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POHaskell readFile type problem
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copied!<p>I have some troubles with Haskell type system.</p> <p>Situation:</p> <ul> <li>Following program is taking list of filenames on the cmdline</li> <li>For all filename it reads its content using the function readFile</li> <li>Content of each file is passed to inputParser (Parsec)</li> <li>Rest is not so important</li> <li>Main problem is in function read_modules</li> <li>First two statements of the 'do' expression are invalid in Haskell type system</li> <li>Problem is conflict between [String] x IO String x [Char] x ...</li> <li>Function 'parse' should take String but when it gets it, it wants IO String suddenly (on the same argument), otherwise it wants String</li> </ul> <p>What do I want:</p> <ol> <li>Read each file content</li> <li>Pass that content to the 'parse' function as third argument</li> </ol> <p>Here is the code.</p> <pre><code>module Main where import System.IO import System.Environment import Text.ParserCombinators.Parsec import InputParser import Data usage :: IO () usage = putStrLn "Usage: x file file file option" parse_modules :: String -> [Char] -> Either ParseError [Module] parse_modules filename input = parse inputParser filename input read_modules :: [String] -> [Module] read_modules [] = []::[Module] read_modules (filename:rest) = do content <- readFile filename -- HERE is the problem modules <- case parse_modules filename content of -- HERE is problem too Left error -> do putStr "parse error at " print error Right out -> out ++ (read_modules rest) return modules use :: [String] -> IO () use args = do init <- last args filenames <- take (length args - 1) args modules <- read_modules filenames return () main :: IO () main = do args <- getArgs if length args < 2 then usage else use args </code></pre> <p>Here are the GHC output errors</p> <pre><code>ghc --make -o x.hs input-parser.hs data.hs [3 of 3] Compiling Main ( x.hs, x.o ) x.hs:19:4: Couldn't match expected type `IO String' against inferred type `[String]' In a stmt of a 'do' expression: content <- readFile filename In the expression: do content <- readFile filename modules <- case parse_modules filename content of { Left error -> do ... Right out -> out ++ (read_modules rest) } return modules In the definition of `read_modules': read_modules (filename : rest) = do content <- readFile filename modules <- case parse_modules filename content of { Left error -> ... Right out -> out ++ (read_modules rest) } return modules -- THIS ERROR is somewhat not important x.hs:30:4: Couldn't match expected type `[Char]' against inferred type `IO Char' Expected type: String Inferred type: IO Char In a stmt of a 'do' expression: init <- last args In the expression: do init <- last args filenames <- take (length args - 1) args modules <- read_modules filenames return () make: *** [x] Error 1 </code></pre> <p>What is the problem:</p> <ul> <li>I cannot understand what I should pass where. I kind of know what I want, but I don't get the syntax or the style.</li> <li>I am haskell greenie (not solving here)</li> <li>TYPES</li> </ul> <p>What are the questions:</p> <ul> <li>How do I fix presented type issue? What should I put into 'parse' function? What 'readFile' gives to me? Are those two types compatible? Isn't there need for some type of conversion?</li> </ul> <p>Relevant weblinks:</p> <ul> <li><a href="http://book.realworldhaskell.org/read/using-parsec.html" rel="nofollow noreferrer">http://book.realworldhaskell.org/read/using-parsec.html</a></li> <li><a href="http://www.zvon.org/other/haskell/Outputprelude/readFile_f.html" rel="nofollow noreferrer">http://www.zvon.org/other/haskell/Outputprelude/readFile_f.html</a></li> <li><a href="http://www.haskell.org/pipermail/haskell/2002-November/010748.html" rel="nofollow noreferrer">http://www.haskell.org/pipermail/haskell/2002-November/010748.html</a></li> </ul> <p>Thank you all for your hints and comments.</p>

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