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POjson parsing in haskell part 2 - Non-exhaustive patterns in lambda
 

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  1. POjson parsing in haskell part 2 - Non-exhaustive patterns in lambda
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    <p>This is actually in continuation of the <a href="https://stackoverflow.com/q/17844223/1045053">question</a> I asked a few days back. I took the applicative functors route and made my own instances.</p> <p>I need to parse a huge number of json statements all in a file, one line after the other. An example json statement is something like this - </p> <pre><code>{"question_text": "How can NBC defend tape delaying the Olympics when everyone has Twitter?", "context_topic": {"followers": 21, "name": "NBC Coverage of the London Olympics (July &amp; August 2012)"}, "topics": [{"followers": 2705, "name": "NBC"},{"followers": 21, "name": "NBC Coverage of the London Olympics (July &amp; August 2012)"}, {"followers": 17828, "name": "Olympic Games"}, {"followers": 11955, "name": "2012 Summer Olympics in London"}], "question_key": "AAEAABORnPCiXO94q0oSDqfCuMJ2jh0ThsH2dHy4ATgigZ5J", "__ans__": true, "anonymous": false} </code></pre> <p><em>sorry for the json formatting. It got bad</em></p> <p>I have about 10000 such json statements and I need to parse them. The code I have written is something like this -</p> <pre><code>parseToRecord :: B.ByteString -&gt; Question parseToRecord bstr = (\(Ok x) -&gt; x) decodedObj where decodedObj = decode (B.unpack bstr) :: Result Question main :: IO() main = do -- my first line in the file tells how many json statements -- are there followed by a lot of other irrelevant info... ts &lt;- B.getContents &gt;&gt;= return . fst . fromJust . B.readInteger . head . B.lines json_text &lt;- B.getContents &gt;&gt;= return . tail . B.lines let training_data = take (fromIntegral ts) json_text let questions = map parseToRecord training_data print $ questions !! 8922 </code></pre> <p>This code gives me a runtime error <code>Non-exhaustive patterns in lambda</code>. The error references to <code>\(Ok x) -&gt; x</code> in the code. By hit and trial, I came to the conclusion that the program works ok till the 8921th index and fails on the 8922th iteration.</p> <p>I checked the corresponding json statement and tried to parse it standalone by calling the function on it and it works. However, it doesn't work when I call <em>map</em>. I don't really understand what is going on. Having learnt a little bit of haskell in "learn haskell for a great good", I wanted to dive into a real world programming project but seem to have got stuck here.</p> <p>EDIT :: complete code is as follows</p> <pre><code>{-# LANGUAGE BangPatterns #-} {-# OPTIONS_GHC -O2 -optc-O2 #-} {-# OPTIONS_GHC -fno-warn-incomplete-uni-patterns #-} import qualified Data.ByteString.Lazy.Char8 as B import Data.Maybe import NLP.Tokenize import Control.Applicative import Control.Monad import Text.JSON data Topic = Topic { followers :: Integer, name :: String } deriving (Show) data Question = Question { question_text :: String, context_topic :: Topic, topics :: [Topic], question_key :: String, __ans__ :: Bool, anonymous :: Bool } deriving (Show) (!) :: (JSON a) =&gt; JSObject JSValue -&gt; String -&gt; Result a (!) = flip valFromObj instance JSON Topic where -- Keep the compiler quiet showJSON = undefined readJSON (JSObject obj) = Topic &lt;$&gt; obj ! "followers" &lt;*&gt; obj ! "name" readJSON _ = mzero instance JSON Question where -- Keep the compiler quiet showJSON = undefined readJSON (JSObject obj) = Question &lt;$&gt; obj ! "question_text" &lt;*&gt; obj ! "context_topic" &lt;*&gt; obj ! "topics" &lt;*&gt; obj ! "question_key" &lt;*&gt; obj ! "__ans__" &lt;*&gt; obj ! "anonymous" readJSON _ = mzero isAnswered (Question _ _ _ _ status _) = status isAnonymous (Question _ _ _ _ _ status) = status parseToRecord :: B.ByteString -&gt; Question parseToRecord bstr = handle decodedObj where handle (Ok k) = k handle (Error e) = error (e ++ "\n" ++ show bstr) decodedObj = decode (B.unpack bstr) :: Result Question --parseToRecord bstr = (\(Ok x) -&gt; x) decodedObj where decodedObj = decode (B.unpack bstr) :: Result Question main :: IO() main = do ts &lt;- B.getContents &gt;&gt;= return . fst . fromJust . B.readInteger . head . B.lines json_text &lt;- B.getContents &gt;&gt;= return . tail . B.lines let training_data = take (fromIntegral ts) json_text let questions = map parseToRecord training_data let correlation = foldr (\x acc -&gt; if (isAnonymous x == isAnswered x) then (fst acc + 1, snd acc + 1) else (fst acc, snd acc + 1)) (0,0) questions print $ fst correlation </code></pre> <p>here's the <a href="http://hr-testcases.s3.amazonaws.com/688/sample.zip" rel="nofollow noreferrer">data</a> which can be given as input to the executable. I'm using ghc 7.6.3. If the program name is ans.hs, I followed these steps.</p> <pre><code>$ ghc --make ans.hs $ ./ans &lt; path/to/the/file/sample/answered_data_10k.in </code></pre> <p>thanks a lot!</p>
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