs = xs == (reverse xs)

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The dot operator.
by bahadýr altan 18 Feb '12

18 Feb '12

Hello. I saw the dot "." operator in some Haskell codes, but I couldn't figure out what it does. I didn't see it in some books and tutorials and couldn't find an explanation on the Internet. Could you tell me what it does? Thanks:)

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my own last function
by bahadýr altan 18 Feb '12

18 Feb '12

Hello everyone, I have just started studying and started trying to solve 99 questions in haskell.org. However I got stuck even in the first question. The first question wants me to write my own last function and I wrote this : q1mylast :: [x] -> x q1mylast [a] = if length [a] <=1 then a else q1mylast (tail [a]) This program doesn't work and it gives this error : Program error: pattern match failure: q1mylast "doaltan" I'll be very glad if you could tell me what's wrong with the code and how can I fix it.. Thanks in advance :)

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